RCAT RCAT-1A Revision A3 User guide

Copyright © 2015 Robot Circuits, LLC
1
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
RCAT
Robotic System Control Board
Serious Power for the Serious Designer
System designer’s manual
Current Version: RCAT-1A Revision A3
Robot Circuits, LLC
[e-mail]
[e-mail]

Copyright © 2015 Robot Circuits, LLC
2
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Table of Contents
RCAT................................................................................................................................................................................ 1
Robotic System Control Board...................................................................................................................................... 1
Introduction..................................................................................................................................................................... 5
So what in the world does “RCAT™”mean? .................................................................................................................. 5
What are we trying to do that other boards cannot?.................................................................................................... 6
RCAT™ Board specifics ..................................................................................................................................................... 8
Power System .............................................................................................................................................................. 8
Schematic ................................................................................................................................................................ 8
Major Subsystems.......................................................................................................................................................... 10
Subsystems –Detailed discussion .................................................................................................................................. 16
LED ............................................................................................................................................................................ 16
Configuration ......................................................................................................................................................... 16
Control................................................................................................................................................................... 16
Schematic .............................................................................................................................................................. 16
System Clock.............................................................................................................................................................. 17
Schematic .............................................................................................................................................................. 17
Notes ..................................................................................................................................................................... 17
Extended Static RAM.................................................................................................................................................. 17
Schematic .............................................................................................................................................................. 17
Notes ..................................................................................................................................................................... 18
TWI Interface ............................................................................................................................................................. 18
Schematic .............................................................................................................................................................. 18
Notes ..................................................................................................................................................................... 19
1M-bit serial EEPROM ................................................................................................................................................ 19
Accelerometer ........................................................................................................................................................... 19
SPI Interface............................................................................................................................................................... 20
Schematic .............................................................................................................................................................. 20
ADC AREF................................................................................................................................................................... 21
Schematic .............................................................................................................................................................. 21
JTAG .......................................................................................................................................................................... 21
Schematic .............................................................................................................................................................. 21
Mosfet Drivers ........................................................................................................................................................... 22
Schematic .............................................................................................................................................................. 22

Copyright © 2015 Robot Circuits, LLC
3
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Alternatives for controlling the mosfets ................................................................................................................. 24
Direct I/O Access........................................................................................................................................................ 25
Primary I/O Connector ........................................................................................................................................... 26
SPI Connector......................................................................................................................................................... 27
USART 0 ................................................................................................................................................................. 27
USART 2 Port –J14 and J7 ...................................................................................................................................... 27
3.3 Volt Input/Output............................................................................................................................................. 28
USART 3 Port –J8................................................................................................................................................... 30
Schematic .............................................................................................................................................................. 31
USART 1 Port –J6................................................................................................................................................... 32
Schematic .............................................................................................................................................................. 32
Final comments about USART direct connections....................................................................................................... 34
Summary of direct access ports.................................................................................................................................. 34
USARTS, and Serial Communications Interfaces ............................................................................................................. 35
RS422/485 Interface ...................................................................................................................................................... 40
Schematic .................................................................................................................................................................. 40
Free RS232 Channel ....................................................................................................................................................... 41
Ethernet System ............................................................................................................................................................ 41
Programming IDE (interactive Development Environment) ........................................................................................ 43
RCAT™ Startup Solution Software .............................................................................................................................. 43
In-Circuit Emulator..................................................................................................................................................... 43
Demonstration Software................................................................................................................................................ 44
RCAT™ Specifications..................................................................................................................................................... 45
Schematic Diagram ........................................................................................................................................................ 46

Copyright © 2015 Robot Circuits, LLC
4
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Warranties
1. (a) Robot Circuits, LLC warrants for a period of one (1) year from the date of sale that the RCAT™ Control Board conforms to and will
perform according to the Specifications.
(b) Robot Circuits, LLC shall not be liable for any defects caused by abuse, neglect, improper use, installation, or modification of the
RCAT™, nor as a result of any customer design or use involving the attachment of any devices, testing apparatus, or other hardware.
(c) Robot Circuits, LLC shall not be liable for any damage or harm, bodily or other, resulting from specifications, designs documentation,
or instructions, verbal or written, not produced by Robot Circuits, LLC.
2. If the RCAT™ Control Board fails to perform to the warranty set forth in section 1 above, then Robot Circuits’ sole liability shall be to
effect the replacement of the board. Replacement board shall be covered under the warranty for the full warranty period.
3. THIS LIMITED WARRANTY IS THE END-USER’S SOLE AND EXCLUSIVE REMEDY AGAINST ROBOT CIRCUITS, LLC WHERE PERMITTED BY LAW.
AND SUBJECT TO SECTION (8). EXCEPT AS SET FORTH ABOVE, PRODUCTS ARE PROVIDED “AS IS” AND “WITH ALL FAULTS”. ROBOT
CIRCUITS, LLC DISCLAIMS ALL OTHER WARRANTIES, EXPRESS OR IMPLIED, REGARDING PRODUCTS, INCLUDING, BUT NOT LIMITED TO,
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
4. The Customer agrees that prior to using any systems that include Robot Circuits, LLC products, the Customer will test such systems and
the functionality of the products as used in such systems. Robot Circuits, LLC may provide technical, applications or design advice, quality
characterization, reliability data or other services. The Customer acknowledges and agrees that providing these services shall not expand
or otherwise alter Robot Circuits, LLC’s warranties, as set forth above, and that no additional obligations or liabilities shall arise from
Robot Circuits, LLC providing such services.
5. Robot Circuits, LLC products are not authorized for use in safety-critical applications where a failure of the Robot Circuits, LLC product
would reasonably be expected to cause severe personal injury or death. Safety-critical applications include, without limitation, life
support devices and systems, equipment or systems for the operation of nuclear facilities and weapons systems. Robot Circuits, LLC
products are neither designed nor intended for use in military or aerospace applications or environments, nor for automotive
applications or the automotive environment. The Customer acknowledges and agrees that any such use of Robot Circuits, LLC products is
solely at the Customer’s risk, and that the Customer is solely responsible for compliance with all legal and regulatory requirements in
connection with such use.
6. The Customer acknowledges and agrees that the Customer is solely responsible for compliance with all legal, regulatory and safety-
related requirements concerning the products and any use of Robot Circuits, LLC products in the Customer’s applications, not-with-
standing any applications-related information or support that may be provided by Robot Circuits, LLC.
7. In no event shall Robot Circuits, LLC be liable to the Customer or any third parties for any special, collateral, indirect, punitive, incidental,
consequential or exemplary damages in connection with or arising out of the products provided hereunder, regardless of whether Robot
Circuits, LLC has been advised of the possibility of such damages. This section will survive the termination of the warranty period.
8. Robot Circuits, LLC may make changes to specifications and product descriptions at any time, without notice. The product information on
the Robot Circuits Web Site is subject to change without notice.
9. Statutory laws. *
(i) some countries, regions, states or provinces do not allow the exclusion or limitation of remedies or of incidental, punitive, or
consequential damages, or the applicable time periods, so the above limitations or exclusions may not apply.
(ii) except to the extent lawfully permitted, this limited warranty does not exclude, restrict or modify statutory rights applicable to where
the product is sold but, rather, is in addition to these rights.
(*) European Consumer Centres provide information on EU-wide consumer laws as well as consumer laws for specific countries:
http://ec.europa.eu/consumers/ecc/contact_en.htm

Copyright © 2015 Robot Circuits, LLC
5
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Introduction
Welcome to the world of RCAT™–We here at Robot Circuits believe you will find that the RCAT™control board
is simply the most versatile, powerful, easy to use, and cost-effective control board on the market today.
This manual will introduce you to the many features that are resident on the board and will guide you through the
process of configuring the board for your specific application. But before we get started, please allow us to give you a
little background on why we even bothered bringing this board into existence…
First things first…
So what in the world does “RCAT™”mean?
Well, RCAT™is an acronym for:
Robotic Controller based on the ATMega™2560 microcontroller by Atmel Corporation.
The RCAT™is, put quite simply, one of the most powerful low-cost single board Microcontroller boards available on the
robotic enthusiast market today.
Consider this summary of the RCAT’s features:
The RCAT™ is based on the Atmel ATMega™2560 Microcontroller which features
o256K of Program Flash ROM
o4K internal EEPROM
o8K internal SRAM
o4 High-speed USARTS
oSPI port
oTWI (I2C) Interface
oWatchdog timer
oMany more features that you can view at Atmel.com
Direct access to 48 of the ATMega™2560’s most crucial I/O ports
Two 30V/400mA Mosfet drivers
128K External Static RAM (directly addressable by the ATMega™2560 as 2 pages of 64K each)
Most communications interfaces feature selectable as 3.3V/5V
Diagnostic 3-color LED
1M-bit Serial EEPROM
14 different communications devices/electrical interfaces/protocols/voltage levels
Power switch connection
Selectable onboard Vcc regulation or 5Vdc primary power input
On-board 3.3Vdc voltage regulator
Easy connection to Network via Serial-Ethernet Server (not included)
On-board USB connector with bridge to serial I/O system
3 channels of RS-232 RX/TX
ADXL345 3-Axis Accelerometer for fast, accurate measurement of inertial and attitudinal situations
Connectors oriented for easy connection to sonars, servo’s or any device requiring typical Gnd/Vcc/Signal
connections

Copyright © 2015 Robot Circuits, LLC
6
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
JTAG connector for instant connection to in-circuit emulators such as Atmel’s ATATMEL-ICE™
Flawless functioning with Atmel AVR Studio (current version 7.0)
Demo code included –contains full AVR Studio solutions featuring driver examples for the RCAT board devices as
well as several peripherals. Code is implemented using freeRTOS, a powerful open-source preemptive
multitasking operating system, with XRAM configured as heap space, fully functioning dynamic memory
management, and demo modules each implemented as scheduled tasks. Best of all, it works and doesn’t take 2
weeks of study just to get installed and running on your system!
One of the major features of the RCAT board is its rich array of communications options. All totaled there are 14 different
serial interfaces available. Although only a subset of these is available at any one time, the flexibility of the
communications system allows you to interface with almost any serial device you can imagine. The 14 interfaces include:
USART 0:
USB 2.0
RS232
RS422/485
USART 1:
Direct Connection at 5V
Indirect Connection at 3.3V
Indirect Connection at 3.3V to Ethernet Converter Interface Connector
USART 2:
Direct Connection at 5V –Full Duplex
Direct Connection at 5V –Half Duplex
Indirect Connection at 3.3V –Full Duplex
Indirect Connection at 3.3V –Half Duplex
RS232
USART 3:
Direct Connection at 5V
Indirect Connection at 3.3V
Uncommitted RS232 (1 channel featuring TX-IN, TX-OUT, RX-IN and RX-OUT access to Transceiver)
What are we trying to do that other boards cannot?
We at Robot Circuits are robotic system designers ourselves. Almost daily we live through the chore of selecting
hardware that is:
easy to use
has the most on-board power without having to buy this shield or that add-on just to get basic needs that are
common with almost every design
offers a sufficient amount of direct connection to the CPU
is a no-brainer to connect to ICE equipment
is fast
has high-current drivers on-board

Copyright © 2015 Robot Circuits, LLC
7
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
includes lots of on-board communications capability (possibly our number one requirement)
is affordable
comes with good documentation
is easy to configure and get running for OUR APPLICATION
comes with actual working examples of useful code
It is not difficult to find project boards on the market that are little more than breakout boards, marketed with a
plethora of add-on boards and so-called “shields”. But what about us who need, say, a board that already comes
STANDARD with extended RAM so that we can implement dynamic memory allocation, or realtime O/S capability?
Sometimes we with real jobs and real deadlines just don’t have time or patience to shop around for what should be “the
basics”.
So, we designed the RCAT™ as an end-run around these issues. The RCAT™ comes STANDARD with:
128K of extended static RAM (divided into two 64K pages that are switchable via an I/O port)
An on-board ADXL345 accelerometer for easy inertial and attitudinal measurement
An onboard three-channel RS-232 transceiver
An onboard RS422/485 interface
An onboard USB bridge
An on board 1M-bit non-volatile RAM
Two onboard 30V/400mA mosfet drivers
A 3.3Vdc Voltage regulator with access to the outside world
In addition, we provide COMPLETE, FUNCTIONAL, LOAD-AND-RUN demonstration software that you can use as a
foundation for your application or just for educational purposes. This software is written with Atmel’s Atmel Studio™ in
mind. All that is required to get you up and running is the RCAT™ control board, a connection to an appropriate power
source, a PC capable of connecting to an ICE (in-circuit-emulator), an ICE (in-circuit-emulator) like the ATATMEL-ICE™,
and a download of Atmel’s free Atmel Studio™ IDE. Once installed, you can simply load our project file and begin
running.
Of all the things that have irritated us at Robot Circuits over the years is downloading somebody’s “demo” code, only to
find that it is useless unless we are using some compiler or ide that is mainstream only to super-geeks, and even then
having to study, study, study just to figure out how to set paths, and compiler flags, and this and that and you name it.
Then –to find that the code doesn’t even do what we needed it to do.
We work in the real world and we needed real information that was relevant to solving real problems.
THAT is what we are striving for at Robot Circuits –to provide you with a “no-games”, professional-grade robotic control
platform that can actually DO SOMETHING!
We hope you find that we have accomplished this by helping you to achieve your goals.
Now…..let’s talk tech…

Copyright © 2015 Robot Circuits, LLC
8
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
RCAT™ Board specifics
Power System
The power input schematic is shown in Figure 1 below:
Schematic
Figure 1
The power input connector, J3, allows several configurations and options for power to the board. The simplest and most
preferable is shown in Figure 2. In this configuration, a regulated 5Vdc is fed to J3 at pin 1. With JP3 installed, this routes
the 5Vdc immediately back out J3 on pins 2 and 5. Pin 2 is a redundant output where you can daisy-chain the 5Vdc to
other devices if desired. Pin 5 is meant for use when you need to switch the board itself on and off (independent of the
power supply). In this case, as show in Figure 2, you can place a switch between pins 5 and 6:
Figure 2

Copyright © 2015 Robot Circuits, LLC
9
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Optionally, you can connect a shorting wire in place of the switch if you will be controlling the RCAT™ power from the
power supply itself.
The RCAT™ can be fitted with an optional onboard 5Vdc regulator (U4) for cases where you do not have 5Vdc already
available. As shipped the RCAT™ does not include this regulator. This is something you would add yourself. The chip
required is the LM7805C –TO220 and can be purchased here. Cost is around 68 cents. Figure 3 shows the land patterns
where this chip installs.
Figure 3
The RCAT™ draws about 500mA when it isn’t driving external peripherals. When driving other peripheral devices such as
devices utilizing the full capability of the mosfets (which can draw up to 400mA each), and sonar ranging modules, etc.,
the current requirements increase accordingly. The LM7805 is good for 1A which will (with adequate heatsinking) suffice
for moderate current requirements, but for most applications we recommend using an external 5Vdc supply rated for
3A which should cover the most extreme cases. Of course, your particular design requirements will drive this decision.
As shown in Figure 2, there exists an onboard 3.3Vdc regulator (U5) that supplies power to the onboard devices
requiring 3.3Vdc. These include the accelerometer (about 10mA max), the USB bridge chip (about 26mA max), and the
5V to 3.3V voltage translators (about 100mA max) for a total consumption around 136mA. The regulator employed is a
LM2937-3.3 that is capable of up to 500mA. Under normal conditions, 136mA does not require a heat sink. However, if
you plan to tap into the 3.3Vdc to power external devices you may need to fit U5 with an appropriate heat sink –not
doing so can damage the RCAT™ and will void your warranty.

Copyright © 2015 Robot Circuits, LLC
10
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Major Subsystems
The RCAT™ control board comprises several major subsystems. Some of these subsystems are broken out to connectors
on the board for access by external user hardware. Others are not. Still others have access to the outside world through
interim circuitry. For example, the extended RAM subsystem is not broken out for external access. This means that CPU
ports PA0 through PA7, PC0 through PC7, PG0 through PG2 and finally PJ7 are dedicated to this subsystem and thus are
not brought out to external connection points. Consider the extended RAM subsystem schematic:
Figure 4
We call such subsystems “dedicated” in that they use CPU ports fully dedicated to nothing other than the subsystem.
As mentioned above, other subsystems have breakout points on various connectors. These may be “direct” connections
to the CPU or they may be “indirect” connections. An example of an “indirect” subsystem is the USB interface subsystem
shown here:
Figure 5
As you can see, CPU ports RXD0(PE0) and TXD0(PE1) connect to the USB bridge chip and then to the external world
through the bridge. Although you have “access”to the ports, it is “indirect” through an onboard device (the CP2102 USB
chip).
The “direct” access subsystem consists of 37 connections broken out on J5 as shown here:

Copyright © 2015 Robot Circuits, LLC
11
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Figure 6
Other ports that provide direct access are presented at other connectors that are configured for use by the alternate
port purposes. For example, connector J4 provides direct access to ports PF4, PF5, PF6, PF7 and the CPU reset input.
However, J4 is laid-out for direct connection to an in circuit emulator for use by the CPU JTAG system:
Figure 7
If you are not using the JTAG interface, these connection points are available for your use.
With the exception of PL0 and PL1 (discussed later), each of these points connects to and only to the port as labeled on
the connector. You are totally in control of these ports.

Copyright © 2015 Robot Circuits, LLC
12
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Ports PL0 and PL1 are a special case. They are shared with the control of the mosfet driver subsystem shown here:
Figure 8
As shown in the diagram on the left, from the CPU chip, ports PL0 and PL1 are connected on the board to the gates of
mosfets Q3 and Q4. However, they are also brought out to connector J5 via a pair of jumpers as shown in the right-hand
diagram. This enables you to control the gates either from the CPU via software or externally, by configuring ports PL0
and/or PL1 as inputs, and installing JP28 and/or JP29, thus bringing connection to the gates out to J5.
The third and final type of subsystem on the RCAT™ board is what we call a “bussed” subsystem. The primary system of
this type is the two-wire-interface (TWI) subsystem. Consider the following schematic:
Figure 9
Here we can see that CPU ports SCL(PD0) and SDA(PD1) interface with the onboard 1M-bit serial EEPROM (U3). But then
the ports are also connected to pullup resistors R5/R5 and then broken out for external access to J2 so that you have
direct access to the TWI subsystem as well.

Copyright © 2015 Robot Circuits, LLC
13
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
A complete list of all the subsystems on the RCAT™ is presented below:
Subsystem Name Type Access Connector Description
And Associated Jumpers
LED Dedicated N/A Onboard Green/Yellow LED controlled by ports
PL6 and PL7
System Clock Dedicated JP1 16MHz Crystal with XTAL1 disconnect via JP1
Extended Static RAM Dedicated N/A A Cypress Semiconductor CY7C1019D-10VXI
RAM chip is provided onboard that makes 128K
bytes of static RAM available to the CPU in two
pages of 64K bytes. Page selection is managed
via dedicated I/O port PJ7. The chip I/O reserves
CPU ports AD0-AD7, A8-A15, and PG0-PG2 for
its exclusive use.
TWI Interface Bussed J2 Provides direct access to CPU ports PD0 and
PD1 (SCL/SDA) via J2 pins 8 and 6 respectively.
Resistors R4 and R5 pull these pins up to Vcc as
required by TWI. Two additional pins, J2/7 and
J2/5, provide access to the TWI bus through
bidirectional 3V level shifters for interfacing to
3V TWI devices.
1M-bit serial EEPROM Bussed J2 M24M01-RMN6TP serial EEPROM controllable
via CPU or external TWI at J2. See TWI details
above and EEPROM datasheet for details. Chip
write enable (/WC) is directly controlled by port
PD4 on CPU chip. This is not accessible outside
the CPU.
Accelerometer Bussed/Direct J5, JP23, JP30 The onboard ADXL345 accelerometer chip
Interfaces with the CPU on the TWI bus. This
means that it can also be accessed by the
external TWI connector J2. Jumper JP23 is used
to select the device address and jumper JP30 is
used to enable/disable the device via its /CS
pin. The interrupt outputs (INT1 and INT2) are
brought out to the CPU at PE4 and PE5
respectively. These are also brought out to J5 at
the PE4/PE5 pins. See the accelerometer
section later in this document for schematic and
detailed usage directions.
SPI Interface Direct J1 Direct access to Ports PB1, PB2, PB3, RST
J1 configured for use in Serial Peripheral
Interface (SPI) (See ATMega™ Manual for
details)

Copyright © 2015 Robot Circuits, LLC
14
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Subsystem Name Type Access Connector Description
And Associated Jumpers
ADC AREF Direct J15 If an external AREF supply is desired for use in
the Analog-to-digital system, it may be injected
at J15 pin 1. Vcc and ground from the board are
also provided at J15/2 and J15/3 respectively.
See ADC details later in this document.
JTAG Direct J4 J4 is laid-out for direct connection via a 5X2
dual-row header to ICE equipment such as the
ATATMEL-ICE™. If not used for this purpose,
direct access to Vcc, Gnd, ports PF4-PF7, and
the /RST pin are available for general purpose
use.
Mosfet Drivers Direct J5, JP26-JP29 Two high-current mosfets enable driving loads
up to 30Vdc at up to 400mA each. On/off
control is determined by the installation of
jumpers JP28 and JP29. Loads are attached on
JP26 and JP27. When the control jumper(s) are
installed, on/off control can be achieved by
either the CPU on ports PL0 or PL1 or by
external control at J5 pins PL0_A and PL1_A. If
the jumpers are NOT installed, then control is
exclusively via the CPU under program control
and J5 PL0_A and PL1_A are not connected to
anything on the board. See schematics later in
the mosfets section for further details.
USART 0 Indirect J9, J11, J16, USART0 can be configured for any one of the
JP14, JP16, following :
JP18, JP19, RS232 (I/O available at J11)
JP24, JP25 USB (I/O available at J9)
RS422/485 (I/O available at J16)
See USART0 configuration details later in this
document for schematic and jumper
configurations.
USART 1 Direct/Indirect J6, J9, J12, USART1 can be configured for any one of the
JP6, JP7 following :
RS232 (I/O available at J12)
Direct connection at 3.3Vdc (I/O available at J6)
Direct connection at 5Vdc (I/O available at J6)
USB available at J9
See USART1 configuration details later in this
document for schematic and jumper
configurations.

Copyright © 2015 Robot Circuits, LLC
15
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Subsystem Name Type Access Connector Description
And Associated Jumpers
USART 2 Direct J7, J14 USART2 can be configured for any one of the
JP5, JP10, JP11 following :
Direct full duplex connection at 3.3Vdc
Direct full connection at 5Vdc
Direct half duplex connection at 3.3Vdc
Direct half connection at 5Vdc
Full duplex mode I/O available at J7
Half duplex mode available at J14
See USART2 configuration details later in this
document for schematic and jumper
configurations.
USART 3 Direct J8, USART3 can be configured for any one of the
JP12, JP13 following :
Direct connection at 3.3Vdc (I/O available at J8)
Direct connection at 5Vdc (I/O available at J8)
See USART3 configuration details later in this
document for schematic and jumper
configurations.
Power supply N/A J3, JP2, JP3, JP4 The RCAT™ board comes configured for
operation from a regulated 5Vdc 3Amp power
supply. It can also be configured for operation
from an unregulated input voltage of 7.5 to
35Vdc by installing an optional LM7805C
regulator in position U4. Please note that
adequate heat sinking is required if you choose
this option and install the regulator. It is up to
the user to provide adequate heat sink. The
RCAT™ has an onboard 3.3Vdc regulator for
powering the onboard chips and systems
requiring 3.3Vdc. This voltage is also available at
several points on the board for use by external
systems. It is incumbent upon the user to not
exceed current draw and to provide proper heat
sink if external 3.3V devices are connected.
Please see the datasheets for the LM2937-3.3
regulator chip for details and specs. Also note
that the 3.3Vdc provided by this chip is NOT the
same as the 3.3Vdc output that is made
available on the USB bridge chip U11.

Copyright © 2015 Robot Circuits, LLC
16
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Subsystems – Detailed discussion
In this section each of the above subsystems is discussed in more detail, including schematics to help you understand
the subsystem design and hopefully guide you to an understanding of how to use each.
LED
The onboard LED is controlled by the CPU from ports PL6 and PL7. In order to use the LED, both of these ports should be
configured as outputs through their associated Data Direction (DDR) registers.
Configuration
Register
Bit
State
DDRL
6
1
DDRL
7
1
Table 1
Control
LED State
PL6
PL7
Off
1
1
Yellow
0
1
Green
1
0
Yel/Grn
0
0
Table 2
Schematic
Figure 10

Copyright © 2015 Robot Circuits, LLC
17
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
System Clock
The onboard 16MHz crystal provides the CPU timing reference.
Schematic
Figure 11
Notes
Although rare, it is possible to “brick” the ATMega™ chip by improperly setting clock values in the programming
apparatus and even sometimes, again, very rarely, as a result of improper voltages or signals being applied to
numerous ports on the CPU. When that occurs, it is necessary to provide an externally generated clock to the
CPU in order to recover. There are plenty of how-tos available online in the event you should brick your board.
But one thing is always necessary –to inject an external clock into the XTAL1 input of the CPU. For this reason,
JP1 is provided so that you can remove the jumper and connect your external clock to the CPU. Hopefully you
will never need this, but in the event it happens, at least you won’t have to unsolder the crystal just to recover
your board.
Extended Static RAM
Schematic
Figure 12

Copyright © 2015 Robot Circuits, LLC
18
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Notes
128Kbytes of external static RAM are available to be used by the CPU. The ATMega™ 2560 only directly
addresses 64Kbytes. In order to make all 128Kbytes available, the RCAT™ board uses port PJ7 as address line
A16. The GCC compiler can be configured to utilize this RAM in a variety of ways. Please consult the
documentation for the IDE to learn the various possibilities. However, in no case is native support for more than
64Kbytes provided, so you will need to manage the paging via your own code. For most applications, 64K is
plenty of space, but the extra page is there if you need it.
The sample solution provided with your board configures most of page 1 of this RAM as heap and uses it in the
dynamic allocation functions such as malloc().
The CPU ports that interface to this RAM are not available for any other purpose on the RCAT™.
TWI Interface
Sometimes referred to as I2C (Philips Semiconductor Corp), the two wire interface (TWI) is a bus communication
system that forms an important communications channel on the RCAT™. Onboard devices that are controlled by
the TWI include:
1M-bit Serial EEPROM
ADXL345 Accelerometer chip
The TWI interface is also broken out for your use. Interface to a 3.3Vdc version as well as a 5Vdc version are
provided.
Schematic
Figure 13

Copyright © 2015 Robot Circuits, LLC
19
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
Notes
Connector J2 provides access to the TWI by you external devices. Pullups R4 and R5 are provided relieving you of
the need to pull the lines to Vcc. Also note the 3.3Vdc to 5Vdc level shifters (Q1/Q2) enable the connection of
3.3Vdc TWI devices to J2 pins 5 and 7. The example software solution provided with your RCAT™ board
configures the TWI for operation at 400KHz, so keep this in mind when connecting your device(s) and using the
example solution provided.
1M-bit serial EEPROM
The RCAT™ board includes a 1M-bit Serial EEProm chip, the STMicro M24M01-RMN6TP, for use however you
desire. This chip is managed via the TWI (described earlier). The schematic for this device is also shown above.
Please consult the manufacturer’s datasheets (M24M01-R) for details on how to properly use this device.
The example software solution provided with your RCAT™ board includes examples of how to read and write
from/to this device. Please note that since it hangs on the TWI, which is accessible from the outside world, you
could turn off any control from the CPU and use your own external control mechanisms to read and write this
chip.
Accelerometer
Onboard the RCAT™ is an ADXL345 accelerometer chip made by Analog Devices. Get datasheet here. The need
for an accelerometer in robotics applications is so commonplace that we decided to relieve our users from the
chore of buying and attaching a separate module dedicated to this purpose. This chip is a very powerful
accelerometer that features 3-axis measurement of attitudinal and inertial situations that make it possible to do
very accurate reckoning of position, tracking of motion and determination of pitch/roll/yaw in almost limitless
ways. The axis orientation of the chip is noted on the silkscreen of the RCAT™ as shown here:
Figure 14

Copyright © 2015 Robot Circuits, LLC
20
RCAT-1A Rev A3 Designer’s manual Serious Power for the Serious Designer
The example software solution provided with your RCAT™ includes code to read this device and calculate pitch
and roll information. Of course, much more complex data manipulation is possible (see the Analog Devices
datasheet for details), but this example will provide you with the basic methods for reading and writing the
various registers on the device.
Like the EEProm, the accelerometer is hung on the TWI system so that you could control it externally is desired.
Referring to the schematic under the TWI section above you see two jumpers associated with this device.
Jumper JP23 determines the address at which the accelerometer is configured:
Position Write addr Read addr
A-B
0x3A
0x3B
B-C
0xA6
0xA7
Table 3
The accelerometer features two interrupt outputs that are actuated in response to certain events inside the
chip. These interrupt outputs are sent to the CPU on PE4 and PE5 as shown in the schematic in the TWI section
above. In addition these interrupts are broken out to connector J5 on positions PE4 and PE5 for monitoring by
your external hardware.
SPI Interface
CPU ports PB1, PB2 and PB3 are brought out to connector J1 as shown below:
Schematic
Figure 15
As shown, access to the CPU /RST pin as well as Vcc and Gnd are also provided. J1 is laid-out to allow easy
connection to programming devices that utilize Serial Peripheral Interface (SPI). If not utilizing the SPI system,
these CPU ports are available for general purpose use.