Atmel AT01607 Installation and operating instructions

APPLICATION NOTE
Atmel AT01607: XMEGA C Schematic Checklist
8-bit Atmel Microcontrollers
Features
•Power supplies
•Reset circuit
•Clocks and crystal oscillators
•PDI
•USB
Introduction
This application note describes a common checklist which should be used when
starting and reviewing the schematics for an Atmel®AVR®XMEGA®C design.
8466A−AVR−06/2013

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Table of Contents
1. Power supplies...................................................................................3
1.1 Power supply connections ................................................................................3
1.2 External analog reference connections.............................................................4
2. External reset circuit...........................................................................4
3. Clocks and crystal oscillators .............................................................5
3.1 External clock source........................................................................................5
3.2 Crystal oscillator................................................................................................6
3.3 Real-time oscillator............................................................................................6
4. PDI interface......................................................................................8
5. USB interface.....................................................................................9
6. Suggested reading...........................................................................10
6.1 Datasheets and manual..................................................................................10
7. Revision History...............................................................................11

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1. Power supplies
1.1 Power supply connections
All power supply pins of the device must be connected to the microcontroller supply.
Both VCC (digital) and AVCC (analog) must be connected to the same microcontroller positive supply, thus ensuring that
they both share an identical supply profile. Likewise both ground pins must be connected to the same microcontroller
ground reference supply.
Figure 1-1. Power supply schematic.
10μF
GND
AVCC
AGND
Close to device
POWER
SUPPLY
10μH100nF
VCC
100nF
10μF
Ferrite
Bead
Close to power
supply Close to device Close to device
Table 1-1. Power supply checklist.
Signal name Recommended pin connection Description
VCC 1.6V to 3.6V Decoupling/filtering capacitors 100nF (1)(2) and 10µF (1)
Decoupling/filtering inductor 10µH (1)(3) Digital supply voltage
AVCC 1.6V to 3.6V
Decoupling/filtering capacitors 100nF (1)(2) and 10µF (1)
Ferrite bead (4) prevents the VCC noise interfering the AVCC Analog supply voltage
GND Ground
Notes: 1. These values are given only as a typical example (that is, ceramic capacitors: 100nF, SMD 0402, X7R,
16V and 10µF, SMD1206, X5R, 6.3V) (that is, inductor: 10µH, 1.2A).
2. Decoupling capacitor should be placed close to the device for each supply pin pair in the signal group, low
ESR caps should be used for better decoupling.
3. Wire wound inductor should be added between the external power and the VCC for power filtering.
4. Ferrite bead has better filtering performance than the common inductor at high frequency. It can be added
between VCC and AVCC for preventing digital noise from entering the analog power. The BEAD should
provide enough impedance (for example, 220Ω at 100MHz, rated current 200mA, that is, Murata
BLM15BB221SN1D) for separating the digital power to the analog power.

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1.2 External analog reference connections
Atmel AVR XMEGA C proposes one ADC using internal references or an external analog reference (AREFA on
PORTA).
The following schematic checklist is only recommended if the design is using the external analog reference. If the
internal reference is used, the circuit is not necessary.
Figure 1-2. External VREF schematic.
EXTERNAL
REFERENCE 1 4.7μF100nF
Close to device
AGND
AREFA
Close to power
supply
Table 1-2. External analog reference checklist.
Signal name Recommended pin connection Description
AREFA 1.0V to AVCC-0.6V for ADC
Decoupling/filtering capacitors 100nF (1)(2) and 4.7µF (1) External reference from AREF pin on PORT A
GND Ground
Notes: 1. These values are given only as a typical example.
2. Decoupling capacitor should be placed close to the device.
2. External reset circuit
The external reset circuit is connected to /RESET pin only if the external reset function is used.
Figure 2-1. External reset circuit example schematic.
RESET
10 kΩ
VCC
Reset
Switch
100 Ω
100nF

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Table 2-1. Reset circuit checklist.
Signal name Recommended pin connection Description
______
RESET
Reset low level threshold voltage
VCC = 2.7 - 3.6V: Below 0.45 × VCC
VCC = 1.6 - 2.7V: Below 0.42 × VCC (VCC = 2.7V included) Reset pin
Notes: The pull-up resistor makes sure that reset does not go low unintended. When the PDI programming and
debugging is used, the reset line is used as clock. The reset pull-up should be 10kΩ or weaker, or be
removed.
The pull-down resistor prevents from overvoltage on the RESET pin when the switch is pressed.
Any reset capacitors should be removed if PDI programming and debugging is used. Other external reset
sources should be disconnected
3. Clocks and crystal oscillators
3.1 External clock source
Figure 3-1. External clock source example schematic.
XTAL2
XTAL1
External Clock
N.C.
Table 3-1. External clock source checklist.
Signal name Recommended pin connection Description
XTAL1 XTAL1 is used as input for an external clock signal Input for inverting oscillator pin 1
XTAL2 Can be left unconnected or used as GPIO

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3.2 Crystal oscillator
Figure 3-2. Crystal oscillator example schematic.
Up to
16 MHz
15pF
15pF
XTAL2
XTAL1
Table 3-2. Crystal oscillator checklist.
Signal name Recommended pin connection Description
XTAL1 Load capacitor 15pF
(1)(2)
External crystal between 0.4MHz to 16MHz
XTAL2 Load capacitor 15pF (1)(2)
Notes: 1. These values are given only as a typical example. Please refer to the crystal datasheet to determine the
capacitor value for the crystal used or refer to the application note “AVR1003: Using the XMEGA Clock
System”.
2. Load capacitors should be placed close to the device and crystal pins.
3.3 Real-time oscillator
The low-frequency crystal oscillator is optimized for use with a 32.768kHz watch crystal. When selecting crystals, load
capacitance and crystal’s equivalent series resistance, ESR must be taken into consideration. Both values are specified
by the crystal vendor.
The Atmel AVR XMEGA C oscillator is optimized for very low power consumption, and thus when selecting crystals, see
Table 3-3 for maximum ESR recommendations on 9pF and 12.5pF crystals.
Table 3-3. Maximum ESR recommendation for 32.768kHz watch crystal.
Crystal CL [pF] Maximum ESR [kΩ]
(1)
9.0 65
12.5 30
Notes: 1. Maximum ESR is typical value based on characterization.
The low-frequency crystal oscillator provides an internal load capacitance of typical 3.0pF. Crystals with recommended
3.0pF load capacitance can be without external capacitors as shown in Figure 3-3.

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Figure 3-3. Real-time oscillator without load capacitor.
32.768kHz
TOSC1
TOSC2
Crystals specifying load capacitance (CL) higher than 3.0pF, require external capacitors applied as described in Figure
3-4.
Figure 3-4. Real-time oscillator with load capacitor.
32.768kHz
18pF
18pF
TOSC1
TOSC2
To find suitable load capacitance for a 32.768kHz crystal, please consult the crystal datasheet.
Table 3-4. External real-time oscillator checklist.
Signal name Recommended pin connection Description
TOSC1 Load capacitor 18pF
(1)(2)
Timer oscillator pin 1
TOSC2 Load capacitor 18pF
(1)(2)
Timer oscillator pin 2
Notes: 1. These values are given only as a typical example. Please refer to the crystal datasheet to determine the
capacitor value for the crystal used or refer to the application notes “AVR1003: Using the XMEGA Clock
System” and “AVR4100: Selecting and testing 32kHz crystal oscillators for Atmel AVR microcontrollers”.
2. Load capacitors should be placed close to the crystal, GND and device oscillator pins.

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4. PDI interface
Figure 4-1. PDI interface example schematic.
PDI_DATA
RESET
VCC
10 kΩ
DATA
GND
N.C.
VCC
CLK
N.C.
3
5
1
4
6
2x3 header
2
PDI_CLK
The connector pinout that is shown in Figure 4-1 mates with Atmel tools like the Atmel AVR JTAGICE 3 and Atmel AVR
ONE!
Table 4-1. PDI port interface checklist.
Signal name Recommended pin connection Description
PDI_CLK
This pull-up resistor makes sure that reset does not go low
unintended. When the PDI programming and debugging is used,
the reset line is used as clock. The reset pull-up should be 10kΩ
or weaker, or be removed.
Any reset capacitors should be removed if PDI programming
and debugging is used. Other external reset sources should be
disconnected.
PDI clock input / reset pin
PDI_DATA PDI_DATA: PDI data input /
output

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5. USB interface
The impedance of the USB differential data line pair is 90Ωto each other and 45Ωto ground. The termination of the line
is included within the Atmel AVR XMEGA C device as serial resistors. To ensure proper signal integrity, the two D+/D-
signals must be closely routed on the PCB (Refer to “AVR1017: XMEGA - USB Hardware Design Recommendations”
application note).
Figure 5-1. Low-cost USB interface example schematic.
VBUS
USB
Connector
VBUS
D+
D-
GND
Shield
GND (Board)
USB_D+
USB_D-
USB
Differential
Data Line Pair
Figure 5-2. Protected USB interface example schematic.
4.5nF
1MΩ
VBUS
USB
Connector
VBUS
D+
D-
GND
Shield
USB Transient
protection
RC Filter
(GND/Shield
Connection) GND (Board)
USB_D+
USB_D-
USB
Differential
Data Line Pair

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Table 5-1. USB interface checklist.
Signal
name Recommended pin connection Description
D+ • The impedance of the pair should be matched on the
PCB to minimize reflections
• USB differential tracks should be routed with the
same characteristics (length, width, number of vias, etc.)
• Signals should be routed as parallel as possible, with
a minimum number of angles and vias
USB full speed / low speed positive
data upstream pin
D- USB full speed / low speed negative
data upstream pin
6. Suggested reading
6.1 Datasheets and manual
The datasheet and the manual contain block diagrams of the peripherals and details about implementing firmware for
the device. The datasheet and the manual are available on http://www.atmel.com/AVR in the Datasheets & Manuals
section.

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7. Revision History
Doc. Rev.
Date
Comments
8466A
06/2013
Initial document release

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