Atmel AT90S2313 User manual

1
Features
•Utilizes the AVR®RISC Architecture
•AVR - High-performance and Low-power RISC Architecture
– 118 Powerful Instructions - Most Single Clock Cycle Execution
– 32 x 8 General Purpose Working Registers
– Up to 10 MIPS Throughput at 10 MHz
•Data and Nonvolatile Program Memory
– 2K Bytes of In-System Programmable Flash
Endurance 1,000 Write/Erase Cycles
– 128 Bytes of SRAM
– 128 Bytes of In-System Programmable EEPROM
Endurance: 100,000 Write/Erase Cycles
– Programming Lock for Flash Program and EEPROM Data Security
•Peripheral Features
– One 8-bit Timer/Counter with Separate Prescaler
– One 16-bit Timer/Counter with Separate Prescaler,
Compare, Capture Modes and 8-, 9- or 10-bit PWM
– On-chip Analog Comparator
– Programmable Watchdog Timer with On-chip Oscillator
– SPI Serial Interface for In-System Programming
– Full Duplex UART
•• Special Microcontroller Features
– Low-power Idle and Power Down Modes
– External and Internal Interrupt Sources
•• Specifications
– Low-power, High-speed CMOS Process Technology
– Fully Static Operation
•Power Consumption at 4 MHz, 3V, 25°C
– Active: 2.8 mA
– Idle Mode: 0.8 mA
– Power Down Mode: <1 µA
•I/O and Packages
– 15 Programmable I/O Lines
– 20-pin PDIP and SOIC
•Operating Voltages
– 2.7 - 6.0V (AT90S2313-4)
– 4.0 - 6.0V (AT90S2313-10)
•Speed Grades
– 0 - 4 MHz (AT90S2313-4)
– 0 - 10 MHz (AT90S2313-10)
Description
The AT90S2313 is a low-power CMOS 8-bit microcontroller based on the AVR RISC
architecture. By executing powerful instructions in a single clock cycle, the
Rev. 0839E–04/99
8-bit
Microcontroller
with 2K bytes
In-System
Programmable
Flash
AT90S2313
Pin Configuration PDIP/SOIC
(continued)

AT90S2313
2
AT90S2313 achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consump-
tion versus processing speed.
The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are directly
connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one single instruction
executed in one clock cycle. The resulting architecture is more code efficient while achieving throughputs up to ten times
faster than conventional CISC microcontrollers.
Figure 1. The AT90S2313 Block Diagram

AT90S2313
3
The AT90S2313 provides the following features: 2K bytes of In-System Programmable Flash, 128 bytes EEPROM,
128 bytes SRAM, 15 general purpose I/O lines, 32 general purpose working registers, flexible timer/counters with compare
modes, internal and external interrupts, a programmable serial UART, programmable Watchdog Timer with internal oscilla-
tor, an SPI serial port for Flash Memory downloading and two software selectable power saving modes. The Idle Mode
stops the CPU while allowing the SRAM, timer/counters, SPI port and interrupt system to continue functioning. The power
down mode saves the register contents but freezes the oscillator, disabling all other chip functions until the next external
interrupt or hardware reset.
The device is manufactured using Atmel’s high density nonvolatile memory technology. The on-chip In-System Program-
mable Flash allows the program memory to be reprogrammed in-system through an SPI serial interface or by a
conventional nonvolatile memory programmer. By combining an enhanced RISC 8-bit CPU with In-System Programmable
Flash on a monolithic chip, the Atmel AT90S2313 is a powerful microcontroller that provides a highly flexible and cost
effective solution to many embedded control applications.
The AT90S2313 AVR is supported with a full suite of program and system development tools including: C compilers, macro
assemblers, program debugger/simulators, in-circuit emulators, and evaluation kits.
Pin Descriptions
VCC
Supply voltage pin.
GND
Ground pin.
Port B (PB7..PB0)
Port B is an 8-bit bi-directional I/O port. Port pins can provide internal pull-up resistors (selected for each bit). PB0 and PB1
also serve as the positive input (AIN0) and the negative input (AIN1), respectively, of the on-chip analog comparator. The
Port B output buffers can sink 20mA and can drive LED displays directly. When pins PB0 to PB7 are used as inputs and
are externally pulled low, they will source current if the internal pull-up resistors are activated. The Port B pins are tri-stated
when a reset condition becomes active, even if the clock is not active.
Port B also serves the functions of various special features of the AT90S2313 as listed on page 48.
Port D (PD6..PD0)
Port D has seven bi-directional I/O port with internal pull-up resistors, PD6..PD0. The Port D output buffers can sink 20 mA.
As inputs, Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port D pins
are tri-stated when a reset condition becomes active, even if the clock is not active.
Port D also serves the functions of various special features of the AT90S2313 as listed on page 53.
RESET
Reset input. A low level on this pin for more than 50 ns will generate a reset, even if the clock is not running. Shorter pulses
are not guaranteed to generate a reset.
XTAL1
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
XTAL2
Output from the inverting oscillator amplifier

AT90S2313
4
Crystal Oscillator
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as an
on-chip oscillator, as shown in Figure 2. Either a quartz crystal or a ceramic resonator may be used. To drive the device
from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven as shown in Figure 3.
Figure 2. Oscillator Connections
Note: When using the MCU Oscillator as a clock for an external device, an HC buffer should be connected as indicated in the figure.
Figure 3. External Clock Drive Configuration
XTAL2
XTAL1
GND
C2
C1
MAX 1 HC BUFFER
HC

AT90S2313
5
Architectural Overview
The fast-access register file concept contains 32 x 8-bit general purpose working registers with a single clock cycle access
time. This means that during one single clock cycle, one ALU (Arithmetic Logic Unit) operation is executed. Two operands
are output from the register file, the operation is executed, and the result is stored back in the register file - in one clock
cycle.
Figure 4. The AT90S2313 AVR RISC Architecture
Six of the 32 registers can be used as three 16-bits indirect address register pointers for Data Space addressing - enabling
efficient address calculations. One of the three address pointers is also used as the address pointer for the constant table
look up function. These added function registers are the 16-bits X-register, Y-register and Z-register.
The ALU supports arithmetic and logic functions between registers or between a constant and a register. Single register
operations are also executed in the ALU. Figure 4 shows the AT90S2313 AVR RISC microcontroller architecture.
In addition to the register operation, the conventional memory addressing modes can be used on the register file as well.
This is enabled by the fact that the register file is assigned the 32 lowermost Data Space addresses ($00 - $1F), allowing
them to be accessed as though they were ordinary memory locations.

AT90S2313
6
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, Timer/Counters,
A/D-converters, and other I/O functions. The I/O memory can be accessed directly, or as the Data Space locations follow-
ing those of the register file, $20 - $5F.
The AVR has Harvard architecture - with separate memories and buses for program and data. The program memory is
accessed with a two stage pipeline. While one instruction is being executed, the next instruction is pre-fetched from the
program memory. This concept enables instructions to be executed in every clock cycle. The program memory is In-system
Programmable Flash memory.
With the relative jump and call instructions, the whole 1K address space is directly accessed. Most AVR instructions have a
single 16-bit word format. Every program memory address contains a 16- or 32-bit instruction.
During interrupts and subroutine calls, the return address program counter (PC) is stored on the stack. The stack is effec-
tively allocated in the general data SRAM, and consequently the stack size is only limited by the total SRAM size and the
usage of the SRAM. All user programs must initialize the SP in the reset routine (before subroutines or interrupts are
executed). The 8-bit stack pointer SP is read/write accessible in the I/O space.
The 128 bytes data SRAM + register file and I/O registers can be easily accessed through the five different addressing
modes supported in the AVR architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
Figure 5. Memory Maps

AT90S2313
7
A flexible interrupt module has its control registers in the I/O space with an additional global interrupt enable bit in the status
register. All the different interrupts have a separate interrupt vector in the interrupt vector table at the beginning of the
program memory. The different interrupts have priority in accordance with their interrupt vector position. The lower the
interrupt vector address the higher the priority.
General Purpose Register File
Figure 6 shows the structure of the 32 general purpose registers in the CPU.
Figure 6. AVR CPU General Purpose Working Registers
All the register operating instructions in the instruction set have direct and single cycle access to all registers. The only
exception is the five constant arithmetic and logic instructions SBCI, SUBI, CPI, ANDI, ORI between a constant and a
register and the LDI instruction for load immediate constant data. These instructions apply to the second half of the regis-
ters in the register file - R16..R31. The general SBC, SUB, CP, AND, OR and all other operations between two registers or
on a single register apply to the entire register file.
As shown in Figure 6, each register is also assigned a data memory address, mapping them directly into the first 32 loca-
tions of the user Data Space. Although the register file is not physically implemented as SRAM locations, this memory
organization provides great flexibility in access of the registers, as the X, Y and Z registers can be set to index any register
in the file.
70Addr.
R0 $00
R1 $01
R2 $02
…
R13 $0D
General R14 $0E
Purpose R15 $0F
Working R16 $10
Registers R17 $11
…
R26 $1A X-register low byte
R27 $1B X-register high byte
R28 $1C Y-register low byte
R29 $1D Y-register high byte
R30 $1E Z-register low byte
R31 $1F Z-register high byte

AT90S2313
8
X-register, Y-register, and Z-register
The registers R26..R31 have some added functions to their general purpose usage. These registers are the address
pointers for indirect addressing of the Data Space. The three indirect address registers X, Y and Z are defined as:
Figure 7. X, Y, and Z Registers
In the different addressing modes these address registers have functions as fixed displacement, automatic increment and
decrement (see the descriptions for the different instructions).
ALU - Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a
single clock cycle, ALU operations between registers in the register file are executed. The ALU operations are divided into
three main categories - arithmetic, logical and bit-functions.
In-System Programmable Flash Program Memory
The AT90S2313 contains 2K bytes on-chip In-System Programmable Flash memory for program storage. Since all
instructions are 16- or 32-bit words, the Flash is organized as 1K x 16. The Flash memory has an endurance of at least
1000 write/erase cycles.
The AT90S2313 Program Counter PC is 10 bits wide, thus addressing the 1024 program memory addresses.
See page 57 for a detailed description on Flash data downloading. See page 10 for the different addressing modes.
EEPROM Data Memory
The AT90S2313 contains 128 bytes of EEPROM data memory. It is organized as a separate data space, in which single
bytes can be read and written. The EEPROM has an endurance of at least 100,000 write/erase cycles. The access
between the EEPROM and the CPU is described on page 38 specifying the EEPROM address register, the EEPROM data
register, and the EEPROM control register.
For the SPI data downloading, see page 65 for a detailed description.
15 0
X - register 7 0 7 0
R27 ($1B) R26 ($1A)
15 0
Y - register 7 0 7 0
R29 ($1D) R28 ($1C)
15 0
Z - register 7 0 7 0
R31 ($1F) R30 ($1E)

AT90S2313
9
SRAM Data Memory
The following Figure 8 shows how the AT90S2313 Data Memory is organized:
Figure 8. SRAM Organization
The 224 Data Memory locations address the Register file, I/O Memory and the data SRAM. The first 96 locations address
the Register File + I/O Memory, and the next 128 locations address the data SRAM.
The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect, Indirect with
Pre-Decrement and Indirect with Post-Increment. In the register file, registers R26 to R31 feature the indirect addressing
pointer registers.
The Direct addressing reaches the entire data address space.
The Indirect with Displacement mode features 63 address locations reach from the base address given by the Y and Z
register.
When using register indirect addressing modes with automatic pre-decrement and post-increment, the address registers X,
Y and Z are used and decremented and incremented.
The 32 general purpose working registers, 64 I/O registers and the 128 bytes of data SRAM in the AT90S2313 are all
directly accessible through all these addressing modes.
Register File Data Address Space
R0 $00
R1 $01
R2 $02
……
R29 $1D
R30 $1E
R31 $1F
I/O Registers
$00 $20
$01 $21
$02 $22
……
$3D $5D
$3E $5E
$3F $5F
Internal SRAM
$60
$61
$62
…
$DD
$DE
$DF

AT90S2313
10
Program and Data Addressing Modes
The AT90S2313 AVR RISC Microcontroller supports powerful and efficient addressing modes for access to the program
memory (Flash) and data memory. This section describes the different addressing modes supported by the AVR architec-
ture. In the figures, OP means the operation code part of the instruction word. To simplify, not all figures show the exact
location of the addressing bits.
Register Direct, Single Register Rd
Figure 9. Direct Single Register Addressing
The operand is contained in register d (Rd).
Register Direct, Two Registers Rd and Rr
Figure 10. Direct Register Addressing, Two Registers
Operands are contained in register r (Rr) and d (Rd). The result is stored in register d (Rd).

AT90S2313
11
I/O Direct
Figure 11. I/O Direct Addressing
Operand address is contained in 6 bits of the instruction word. n is the destination or source register address.
Data Direct
Figure 12. Direct Data Addressing
A 16-bit Data Address is contained in the 16 LSBs of a two-word instruction. Rd/Rr specify the destination or source
register.

AT90S2313
12
Data Indirect with Displacement
Figure 13. Data Indirect with Displacement
Operand address is the result of the Y or Z-register contents added to the address contained in 6 bits of the instruction
word.
Data Indirect
Figure 14. Data Indirect Addressing
Operand address is the contents of the X, Y or the Z-register.

AT90S2313
13
Data Indirect With Pre-Decrement
Figure 15. Data Indirect Addressing With Pre-Decrement
The X, Y or the Z-register is decremented before the operation. Operand address is the decremented contents of the X, Y
or the Z-register.
Data Indirect With Post-Increment
Figure 16. Data Indirect Addressing With Post-Increment
The X, Y or the Z-register is incremented after the operation. Operand address is the content of the X, Y or the Z-register
prior to incrementing.

AT90S2313
14
Constant Addressing Using the LPM Instruction
Figure 17. Code Memory Constant Addressing
Constant byte address is specified by the Z-register contents. The 15 MSBs select word address (0 - 1K), the LSB selects
low byte if cleared (LSB = 0) or high byte if set (LSB = 1).
Indirect Program Addressing, IJMP and ICALL
Figure 18. Indirect Program Memory Addressing
Program execution continues at address contained by the Z-register (i.e., the PC is loaded with the content of the
Z-register).

AT90S2313
15
Relative Program Addressing, RJMP and RCALL
Figure 19. Relative Program Memory Addressing
Program execution continues at address PC + k + 1. The relative address k is -2048 to 2047.
Memory Access and Instruction Execution Timing
This section describes the general access timing concepts for instruction execution and internal memory access.
The AVR CPU is driven by the System Clock Ø, directly generated from the external clock crystal for the chip. No internal
clock division is used.
Figure 20 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the
fast-access register file concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding
unique results for functions per cost, functions per clocks, and functions per power-unit.
Figure 20. The Parallel Instruction Fetches and Instruction Executions
Figure 21 shows the internal timing concept for the register file. In a single clock cycle an ALU operation using two register
operands is executed, and the result is stored back to the destination register.
System Clock Ø
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
T1 T2 T3 T4

AT90S2313
16
Figure 21. Single Cycle ALU Operation
The internal data SRAM access is performed in two System Clock cycles as described in Figure 22.
Figure 22. On-Chip Data SRAM Access Cycles
System Clock Ø
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
T1 T2 T3 T4
System Clock Ø
WR
RD
Data
Data
Address Address
T1 T2 T3 T4
Prev. Address
Read Write

AT90S2313
17
I/O Memory
The I/O space definition of the AT90S2313 is shown in the following Table 1:
Note: Reserved and unused locations are not shown in the table.
Table 1. AT90S2313 I/O Space
Address Hex Name Function
$3F ($5F) SREG Status Register
$3D ($5D) SPL Stack Pointer Low
$3B ($5B) GIMSK General Interrupt MaSK register
$3A ($5A) GIFR General Interrupt Flag Register
$39 ($59) TIMSK Timer/Counter Interrupt MaSK register
$38 ($58) TIFR Timer/Counter Interrupt Flag register
$35 ($55) MCUCR MCU general Control Register
$33 ($53) TCCR0 Timer/Counter 0 Control Register
$32 ($52) TCNT0 Timer/Counter 0 (8-bit)
$2F ($4F) TCCR1A Timer/Counter 1 Control Register A
$2E ($4E) TCCR1B Timer/Counter 1 Control Register B
$2D ($4D) TCNT1H Timer/Counter 1 High Byte
$2C ($4C) TCNT1L Timer/Counter 1 Low Byte
$2B ($4B) OCR1AH Output Compare Register 1 High Byte
$2A ($4A) OCR1AL Output Compare Register 1 Low Byte
$25 ($45) ICR1H T/C 1 Input Capture Register High Byte
$24 ($44) ICR1L T/C 1 Input Capture Register Low Byte
$21 ($41) WDTCR Watchdog Timer Control Register
$1E ($3E) EEAR EEPROM Address Register
$1D ($3D) EEDR EEPROM Data Register
$1C ($3C) EECR EEPROM Control Register
$18 ($38) PORTB Data Register, Port B
$17 ($37) DDRB Data Direction Register, Port B
$16 ($36) PINB Input Pins, Port B
$12 ($32) PORTD Data Register, Port D
$11 ($31) DDRD Data Direction Register, Port D
$10 ($30) PIND Input Pins, Port D
$0C ($2C) UDR UART I/O Data Register
$0B ($2B) USR UART Status Register
$0A ($2A) UCR UART Control Register
$09 ($29) UBRR UART Baud Rate Register
$08 ($28) ACSR Analog Comparator Control and Status Register

AT90S2313
18
All AT90S2313 I/O and peripherals are placed in the I/O space. The I/O locations are accessed by the IN and OUT instruc-
tions transferring data between the 32 general purpose working registers and the I/O space. I/O registers within the
address range $00 - $1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of sin-
gle bits can be checked by using the SBIS and SBIC instructions. Refer to the instruction set chapter for more details.
When using the I/O specific commands IN, OUT the I/O addresses $00 - $3F must be used. When addressing I/O registers
as SRAM, $20 must be added to this address. All I/O register addresses throughout this document are shown with the
SRAM address in parentheses.
For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses
should never be written.
Some of the status flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate on
all bits in the I/O register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instructions
work with registers $00 to $1F only.
The I/O and peripherals control registers are explained in the following sections.
Status Register - SREG
The AVR status register - SREG - at I/O space location $3F ($5F) is defined as:
•Bit 7 - I: Global Interrupt Enable
The global interrupt enable bit must be set (one) for the interrupts to be enabled. The individual interrupt enable control is
then performed in separate control registers. If the global interrupt enable bit is cleared (zero), none of the interrupts are
enabled independent of the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt has
occurred, and is set by the RETI instruction to enable subsequent interrupts.
•Bit 6 - T: Bit Copy Storage
The bit copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T bit as source and destination for the operated bit.
A bit from a register in the register file can be copied into T by the BST instruction, and a bit in T can be copied into a bit in
a register in the register file by the BLD instruction.
•Bit 5 - H: Half Carry Flag
The half carry flag H indicates a half carry in some arithmetic operations. See the Instruction Set description for detailed
information.
•Bit 4 - S: Sign Bit, S = N ⊕V
The S-bit is always an exclusive or between the negative flag N and the two’s complement overflow flag V. See the Instruc-
tion Set Description for detailed information.
•Bit 3 - V: Two’s Complement Overflow Flag
The two’s complement overflow flag V supports two’s complement arithmetics. See the Instruction Set Description for
detailed information.
•Bit 2 - N: Negative Flag
The negative flag N indicates a negative result after the different arithmetic and logic operations. See the Instruction Set
Description for detailed information.
•Bit 1 - Z: Zero Flag
The zero flag Z indicates a zero result after the different arithmetic and logic operations. See the Instruction Set Description
for detailed information.
•Bit 0 - C: Carry Flag
The carry flag C indicates a carry in an arithmetic or logic operation. See the Instruction Set Description for detailed
information.
Note that the status register is not automatically stored when entering an interrupt routine and restored when returning from
an interrupt routine. This must be handled by software.
Bit 76543210
$3F ($5F) I T H S V N Z C SREG
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0

AT90S2313
19
Stack Pointer - SP
An 8-bit register at I/O address $3D ($5D) forms the stack pointer of the AT90S2313. 8 bits are used to address the
128 bytes of SRAM in locations $60 - $DF.
The Stack Pointer points to the data SRAM stack area where the Subroutine and Interrupt Stacks are located. This Stack
space in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are
enabled. The stack pointer must be set to point above $60. The Stack Pointer is decremented by one when data is pushed
onto the Stack with the PUSH instruction, and it is decremented by two when an address is pushed onto the Stack with
subroutine calls and interrupts. The Stack Pointer is incremented by one when data is popped from the Stack with the POP
instruction, and it is incremented by two when an address is popped from the Stack with return from subroutine RET or
return from interrupt RETI.
Reset and Interrupt Handling
The AT90S2313 provides 10 different interrupt sources. These interrupts and the separate reset vector, each have a sepa-
rate program vector in the program memory space. All the interrupts are assigned individual enable bits which must be set
(one) together with the I-bit in the status register in order to enable the interrupt.
The lowest addresses in the program memory space are automatically defined as the Reset and Interrupt vectors. The
complete list of vectors is shown in Table 2. The list also determines the priority levels of the different interrupts. The lower
the address the higher is the priority level. RESET has the highest priority, and next is INT0 - the External Interrupt Request
0, etc.
Bit 76543210
$3D ($5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value 0 0 0 0 0 0 0 0
Table 2. Reset and Interrupt Vectors
Vector No. Program Address Source Interrupt Definition
1 $000 RESET Hardware Pin, Power-on Reset and Watchdog Reset
2 $001 INT0 External Interrupt Request 0
3 $002 INT1 External Interrupt Request 1
4 $003 TIMER1 CAPT1 Timer/Counter1 Capture Event
5 $004 TIMER1 COMP1 Timer/Counter1 Compare Match
6 $005 TIMER1 OVF1 Timer/Counter1 Overflow
7 $006 TIMER0 OVF0 Timer/Counter0 Overflow
8 $007 UART, RX UART, RX Complete
9 $008 UART, UDRE UART Data Register Empty
10 $009 UART, TX UART, TX Complete
11 $00A ANA_COMP Analog Comparator

AT90S2313
20
The most typical and general program setup for the Reset and Interrupt Vector Addresses are:
Address Labels Code Comments
$000 rjmp RESET ; Reset Handler
$001 rjmp EXT_INT0 ; IRQ0 Handler
$002 rjmp EXT_INT1 ; IRQ1 Handler
$003 rjmp TIM_CAPT1 ; Timer1 Capture Handler
$004 rjmp TIM_COMP1 ; Timer1 Compare Handler
$005 rjmp TIM_OVF1 ; Timer1 Overflow Handler
$006 rjmp TIM_OVF0 ; Timer0 Overflow Handler
$007 rjmp UART_RXC ; UART RX Complete Handler
$008 rjmp UART_DRE ; UDR Empty Handler
$009 rjmp UART_TXC ; UART TX Complete Handler
$00a rjmp ANA_COMP ; Analog Comparator Handler
;
$00b MAIN: ldi r16,low(RAMEND); Main program start
$00c out SPL,r16
$00d <instr> xxx
… … … …
Reset Sources
The AT90S2313 has three sources of reset:
• Power-On Reset. The MCU is reset when the supply voltage is below the power-on reset threshold (VPOT).
• External Reset. The MCU is reset when a low level is present on the RESET pin for more than 50 ns.
• Watchdog Reset. The MCU is reset when the Watchdog timer period expires and the Watchdog is enabled.
During reset, all I/O registers are then set to their initial values, and the program starts execution from address $000. The
instruction placed in address $000 must be an RJMP - relative jump - instruction to the reset handling routine. If the
program never enables an interrupt source, the interrupt vectors are not used, and regular program code can be placed at
these locations. The circuit diagram in Figure 23 shows the reset logic. Table 3 defines the timing and electrical parameters
of the reset circuitry.
Figure 23. Reset Logic