AVR Assembler. Microcontroller VL Thomas Nowak TU Wien

Transcription

1 AVR Assembler Microcontroller VL Thomas Nowak TU Wien 1

2 Overview ATmega16 Architecture Instructions Simple Example Program ATmega16 Control Registers Example: Digital I/O Example: Timer 2

3 ATmega16 Architecture 16 MHz RISC processor registers are 8 bit wide 32 general purpose registers (GPRs) 32 I/O registers status register (SR) contains status of last instruction (for conditional jumps) program counter (PC) 3

4 ATmega16 Architecture stack pointer (SP) used to store return address (RA) during subroutine calls and interrupts stack accessible by commands push and pop stack grows towards lower memory regions (i.e. SP is decreased after a push instruction) Harvard architecture separate memory areas for code (.text, 16K flash memory) and data (.eeprom, 512B EEPROM for constants, and.data, 1K SRAM) 4

5 Instructions Meta instructions 1/3.equ defines an alias often used to alias registers.equ temp_reg,0x02 aliases register 2 awareness of the context in which the alias is used is necessary andi r1,temp_reg means r1 := r1 AND 0x02 and r1,temp_reg means r1 := r1 AND r2.section specifies in which memory the following should be placed.section.text 5

6 Instructions Meta instructions 2/3.byte and.word write given byte/word (16bit) to current memory address.org specifies a memory address.org 0x0004 sets the current memory address to 0x0004 implicit increment of current memory address on subsequent instructions.org 0x0004.byte 0x01.byte 0x03 puts value 0x01 to address 0x0004 and 0x03 to 0x0005 6

7 Instructions Meta instructions 3/3.NOLIST.INCLUDE def.s.list includes file def.s without adding the contents to the list file (useful for definition files) label: defines a label at current memory location.global exports a label for use in other files comment lines start with ; 7

8 Instructions Arithmetic/logical instructions 1/2 have the form and r1, r2 i.e. r1 := r1 AND r2 often have a immediate variant for use with constants andi r1, 0xF0 masks the 4 upper bits of r1 do not work on all registers they do not work on I/O registers see AVR Instruction Set for details (Operands) 8

9 Instructions Arithmetic/logical instructions 2/2 adc (add with carry) uses the carry bit in the status word that is updated on every add instruction some instruction are also available for processing of words (2 bytes) adiw r4:r3,5 adds 5 to the word that has r4 as its high and r3 as its low byte 9

10 Instructions Jump instructions 1/2 rjmp label jumps to label (unconditionally) for really long jump distances, jmp would have to be used (rjmp can address the nearest 4096 words relative to the rjmp instruction) rcall perform calls subroutine perform, i.e. pushes the current PC value on the stack (RA) and jumps to label perform use call for long jump distances ret returns from subroutine, i.e. pops the RA from the stack into PC 10

11 Instructions Jump instructions 2/2 reti returns from an interrupt service routine (ISR). Same as ret, additionally the global interrupt enable flag (IE) is set (it is cleared by default on entry to an ISR) breq, brne,... jump iff certain bits in the SR are set/cleared breq label (branch if equal) jumps to label if the zero flag in the SR is set use cp r1,r2 (compare) to update the status word without changing any other registers jump distance is at most 64 words in each direction 11

12 Instructions Data transfer instructions 1/2 mov r1, r2 r1 := r2 in r3, PINA copies contents of I/O register PINA to GPR r3 this is often necessary because most operations only work on GPRs out PORTA, r6 copies contents of GPR r6 to I/O register PORTA push, pop 12

13 Instructions Data transfer instructions 2/2 st X,r4 stores content of register r4 to memory location contained in word X (XH:XL) X, Y and Z are words consisting of two GPRs each X is r27:r26, Y is r29:r28, Z is r31:r30 postincrement and predecrement are available ld r5,x loads contents of memory location contained in word X into register r5 postincrement and predecrement are available direct memory addressing through the commands lds and sts 13

14 Instructions Bit instructions 1/2 lsl, lsr, asr, rol, ror shift/roll sec, clc, sen, cln, sei,... set/clear individual bits in the SR (e.g. clc clears the carry bit) sbi, cbi set/clear bit in I/O register sbi PORTA,4 sets bit 4 in PORTA sbr, cbr set/clear bits in GPR second operand is a bit mask sbr r3, 0xF0 sets bits 7..4 in r3 14

15 Instructions Bit instructions 2/2 bst r1,3 stores bit 3 of r1 to the T-Flag (SR) bld r4,5 loads T-Flag into bit 5 of r4 Misc instructions nop no operation sleep change to sleep mode as specified in the MCUCR register 15

16 Simple Example Program program simply turns on all LEDs 16

17 Simple Example Program ; simple example program.nolist.include Includes/m16def.inc.LIST ; alias registers.equ temp, 0x10.section.text.org 0x0000 ; set port C to output (LEDs) ldi temp, 0xFF out DDRC, temp ; activate all LEDs on port C ldi temp, 0xFF out PORTC, temp infinite_loop: rjmp infinite_loop 17

18 ATmega16 Control Registers Many features of the microcontroller are configured through special control registers e.g. to set the timer mode (normal, PWM,...) of Timer1, you have to set the right bits (WGM13:0) in TCCR1A and TCCR1B these are I/O registers some share a common I/O location (e.g. UBRRH/UCSRC) Check the manual for details 18

19 Example: Digital I/O program turns on all LEDs if SW1 is on and turns them off otherwise 19

20 Example: Digital I/O ; digital i/o example program.nolist.include Includes/m16def.inc.LIST ; alias registers.equ temp, 0x10.equ leds, 0x11.section.text.org 0x0000 ; set port C to output and initialize with 0 (LEDs) ldi temp, 0xFF out DDRC, temp ldi leds, 0x00 out PORTC, leds 20

21 Example: Digital I/O ; set port A bit 4 to input and activate pull-up (SW1) cbi DDRA, 4 sbi PORTA, 4 ; main loop. poll PINA bit 4 and activate or deactivate ; the leds accordingly loop_start: in temp, PINA bst temp, 4 brtc switch_on switch_off: ldi leds, 0x00 rjmp loop_end switch_on: ldi leds, 0xFF loop_end: out PORTC, leds rjmp loop_start 21

22 Example: Timer program switches state of LEDs on overflow of Timer1 22

23 Example: Timer ; timer example program.nolist.include Includes/m16def.inc.LIST ; alias registers.equ temp, 0x10.equ leds, 0x11.section.text.org 0x0000 rjmp main ; install timer ISR.org OVF1addr *2 rjmp overflow main: ; set port C to output and initialize with 0 (LEDs) ldi temp, 0xFF out DDRC, temp 23

24 Example: Timer ldi leds, 0x00 out PORTC, leds ; set global interrupt flag sei ; initialize timer ldi temp, 0x00 out TCCR1A, temp ldi temp, 1<<CS12 out TCCR1B, temp in temp, TIMSK sbr temp, 1<<TOIE1 out TIMSK, temp infinite_loop: rjmp infinite_loop 24

25 Example: Timer ; interrupt service routine overflow: cpi leds, 0x00 breq was_zero was_ff: ldi leds, 0x00 rjmp ovf_end was_zero: ldi leds, 0xFF ovf_end: out PORTC, leds reti 25