Part 2: Programming in assembler
Why?
I need to get rid of that library and gain control of the entire CPU. So my idea is to rewrite the all code ... in assembly (or assembler).
A quick note here: I did program in assembly over 30 years ago, at University (8085, 68000) as well as for personnal projects (6502, 6809, 8086, a little bit of Z80). But since then, I turned to other languages, mostly to C/C++. So my assembly is a little bit rusty.
Pros and cons
- Assembly is simple: programming in assembly is like playing with Lego (which I did a lot!). It consists of assembling small block (operational codes, or opcodes) together. This is why it's called ... assembly! Each opcode does a very limited operation, well described in the datasheet.
- Nothing is hidden: no library, no help, you'll have to write everything from scratch. Not totally true, since we can find some headers files online, which I used.
- The datasheet is the reference: it contains everything, even some examples in assembly.
- The lack of documentation problem: mainly because it is used by a minority, and several toolsets exist, you cannot count on finding a quick answer when an problem arises. Actually, you are mostly on your own.
Also, most of the doc online is outdated. However, the ATTiny4313/2313 datasheet is the reference source of information, as well as the Avr Instruction Set manual. Finally, I found some valuable information in avr-libc documentation. - Syntax is not consistent: since there are several assembler (the compilator), each one has its own implementation, hence slight differences on the syntax. I use avr-gcc.
- Assembly is not portable: true, but in my case, not an issue. Also, the opcodes are coherent between all avr-based chips.
Some notes
- Contrary to all CPU I used in the past, ATTiny is based on Harvard architecture where code and data are separated.
- The interrupt vectors are located between 0x000 and 0x0014. If a vector is not used, the memory can be re-used for code.
The tools
I have installed:
avr-gcc(documentation)$ avr-gcc --version avr-gcc (GCC) 5.4.0
avr-asAKAgas(GNU Assembler)$ avr-as --version GNU assembler (GNU Binutils) 2.26.20160125
simulavr$ simulavr --version SimulAVR 1.2dev
$ dmesg | grep tty [320448.786051] cdc_acm 5-2:1.0: ttyACM0: USB ACM device $ export USBDEVICE=/dev/ttyACM0The Makefile
.PHONY: \
vars \
help \
clean \
compile \
dump \
upload \
simul \
fuses \
check-fuses
# Hack to get the directory this makefile is in:
MKFILE_PATH := $(lastword $(MAKEFILE_LIST))
MKFILE_DIR := $(notdir $(patsubst %/,%,$(dir $(MKFILE_PATH))))
MKFILE_ABSDIR := $(abspath $(MKFILE_DIR))
AVR_PATH := /snap/arduino/85/hardware/tools/avr
AVR_BIN_PATH := $(AVR_PATH)/bin
# Hack to get all *.h files into compile dependencies:
HEADERS = $(shell find $(MKFILE_DIR) -name "*.h")
BUILDTMP ?= $(MKFILE_DIR)/build-tmp
OPTIMIZATION ?= -Os
AVRAS ?= $(AVR_BIN_PATH)/avr-as -Wall -al -v --listing-rhs-width=80 -mmcu=$(DEVICE) -I$(MKFILE_DIR)
AVRGCC ?= $(AVR_BIN_PATH)/avr-gcc -Wall -nostartfiles -mmcu=$(DEVICE) -I$(MKFILE_DIR)
AVRCC ?= $(AVRGCC)
AVRDUDE ?= $(AVR_BIN_PATH)/avrdude
LD ?= ld
#---------------------------------------------------------
# AVRDUDE_FLASHARG:
# This preserves the chip memory when updating the fuses.
# To erase the chip when setting fuses, do:
#
# make AVRDUDE_FLASHARG=-e fuses
#
AVRDUDE_FLASHARG ?= -D
#---------------------------------------------------------
AVR_SIZE ?= $(AVR_BIN_PATH)/avr-size
AVR_OBJCOPY ?= $(AVR_BIN_PATH)/avr-objcopy
AVR_OBJDUMP ?= $(AVR_BIN_PATH)/avr-objdump
DEVICE := attiny4313
ARCHITECTURE := "avr:25"
CLOCK := 8000000L
PROGRAMMER := stk500v1
BAUD := 19200
SRC := blink.S
OBJ := $(BUILDTMP)/$(SRC:S=o)
ELF := $(BUILDTMP)/$(SRC:S=elf)
HEX := $(BUILDTMP)/$(SRC:S=hex)
EEP := $(BUILDTMP)/$(SRC:S=eep)
FUSE_EXT := 0xff
FUSE_HIGH := 0x9f
FUSE_LOW := 0xcf
AVRDUDE_CONF := $(AVR_PATH)/etc/avrdude.conf
AVRDUDE_OPTS := -C $(AVRDUDE_CONF) -p$(DEVICE) -c$(PROGRAMMER) -P$(USBDEVICE) -b$(BAUD)
USBDEVICE := $(shell dmesg | awk '/tty/ {gsub(/:/,"",$$4);A=$$4} END{print "/dev/"A}')
# Misc target info:
help_spacing := 12
.DEFAULT_GOAL := compile
#---------------------------------------------------------
# Ensure temp directories.
#
# In order to ensure temp dirs exit, we include a file
# that doesn't exist, with a target declared as PHONY
# (above), and then have the target create our tmp dirs.
#---------------------------------------
-include ensure-tmp
ensure-tmp:
@mkdir -p $(BUILDTMP)
vars: ## Print relevant environment vars
@printf "%-20.20s%s\n" "MKFILE_PATH:" "$(MKFILE_PATH)"
@printf "%-20.20s%s\n" "MKFILE_DIR:" "$(MKFILE_DIR)"
@printf "%-20.20s%s\n" "MKFILE_ABSDIR:" "$(MKFILE_ABSDIR)"
@printf "%-20.20s%s\n" "BUILDTMP:" "$(BUILDTMP)"
@printf "%-20.20s%s\n" "OPTIMIZATION:" "$(OPTIMIZATION)"
@printf "%-20.20s%s\n" "AVRAS:" "$(AVRAS)"
@printf "%-20.20s%s\n" "AVRGCC:" "$(AVRGCC)"
@printf "%-20.20s%s\n" "AVRCC:" "$(AVRCC)"
@printf "%-20.20s%s\n" "AVRDUDE:" "$(AVRDUDE)"
@printf "%-20.20s%s\n" "AVRDUDE_OPTS:" "$(AVRDUDE_OPTS)"
@printf "%-20.20s%s\n" "AVR_SIZE:" "$(AVR_SIZE)"
@printf "%-20.20s%s\n" "AVR_OBJCOPY:" "$(AVR_OBJCOPY)"
@printf "%-20.20s%s\n" "AVR_OBJDUMP:" "$(AVR_OBJDUMP)"
@printf "%-20.20s%s\n" "DEVICE:" "$(DEVICE)"
@printf "%-20.20s%s\n" "CLOCK:" "$(CLOCK)"
@printf "%-20.20s%s\n" "PROGRAMMER:" "$(PROGRAMMER)"
@printf "%-20.20s%s\n" "USBDEVICE:" "$(USBDEVICE)"
@printf "%-20.20s%s\n" "BAUD:" "$(BAUD)"
@printf "%-20.20s%s\n" "SRC:" "$(SRC)"
@printf "%-20.20s%s\n" "ELF:" "$(ELF)"
@printf "%-20.20s%s\n" "EEP:" "$(EEP)"
@printf "%-20.20s%s\n" "HEX:" "$(HEX)"
help: ## Print this makefile help menu
@echo "TARGETS:"
@grep '^[a-z_\-]\{1,\}:.*##' $(MAKEFILE_LIST) \
| sed 's/^\([a-z_\-]\{1,\}\): *\(.*[^ ]\) *## *\(.*\)/\1:\t\3 (\2)/g' \
| sed 's/^\([a-z_\-]\{1,\}\): *## *\(.*\)/\1:\t\2/g' \
| awk '{$$1 = sprintf("%-$(help_spacing)s", $$1)} 1' \
| sed 's/^/ /'
@printf "\nUsage:\n make \\\n %s \\\n %s \\\n %s \\\n %s\n" \
"USBDEVICE=/dev/cu.usbserial-1234" \
"SRC=my_source.c" \
"DEVICE=%lt;mcu>" \
"%lt;make target>"
vpath %.o $(BUILDTMP)
vpath %.eep $(BUILDTMP)
vpath %.elf $(BUILDTMP)
vpath %.hex $(BUILDTMP)
$(OBJ): $(SRC)
$(AVRAS) -mmcu=$(DEVICE) -o $@ $%lt;
$(ELF): $(OBJ)
$(AVRGCC) -mmcu=$(DEVICE) -L$(BUILDTMP) $(LDFLAGS) -o $@ $%lt;
$(HEX): $(ELF)
$(AVR_OBJCOPY) -O ihex -R .eeprom --preserve-dates $%lt; $@
$(AVR_OBJDUMP) --architecture=$(ARCHITECTURE) -D $@
$(EEP): $(ELF)
$(AVR_OBJCOPY) -O ihex -j .eeprom --set-section-flags=.eeprom=alloc,load --no-change-warnings --change-section-lma=.eeprom=0 --preserve-dates $%lt; $@
clean: ## Clean build artifacts
rm -rf $(BUILDTMP)/*
rm -vf *.s
compile: $(EEP) $(HEX)
$(AVR_SIZE) -A $(ELF)
link: compile ## Link compilation artifacts and package for upload
upload: $(HEX) ## Upload (NOTE: USBDEVICE must be set)
ifndef USBDEVICE
$(error 'USBDEVICE not defined! Please set USBDEVICE env var!')
endif # USBDEVICE
$(AVRDUDE) -v $(AVRDUDE_OPTS) -Uflash:w:$(HEX):i
fuses: ## Flash the fuses
ifndef USBDEVICE
$(error 'USBDEVICE not defined! Please set USBDEVICE env var!')
endif # USBDEVICE
$(AVRDUDE) -v $(AVRDUDE_OPTS) -D \
-Uefuse:w:$(FUSE_EXT):m \
-Uhfuse:w:$(FUSE_HIGH):m \
-Ulfuse:w:$(FUSE_LOW):m
check-fuses: ## Verify device signature and check fuse values
$(AVRDUDE) $(AVRDUDE_OPTS)
simul: $(ELF)
simulavr -d attiny2313 -t attiny2313 -f $(ELF)
mo4: mo4.S tn4313def.h attiny4313_registers.h
avr-gcc -mmcu=attiny4313 $^
dump: $(HEX)
@echo "Dump of Flash"
$(AVRDUDE) $(AVRDUDE_OPTS) -U flash:r:$(MKFILE_DIR)/flash.bin:r
@$(AVR_OBJDUMP) --architecture=$(ARCHITECTURE) --demangle --disassemble --source --wide $(MKFILE_DIR)/flash.bin
Blink in assembly
#include "tn4313def.h" ; I found this header file on Internet; it is mostly correct.
; *******************************************************************************************
; Macros
; *******************************************************************************************
; These macros fixes the address shift between I/O and registers
; When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used.
; When addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these addresses.
.macro STORE addr,reg
.if \addr < 0x60
out \addr,\reg
.else
sts \addr + 0x20,\reg
.endif
.endm
.macro LOAD reg,addr
.if \addr < 0x60
in \reg,\addr
.else
lds \reg + 0x20,\addr
.endif
.endm
; Calculate the delay value
.equ CLOCK_FREQ, 8000000 ; 8 MHz
.equ PRESCALER, 1024 ; / 1024
.equ DELAY_VALUE, (CLOCK_FREQ / (2 * PRESCALER)) / 16
; We use a 10 LED bargraph. MIDI_LED must flash every time LOOP_LED changes states. CTRL_LED is the control LED (always ON)
.equ MIDI_LED, PIND6 ; Timer 1 (compare with OCR1A)
.equ LOOP_LED, PIND4 ; Software loop
.equ CTRL_LED, PIND0
.equ V17, 0x01
.text
Reset: rjmp main
INT0addr: rjmp Reset
INT1addr: rjmp Reset
ICP1addr: rjmp Reset
OC1Aaddr: rjmp OC1Aaddr_routine
OVF1addr: rjmp Reset
;
; Starting from this address, it's not necessary to declare the interrupt vectors
; since I don't use them. I can start the program here. However, since I have enough
; program memory (4K - 40 bytes - stack), I prefer to declare the vectors. They all go
; to Reset, hence to main.
;
OVF0addr: rjmp Reset
URXCaddr: rjmp Reset
UDREaddr: rjmp Reset
UTXCaddr: rjmp Reset
ACIaddr: rjmp Reset
PCIBaddr: rjmp Reset
OC1Baddr: rjmp Reset
OC0Aaddr: rjmp Reset
OC0Baddr: rjmp Reset
USI_STARTaddr: rjmp Reset
USI_OVFaddr: rjmp Reset
ERDYaddr: rjmp Reset
WDTaddr: rjmp Reset
PCIAaddr: rjmp Reset
PCIDaddr: rjmp Reset
; =========================================================
; Main routine
; =========================================================
main: cli ; disable interrupts
ldi r16, lo8(RAMEND) ; Set stack pointer to end of ram
STORE SPL, r16
;ldi r16, hi8(RAMEND) ; SPH doesn't exist on this CPU
;STORE SPH, r16
ldi r16, (1<<PUD) ; Disable Pull-Up resistors globally
STORE MCUCR,r16 ; Update the MCU Control Register
; We select the pins of Port D that must be set as outputs
ldi r16, (1<<MIDI_LED)|(1<<LOOP_LED)|(1<<CTRL_LED)
STORE DDRD, r16 ; ... and set the data direction of port B to "out"
; ---------------------------------
; Configure Timer 1
; ---------------------------------
; Configure Timer/Counter 1 to mode 4 (CTC) and prescaler / 1024
; TOP value = OCR1A
;TOV1 Flag Set on = MAX
; | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
; +------+------+------+------+------+------+------+------+
; TCCR1A = |COM1A1|COM1A0|COM1B1|COM1B0| - | - | WGM11| WGM10|
; | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
; TCCR1B = |ICNC1 |ICES1 | - | WGM13| WGM12| CS12 | CS11 | CS10 |
; | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | = mode 4, prescaler /1024
; TCCR1C = |FOC1A |FOC1B | - | - | - | - | - | - |
; | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
ldi r16, hi8(DELAY_VALUE) ; Prepare the values for OCR1A
ldi r17, lo8(DELAY_VALUE)
Very important here: 16 bits registers must be loaded high byte first. Otherwise, only the lowest byte is loaded, leading to unexpected results
; WRONG, NOT WHAT YOU'D EXPECT
; STORE OCR1AL, r16 <--- lowest byte will be loaded
; STORE OCR1AH, r17 <--- but not the highest byte; OCR1A contains 0x00FF not 0xFFFF.
; This is the good way
STORE OCR1AH, r16 ; High byte must be written first ...
STORE OCR1AL, r17 ; .. when setting a 16 bits register.
; Configure the Timer/Counter 1 Control Registers
clr r16
STORE TCCR1A, r16
STORE TCCR1C, r16
ldi r16,(1<<WGM12)|(1<<CS10)|(1<<CS12) ; Set CS10 and CS12 for prescaler of 1024
STORE TCCR1B, r16
; Configure the OCF1A Interrupt Mask Register
LOAD r16, TIFR
ldi r16, (1<<OCIF1A)
STORE TIFR, r16
; Configure the Timer/Counter Interrupt Mask Register
ldi r16, (1<<OCIE1A)
STORE TIMSK, r16
sei ; Set Global Interrupt Enable
; ---------------------------------
; Starting the show
; ---------------------------------
; Light on the Control LED
ldi r16, (1<<CTRL_LED)|(1<<LOOP_LED)
STORE PORTD, r16
oloop: ; outer loop
ldi r17, V17 ; Initialize our software counter
ldi r18, 0xff ; Initialize our software counter
ldi r19, 0xff ; Initialize our software counter
iloop: ; inner loop
dec r17 ; decrement r20
brne iloop
ldi r17, V17 ; reset r20
dec r18 ; decrement r21
brne iloop
ldi r17, V17 ; reset r20
ldi r18, 0xff ; reset r21
dec r19 ; decrement r22
brne iloop
ldi r23, (1<<LOOP_LED)
rcall toggle ; Toggle the LOOP LED
rcall start_timer
rjmp oloop ; Restart the loop
; =========================================================
; Sub-routines
; =========================================================
toggle: ; Toggle the LED in register r23
LOAD r16, PORTD ; Load content of Port D
eor r16, r23 ; Exclusive OR to toggle the bit
STORE PORTD, r16 ; Write Port D
ret
start_timer:
; Disable interrupts
cli
; Reset the TCNT1 counter
clr r16
clr r17
STORE TCNT1H, r16
STORE TCNT1L, r17
; Set prescaler = 1024
LOAD r16, TCCR1B
ldi r16, (1<<WGM12)|(1<<CS10)|(1<<CS12)
STORE TCCR1B, r16
; Light the MIDI LED on
LOAD r16, PORTD
sbr r16, (1<<MIDI_LED)
STORE PORTD, r16
; Enable interrupts
sei
ret
stop_timer:
; Set prescaler = 0 -> stop the timer
LOAD r16, TCCR1B
ldi r16, (1<<WGM12)
STORE TCCR1B, r16
; Light the MIDI LED off
LOAD r16, PORTD
cbr r16, (1<<MIDI_LED)
STORE PORTD, r16
ret
; =========================================================
; Interrupt Service Routine (ISR)
; =========================================================
OC1Aaddr_routine: ; Timer 1: interrupt when TCNT1 = OCR1A
cli ; Disable interrupt
rcall stop_timer
sei ; Enable interrupt
reti ; Return from ISR
Compiling and uploading
make && make uploadAnd yes, it blinks!
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