AVR Assembly program Shift Register and 7 segment LED

Websearch is a good way to find circuits to build and code to try. I found this example bundled in a simulator program and it comes with some software.

Change the LED bar to a 7 segment display and load the hex file. The example comes with software that runs a single bit down the segments. This is AVR assembly language that you can program with MicroChip Studioor Avra.

Assembly Language

We say that Assembly language is low-level compared to PyThon that imports complex functions. In assembly we push numbers into registers and increment them, copying the actions we saw in Shift Register project.

Look through the code running on the demo, it toggles bits in the PORTB register hardware. Find DMS_OUTER: DELAYTEMP, 20 and increase the number to slow the speed of the demo.

;Chip Model: MEGA328P
;Assembler header file
.INCLUDE "m328pdef.inc"

;SREG bit names (for AVR Assembler compatibility, GCBASIC uses different names)
#define I 7
#define T 6
#define H 5
#define S 4
#define V 3
#define N 2
#define Z 1
#define C 0

;********************************************************************************

;Set aside memory locations for variables
.EQU	BITOUT=256
.EQU	INDEX=257

;********************************************************************************

;Register variables
.DEF	DELAYTEMP=r25
.DEF	DELAYTEMP2=r26
.DEF	SysBitTest=r5
.DEF	SysCalcTempA=r22
.DEF	SysValueCopy=r21
.DEF	SysWaitTempMS=r29
.DEF	SysWaitTempMS_H=r30
.DEF	SysTemp1=r0

;********************************************************************************

;Vectors
;Interrupt vectors
	nop
	rjmp	BASPROGRAMSTART ;Reset
	nop
	reti	;INT0
	nop
	reti	;INT1
	nop
	reti	;PCINT0
	nop
	reti	;PCINT1
	nop
	reti	;PCINT2
	nop
	reti	;WDT
	nop
	reti	;TIMER2_COMPA
	nop
	reti	;TIMER2_COMPB
	nop
	reti	;TIMER2_OVF
	nop
	reti	;TIMER1_CAPT
	nop
	reti	;TIMER1_COMPA
	nop
	reti	;TIMER1_COMPB
	nop
	reti	;TIMER1_OVF
	nop
	reti	;TIMER0_COMPA
	nop
	reti	;TIMER0_COMPB
	nop
	reti	;TIMER0_OVF
	nop
	reti	;SPI_STC
	nop
	reti	;USART_RX
	nop
	reti	;USART_UDRE
	nop
	reti	;USART_TX
	nop
	reti	;ADC
	nop
	reti	;EE_READY
	nop
	reti	;ANALOG_COMP
	nop
	reti	;TWI
	nop
	reti	;SPM_READY

;********************************************************************************

;Start of program memory page 0
nop
BASPROGRAMSTART:
;Initialise stack
	ldi	SysValueCopy,high(RAMEND)
	out	SPH, SysValueCopy
	ldi	SysValueCopy,low(RAMEND)
	out	SPL, SysValueCopy
;Call initialisation routines
	rcall	INITSYS

;Start of the main program
	rcall	INIT595
SysDoLoop_S1:
	ldi	SysValueCopy,1
	sts	BITOUT,SysValueCopy
	rcall	SHIFTBIT
	ldi	SysWaitTempMS,244
	ldi	SysWaitTempMS_H,1
	rcall	Delay_MS
	ldi	SysValueCopy,0
	sts	INDEX,SysValueCopy
SysForLoop1:
	lds	SysTemp1,INDEX
	inc	SysTemp1
	sts	INDEX,SysTemp1
	ldi	SysValueCopy,0
	sts	BITOUT,SysValueCopy
	rcall	SHIFTBIT
	ldi	SysWaitTempMS,244
	ldi	SysWaitTempMS_H,1
	rcall	Delay_MS
	lds	SysCalcTempA,INDEX
	cpi	SysCalcTempA,7
	brlo	SysForLoop1
SysForLoopEnd1:
	rjmp	SysDoLoop_S1
SysDoLoop_E1:
BASPROGRAMEND:
	sleep
	rjmp	BASPROGRAMEND

;********************************************************************************

Delay_MS:
	inc	SysWaitTempMS_H
DMS_START:
	ldi	DELAYTEMP2,254
DMS_OUTER:
	ldi	DELAYTEMP,20
DMS_INNER:
	dec	DELAYTEMP
	brne	DMS_INNER
	dec	DELAYTEMP2
	brne	DMS_OUTER
	dec	SysWaitTempMS
	brne	DMS_START
	dec	SysWaitTempMS_H
	brne	DMS_START
	ret

;********************************************************************************

INIT595:
	sbi	DDRB,5
	sbi	DDRB,4
	sbi	DDRB,3
	sbi	DDRB,2
	sbi	DDRB,1
	cbi	PORTB,5
	cbi	PORTB,4
	cbi	PORTB,3
	cbi	PORTB,2
	cbi	PORTB,1
	sbi	PORTB,5
	ret

;********************************************************************************

INITSYS:
	ldi	SysValueCopy,0
	out	PORTB,SysValueCopy
	ldi	SysValueCopy,0
	out	PORTC,SysValueCopy
	ldi	SysValueCopy,0
	out	PORTD,SysValueCopy
	ret

;********************************************************************************

SHIFTBIT:
	cbi	PORTB,3
	cbi	PORTB,4
	cbi	PORTB,1
	lds	SysBitTest,BITOUT
	sbrc	SysBitTest,0
	sbi	PORTB,1
	sbi	PORTB,4
	sbi	PORTB,3
	ret

;********************************************************************************

Assembly is like punching the buttons with your hands.

Upload Binary Hex File

The hexadecimal file contains the ones and zeros for our program. We build it on a PC and move it to a target MCU. The file provided by the simulator works on real Uno boards. Program a Uno normally or follow Arduino IDE Upload with external programmer.

With the simulation right-click on the processor and look for load firmware. Folder explorer to the demo hex file.

Development Hardware

Look at our development hardware and software. ATmega328 processor, standard shift-register, 7segment LED. We have a circuit that blinks segments a to g and dot.

Read the pins, trace the leads, make a Connection Table. Expect the names on one device to be different on the next. Here are some of the signals we are using.

This table needs some explanation. Blank means not applicable.

The ATmega processor chip a has metal pins named B1, B2, B3, B4, B5, AVR Assembly language calls these pins PORTB, 1 etc. Arduino Corp uses pin9, pin10, pin11, pin12, pin13.

Wires connect our processor to an LS74HC595 shift register. It works like the hands-on in Shift Register Project. Data Strobe, Output Enable, Clock, Latch and Reset are connected. But the names are slightly different. > is a clocking symbol.

We need a data input and a clock. Maybe we want a latch signal every 8 clock pulses to handle bytes. Our assembly demo holds latch and reset high to keep the shift register running. Output enable is kept low and the screen stays on.

Next Steps

Lots of options. Websearch for assembly programs and there are plenty using this display. Now that you know which pins are used you can modify that code to run.

High level languages such as C++. This is the same processor as in the Arduino Uno.