LED Blink in assembly with timer1 output compare interrupt not working

Question

I was trying to have some fun with avr-assembly on my arduino and I tried the following codes to blink my LED

first I tried the ISR approach and when It didn't work I tried the CTC mode one, both codes are listed respectively. on the first one the LED lightsup and stay that way (no blinking) on the second one it just stays off

one more thing to note is on trying to load OCR1AL first The led would blink nicely if my hand got near PORTD pins for some reason,

I guessed it's some grounding problem and It's fixed when I pulled high all PORTD pins and it disappeared completely when I figured out that I need to load (the high byte) OCR1AH first

.nolist
.include "m328Pdef.inc" 
.list
.org 0x0000
jmp Init
.org 0x0016
jmp TIMER1_COMPA
.EQU LED_PIN = PB1
Init:
 ;ldi r16, 255
 ;out DDRD, r16
 ;out PORTD, r16
 ;First configure The led pin as output and initilize it to high
 sbi DDRB, LED_PIN
 sbi PORTB, LED_PIN
 ;Second Enable Global Interrupts
 sei
 ;Enable the Output compare Interrupt in the TIMSK1 Register
 ldi r16, 0b00000010 
 sts TIMSK1, r16
 ; Reset the TCCR1A 
 ldi r16, 0b00000000
 sts TCCR1A, r16
 ; Set the prescalar to 256 in the Timer Control 1B register
 ldi r16, 0b00000111 
 sts TCCR1B, r16
 ;Now load the value for Output compare register 31250 or 01111011 00100000 or 15625 =わ 00111101 00001001
 ldi r16, 0b00111101
 ldi r17, 0b00001001 
 sts OCR1AH, r16
 sts OCR1AL, r17
LOOP:
 rjmp LOOP
TIMER1_COMPA:
 ;First Reset the coutner 
 ldi r16, 0
 sts TCNT1H, r16
 sts TCNT1L, r16
 ;Second Toggle the Pin (LED) state
 sbi PINB, LED_PIN
 reti

CTC:

.nolist
.include "m328Pdef.inc" 
.list
.org 0x0000
jmp Init
.org 0x0016
jmp TIMER1_COMPA
.EQU LED_PIN = PB1
Init:
 ;ldi r16, 255
 ;out DDRD, r16
 ;out PORTD, r16
 ;First configure The led pin as output and initilize it to high
 sbi DDRB, LED_PIN
 sbi PORTB, LED_PIN
 ;Second Enable Global Interrupts
 sei
 ;Enable the Output compare Interrupt in the TIMSK1 Register
 ldi r16, 0b00000010 
 sts TIMSK1, r16
 ; Reset the TCCR1A 
 ldi r16, 0b01000000
 sts TCCR1A, r16
 ; Set the prescalar to 256 in the Timer Control 1 registers
 ldi r16, 0b00001111 
 sts TCCR1B, r16
 ;Now load the value for Output compare register 31250 or 01111011 00100000 or 15625 =わ 00111101 00001001
 ldi r16, 0b00111101
 ldi r17, 0b00001001 
 sts OCR1AH, r16
 sts OCR1AL, r17
LOOP:
 rjmp LOOP
TIMER1_COMPA:
 ;First Reset the coutner 
 ;ldi r16, 0
 ;sts TCNT1H, r16
 ;sts TCNT1L, r16
 ;;Second Toggle the Pin (LED) state
 ;sbi PINB, LED_PIN
 reti

Answer 1

I am not set up to test your code right now, perhaps later. But:

 ; Set the prescalar to 256 in the Timer Control 1B register
 ldi r16, 0b00000111 
 sts TCCR1B, r16

When set to ones, the low order three bits of TCCR1B, select an external clock source at the T1 pin, for clocking on a rising edge. Probably what's happening is that when you finger is near enough the T1 pin you are capacitively coupling mains frequency to it causing it to tick.

/256 prescaler would have 100 in the lower three bits.

If I manage to get a proper environment up to test your code, I'll look into it a bit more thoroughly.

• Mahmoud Salah: OPS, Yup that Was it although I read the datasheet and saw that it was 100 don't know how it ended up as 111. anyway as a suggestion for an environment I use avrdude and avra assembler on linux mint terminal with the arduino uno
• Mahmoud Salah: and the main frequency in my country is around 60 HZ and the LED clearly didn't blink on that frequency it was fast but not that fast at all. How is that? is there an explanation for the altered frequency?
• timemage: Well, the frequency of what appears on T1 is being effectively divided by what you're doing with your ISR/TCNT1/OCRA1A; one cycle on T1 is not one execution of sbi PINB, LED_PIN, and each toggling corresponds to just the high or just the low period of the LED cycle. Then, you might also have high frequency components being seen at T1. If you wanted to experiment with that, the thing to do would be to feed a solid 60Hz into T1 to eliminate the unknown components there. By the way, your ISR should probably be preserving r16.