VERY SIMPLE TIMER

VERY SIMPLE TIMER in ASM by RETRODAN

QUOTE

I'm sorry I'm a slow learner... I don't know which ports and registers to properly setup parameters for my RTC

PRELUDE:

In my latest Butterfly Bootloader (Cricket) I wanted to totally change the upload logic. I got tired of holding down the joystick in the middle postion and I think it's wearing out. I designed it mostly for my own personal use and find it very difficult to use anything else now that I am used to how much easier it is to use.

What I wanted was when the Cricket was RESET it would "chirp" once then calibrate the OSciallator then "chirp" again and automatically wait for you to either upload a new program or to move the joystick to launch a program. After a reasonable amount of time it would "chirp" one last time to let you know it was going to sleep to conserve battery.

I used a timer to do this, and perhaps this listing will help you to get started with your RTC in ASM. The only lines you really need to worry about are the first three and four near the end that read the clock and exit when high byte gets to 255.

THE MATH:

Here is how the Math works out:

The clock speed I chose to use for this part of program is 8Mhz and writing a 5 to TCCRB1 set the "prescaler" to 1024.

That mean that the timer will only "tick" after 1024 clock cycles.

CODE

Tick = PreScale / 8Mhz = 1,024/8,000,000 = 0.000128 Sec

Since I am using 2 bytes for the clock/timer the lower-byte is going to "roll-over" and increment the high-byte when it hits 256:

CODE

HighByteTick = 256 x 0.000128 = 0.032768 Sec

Since I Exit when HighByte gets to 255:

CODE

Exit=255 x 0.032768 = 8.35 Seconds

When I initially worked this out, I figured the 8 seconds was way too short to start uploads, and was going to extend it, but it turns out that 8 seconds is ideal.

THE PROGRAM SEGMENT:

CODE

;----------------------------------;
; BOOT SENSORY LOOP ;
; CHECK JOYSTICK & UART THEN SLEEP;
;----------------------------------;
STS TCCR1B,FIVE;PRESCALE /1024
STS TCNT1L,ZERO;START AT ZERO
STS TCNT1H,ZERO

BOOT_LOOP: CLR TMP
SBIS PINB,6 ;JOYSTICK UP
RJMP START_8MHZ
SBIS PINB,7 ;JOYSTICK DOWN
RJMP START_2MHZ
SBIS PINE,2 ;JOYSTICK LEFT
RJMP START_4MHZ
SBIS PINE,3 ;JOYSTICK RIGHT
RJMP START_1MHZ

LDS TMP,TCNT1L ;CHECK CLOCK
LDS TMP,TCNT1H
CPI TMP,255
BREQ GO_SLEEP

LDS TMP1,UCSR0A;CHECK UART
SBRS TMP1,RXC
RJMP BOOT_LOOP ;NO UART THEN LOOP

IN CONCLUSION:

So I don't profess to be an expert on using timers in ASM, but I hope that you can work-out your own MATH using this as an example, and get you on-your-way to designing your own RTC.

ADDENDUM:

Another example of the Math using the same clock and prescaler:

CODE

1 Sec = Clock/PreScaler = 8,000,000/1024 = 7812.5 Ticks
1 Sec= 7812.5 / 256 = 30.5 HighByteTicks

So if you waited for HighByte of clock to reach 30 or 31 you would have approx. 1 second but this not going to be very accurate.

2 Seconds for 71 HighByteTicks in not too far off.

If you set the prescaller to 256 instead of 1024 by writing a 4 (instead of 5) to TCCR1B then:

CODE

1Sec= 8000000/256 = 31250 Ticks
1Sec= 31250 / 256 = 122.07 HighByteTicks

So by waiting for high byte to reach 122 in this example you are getting about a second also.