TIMERS AND SQUARE WAVE GENERATOR (CLK/ 16 BIT NUMBER)

ALJ-1108 · Joined: 08 Nov 2005 Posts: 7 Location: USA RICHMOND,TEXAS

Hello All,
I've been playing with this Atmega 2561 on a Kanda board for the better part of a week now. Wondering if anyone has anything on setting up a the timer1 for a simple Clk divided by a 16 bit number to give you a known frequency output????? I'm so darn use to the 82c54 its ridiculous but I want to know if the timers in the mpu itself are of any use...
So far all I got is:

'TIMER FOR FREQUENCY OUTPUT
Config Timer1 = Timer,Prescale = 1,Compare A = Toggle,Clear Timer = 1
Timer1 = 0
Config Portb = Output
Start Timer0
Strt:
Compare1a = 0
Goto Strt

Which on a 16mhz clock is 8mhz but when playing with the Compare1a the values are screwed...

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

You start timer1 at 0, then set the compare1 reg to 0 in the loop. So you get an immediate toggle every time the timer starts from 0. Maybe what you want, maybe not.
But its slightly better this way:

ALJ-1108 · Joined: 08 Nov 2005 Posts: 7 Location: USA RICHMOND,TEXAS

Yea I screwed up on the cutting and posting....
It was like this
'TIMER FOR FREQUENCY OUTPUT
Config Timer1 = Timer,Prescale = 1,Compare A = Toggle,Clear Timer = 1
Timer1 = 0
Config Portb = Output
Start Timer1
Strt:
Compare1a = 1143 ' which 1143 is the count of 7Khz
Goto Strt
' The wierd is that the toggle divides it by 2
' So basically (16MHZ * 1 * 7Khz)/2
' Where 16mhz is the base clk , 1 is the prescale , 7Khz is the frequecy out

mattcro · Joined: 03 Oct 2007 Posts: 327 Location: Scotland

The AVR timers are pretty complicated and powerful - there are several ways to do what you want, but this is probably the simplest to understand (honest!). It takes a fair bit of reading through the data sheet and examples to get a good understanding of what's going on under the hood.

What you have in your latest code is a counter counting up from 0 to (in your case) 1143 at 16MHz, then resetting (rolling over) to 0 again, i.e. 14000 times a second. As it resets, it flips the state of (toggles) the Compare A output pin (OC1A) to produce a square wave.

The frequency of this square wave is half of the counter rollover frequency since the output pin only toggles once for every 1143 counts, and it takes two rollovers to produce one cycle of the output square wave.

You just have to take account of this in your frequency calculations.