ATMega 168 Fast PWM with Timer 2

autoguider · Joined: 24 Sep 2007 Posts: 82 Location: Aachen

Dear All,
I cannot explain why my circuit does not work.
I want to use Timer 2 to generate a refernce voltage to control the dutycycle of a stepper motor.
The µC is PDIP casing. OC2A is on pin 17.
Pin is connected to a 150 kOhm + 15 kOhm + 10 µF RC combination. So the current is small.
By setting OCR2A =100 I expect appr. 2.2 V at pin 17. I measure 0V ( digital multimeter).
Even on the RC combination voltage is 0 V.

The buzzer's beeping shows that the program is running.

Do you have an idea why it does not work ?

Please see the attached program. I included also the description of the TCCR registers.

best regards

(BASCOM-AVR version : 2.0.7.7 )

Dave · Joined: 05 Feb 2005 Posts: 314 Location: OR

You don't have a clk source selected in TCCR2B= &B00000000

Bits 0,1 & 2 are the clk select bits. Zero stops the timer.

autoguider · Joined: 24 Sep 2007 Posts: 82 Location: Aachen

Hi Dave,

many thanks for the useful advice.

I changed the registers now to:

TCCR2A= &B10000011 ' Fast PWM, 1 0 Clear OC2A on Compare Match, set OC2A at BOTTOM
'
TCCR2B= &B00000001 ' No prescaling

OCR2a=128

Now the multimeter shows a voltage of nearly 5 V at the OC2A pin.
I expect 2.5 V with the OCR2A = 128 setting.

It seems that the value of the OCR2A register = 50 is some kind of threshold.
With OCR2A = 50 or less the voltage on the pin is 0V.
With OCR2A = 51 or higher the voltage on the pin is 5V.

This is strange to me.

Are there some other registers that have to be modified ?

best regards

Christian

Dave · Joined: 05 Feb 2005 Posts: 314 Location: OR

Hi Christian,

Remove "Start Timer2".

See help:" The START TIMER statement will only work correctly when you have selected a clock source or pre-scaler value with the CONFIG TIMER statement."

Dave

autoguider · Joined: 24 Sep 2007 Posts: 82 Location: Aachen

Dear dave,
now it works.

This is my short program for testing.

dim j as integer

Config PORTC.3 = Output ' For Buzzer
Config Portb.3 = Output ' Output OC2A

TCCR2A= &B10000011 ' Fast PWM, 1 0 Clear OC2A on Compare Match, set OC2A at BOTTOM
'
TCCR2B= &B00000001 ' No prescaling

OCR2a=10 'Initial value

Portc.3=1 ' Initial loud beep
wait 1
portc.3=0

do
portc.3=1 'acoustic signal indicating program running
waitms 4
portc.3=0

j= ocr2a
J=j +10
if j >254 then J=1
ocr2A =j
wait 5
loop

many thanks and best regards

Christian