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Generated PWM

 
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loops0901



Joined: 01 Apr 2012
Posts: 5
Location: Malaysia

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Generated PWM
PostPosted: Thu Jun 07, 2012 11:41 am     Reply with quote
Hi,
I am using PIC16F877 to generate PWM signal. I found that the generated signal is having Vmax=2.40V and Vmin=-2.56V . The waveform as shown as image below:


Is this normal? As i know, the PWM signal should be from 0V to 5V. But the generated PWM is from -2.56V to 2.40V.

I am using the following code:
Code:
#include <16F877.h>
#fuses HS, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 20000000)

main()
{
output_low(PIN_C1);   // Set CCP2 output low
output_low(PIN_C2);   // Set CCP1 output low

setup_ccp1(CCP_PWM);  // Configure CCP1 as a PWM
setup_ccp2(CCP_PWM);  // Configure CCP2 as a PWM

setup_timer_2(T2_DIV_BY_16, 124, 1);  // 500 Hz     
set_pwm1_duty(31);                    // 25% duty cycle on pin C2
set_pwm2_duty(62);                    // 50% duty cycle on pin C1

while(1);
}


I am using this generated PWM signal for L298 to control my peltier. Voltage supply is 12V. When i supply 90% duty cycle to L298, the output voltage that i measured for peltier is only equal to 7.8V. I am wondering whether this generated PWM is the factor or not.
Thanks
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Thu Jun 07, 2012 12:18 pm     Reply with quote
You have the scope in AC input coupling mode. Push the AC/DC button
and put it in DC mode. Keep it there.
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