PWM on 16F877

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Put an upper case 'L' after the 500. ie., make it be 500L.

jksor1234 · Joined: 18 Jan 2007 Posts: 9

What does the upper case L do?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

It tells the compiler that the number is a 'long' integer (greater than
8-bits). When the compiler sees a 'long' value as the duty cycle
parameter, it switches to use 10-bit PWM mode instead of 8-bit.
Here are several threads that discuss this:
http://www.ccsinfo.com/forum/viewtopic.php?t=26751
http://www.ccsinfo.com/forum/viewtopic.php?t=24055
http://www.ccsinfo.com/forum/viewtopic.php?t=22119
http://www.ccsinfo.com/forum/viewtopic.php?t=9927

vsmguy · Joined: 13 Jan 2007 Posts: 91

PWMWizard tells me to write :

set_pwm1_duty(500l);

for the above example...

but from what you said, I should call

set_pwm1_duty(124);

I have posted a new thread to this effect, could you (PCM) have a look at it ?

jksor1234 · Joined: 18 Jan 2007 Posts: 9

The L is needed to tell the compiler you want a 10-bit PWM... otherwise it will assume 8-bit.

From the equations I posted earlier, a 50% duty cycle needs a value of 125... set_pwm1_duty(125);

125 Dec = 0111 1101 Binary (an 8-bit number)
500 Dec = 01 1111 0100 Binary (a 10-bit number)

The 125 is just shifted left two, to make 500. Both values would set the PWM to 50%, except 500 would give the PWM 10-bits of accuracy and 125 would only give 8-bits.

Aquila · Joined: 25 Feb 2007 Posts: 1 Location: Turkiye

Hi I'm a student from mechanical engineering department. I don't know a lot of things about electronics. Could you tell me how can I build control circuit for two DC motor. I have to use 16f877; Motors are 12 V 550 rpm and one of them use 9-10 Ampere in start and full load. Must I use transistor or relay in this situation? I can't find any useful links:sad:

I'm sorry that I took your precious time

thanks...

Ttelmah · Guest

What are you going to 'control'?. Just on/off, or speed?. Forward/reverse?. Braking?. The 'odds' are that you want all of these. In which case, the 'answer' is an 'H-bridge' drive.
Most of the robotics sites, will offer some information about these. Remember that the trap diodes are essential if you are not going to destroy electronics when the drives go 'off'. Look at the 'robotpower' site, for some off the shelf drives, in particular, the Scorpion XL, is commonly used in a lot of robotic designs this sort of size.

Best Wishes

disenteako · Guest

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

killer_fighting · Joined: 22 Feb 2012 Posts: 9

I am using PIC16F848A and I want to get 25kHz through PWM. How can I do that?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

1. There is no PIC with that part number. What's the real part number ?

2. What is the oscillator frequency of your PIC ?

killer_fighting · Joined: 22 Feb 2012 Posts: 9

I am sorry. I am using PIC16F648A and my oscillator frequency is 4MHz. I would like to generate a 25 kHz PWM signal.

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

Data sheet. Data sheet. Data sheet.
Table 9-3 (just divide the frequencies by 5, since you are clocking at 4MHz, and the values in this are for 20MHz), and paragraph 9.3.1.
Yes you can get 25Khz. PR2=39. You will have a PWM range of 0 to 159. 7.3bits.

Best Wishes

killer_fighting · Joined: 22 Feb 2012 Posts: 9

So I should write like that, right? Please refer to the bold part.

#include <16F648A.h>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 4000000)

main()
{
output_low(PIN_B3); // Set CCP1 output low

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

setup_timer_2(T2_DIV_BY_5, 39, 1); // 25kHz
set_pwm1_duty(31); // 25% duty cycle on pin C2
set_pwm2_duty(62); // 50% duty cycle on pin C1

while(1); // Prevent PIC from going to sleep (Important !)
}

Thanks for your help. Sorry, I am the beginner for PIC user.

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

No. 'T2_DIV_BY_5'. no such setting. Would give 5KHz anyway.
The point about the division by five I mention, is when _looking at the frequencies given by table 9-3_, the frequencies _you_ will get, will be 1/5 of the shown values, because the table is calculated for a 20MHz master clock.
T2_DIV_BY_1

Your 50%, and 25% values are a fraction high. 0 to 159. What is 1/2 and 1/4 of this?.

Also remember as has already been said in this thread several times to use the full resolution of the PWM generate, you _must_ have 'L' on the numbers if they are less than 255.

Best Wishes