8 bits CCP PWM on 16F18324

mvanvliet · Joined: 02 Jun 2009 Posts: 123 Location: The Netherlands

In my opinion it was in the 16F1823/1824 series that the CCP PWM module was standard 8 bits and after using a 16 bits variable or placing an 'L' behind the variable the CCP PWM is working at 10 bits. But now I'm using the 16F18324 and this module is always using 10 bits. Because I'm migrating an old program for 16F1824 to 16F18324 I would like to keep as much the same as possible, but for me it isn't working. Does anyone know how to use an 8 bits CCP PWM module instead of the 10 bits?

CCS version: 5.062

Test program (so this is not my final program!):

Ttelmah · Joined: 11 Mar 2010 Posts: 19561

This is because of the fundamentally 'different' PWM on this chip.

On the standard PIC's, there is an 8bit PWM register, and then two separate 'extra' low bits stored into another register. Using an int8, only accesses the main 8bit register.
On this PIC, there are instead two PWM registers giving you full 16bit PWM control. To actually use it in a mode 'akin' to the older chip, you'd have to multiply your PWM duty value by 4.

If you look in the header file for the chip, you will find the duty prototype, only accepts an int16.

mvanvliet · Joined: 02 Jun 2009 Posts: 123 Location: The Netherlands

The multiply by 4 was also my quick solution. Unfortunately there is no other solution.

Ttelmah · Joined: 11 Mar 2010 Posts: 19561

The compiler will code a *4, as a binary shift, so very quick, but it is fundamental to the nature of the chip.

With the standard 10bit PWM, you have

rrrrrrrrxx

Where 'r' are bits in the register, and 'xx' are the two extra bits stored separately.

While with this pwm, you have

hhhhhhhhrrrrrrrr

Stored in two registers (h is the high register).

If you code the pwm to use 1/4 the divider, and then a 4* larger prescaler, the LSb of the value will give the same result as you get from the 10bit PWM.

setup_timer_4(T4_DIV_BY_4,63,1);