Tuesday, July 20, 2010

PWM Configuration for PIC16F877 microntroller

PWM can be used to control a motor driver or generate sound samples.

The register specifically used to configure PWM are the TRISC, PR2, CCP1CON, CCP2CON, CCPR1L, CCPR2L and T2CON registers. There are two PWM channels on the PIC16F877. These modules are coined CCP1 et CCP2. These modules are implemented on pins 16 and 17 of the PIC16F877. There are two ways to connect these pins to a L293D H-Bridge. I recommend to connect these to the L293D Chip Enable pins. PWM then controls the percentage of time the chip is enabled. 

There are two parameters that affect the behavior of the PWM. The first one is the frequency of the pulses, the second is the length of these pulses. The length of the pulse is also called the duty cycle, e.g. the percentage of time the motor is enabled. In an ideal environment, if a motor normally exhibit 100 RPM, then it should do 25 RPM with a 25% duty cycle

It is important to experiment with these parameters to find the configuration that is right for your application because no two motors are the same. Some motors are better when the frequency is low (between 2 000-5 000 Hz). Other motors are best when the frequency is high, say between 15 000 and 20 000 Hz. Note that a low frequency (between 1 000 and 8 000 Hz) will produce an audible sound that can be very annoying. I have tested three robots with varying weight (500 g to 15 kg) and with different motors and have found that a frequency of 15 000 to 20 000 Hz gives good results. I have never seen any robot move with a duty cycle of less than 50%. If you want to do this, you have to find ways to control PWM in software, i.e. for a duty cycle of 25%, use a 50% duty cycle and turn PWM 50% of the time. Ideally, you will find motors that are not too fast at 50% and fast enough at 100%. 

The TRISC register must be initialized so that RC1 et RC2 are identified as output ports (bits 1 and 2 of TRISC must be set to 0). CCP1CON and CCP2CON are used to configure CCP1 and CCP2 modules mode. A value of 12 configures these modules for PWM mode.

Timer2 is used by CCP1 and CCP2 modules for PWM purposes. Timer2 prescaler is configured using bits TCKPS0 et TCKPS1 of T2CON register (bits 0 and 1). A value of 0 for these two bits configures a prescaler of 1. Bit TMR2ON (bit 2) must be initialized to 1 to start Timer2.PR2 register controls the pulse frequency. The formula used to calculate the pulse period is:(PR2 + 1) * 4 * TOSC * (Timer2 prescaler value).Pulse frequency is 1 / period. If PR2 is equal to 255 and the PIC runs at 20 MHz, pulse frequency is 19.53 kHz with a Timer2 prescaler of 1, i.e. 1 / ((255 + 1) * 4 * 1 / 20 000 000 * 1).

The CCPR1L et CCPR2L registers configure the "duty cycle". The "duty cycle" is relative to PR2. If PR2 is 100 and CCPR1 is 50, then the "duty cycle" is effectively 50% (half of PR2).
CCS provides macros to configure these registers appropriately. Here is an example program that shows how to configure PWM with CCS:


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