Specifications

4.6.10 Initialisation of Ports and Interrupts

There are several different subroutines used to initialise ports and define interrupt

control. One exists for each of the following: Pulse width modulation, port P; Port A;

Key wake up Port G and the serial port. It is essential that these initialisation

subroutines are run at the start of the program before any actual running commences.

This ensures the interrupts are set ready to work, the serial port is communicating and

that all input and output is taken care of.

The Pulse Width Modulation (PWM) is the method of controlling the speed of the

motors. The Motorola HC12 has four channels devoted to PWM and can easily be set

using the following code. Each of the bytes moved into the PWM control registers

provides a certain setting required for the desired use of the PWM channels. Clear

explanation of each bit can be found in the data sheet for the Motorola HC12. First

specification set is the pre-scaler. This is used to slow down the clock for use as the

PWM. Extremely high frequencies are not desired as they create excess power loss and

heating through the H Bridge switching. Used here is a pre-scaler of 128 which means

the frequency of the PWM will be 128 times smaller than the frequency of the

microcontroller clock. The PWPOL byte takes care of which clock to use for each

channel and also the polarity of the duty cycle for each cycle. PWEN simply enables all

the selected lines to be output regardless of the data direction register. PWCTL sets

port P to have an active pull up device enabled.

;***************************************************************************

;SETUP PWM Channels

INITPWM MOVB #%00111111,PWCLK ;PRESCALER OF /128

MOVB #%00001111,PWPOL ;PPOL

MOVB #%00001111,PWEN ;ENABLE ALL OUTPUTS

MOVB #%00000010,PWCTL ;PULL UP DEVICE ENABLED

SETPWM MOVB #$FF,PWPER0 ;|

MOVB #$FF,PWPER1 ;|PERIOD FOR PWM

MOVB #$FF,PWPER2 ;|

MOVB #$FF,PWPER3 ;|