Pulse-Width Modulation (PWM)

what is it?

pulse-width modulation (PWM) is literally what it sounds like, the width of the pulse and how it is modulated. PWM allows the user to generate a square signal with a user determined frequency/period and duty cycle.

the PWM frequency is calculated with equation:

Fpwm = Ftpmck/(TPMMOD + 1)

Fpwm is the frequency of the PWM.
Ftpmck is the frequency of the TPM increments in Hz.
TPMMOD is the modulo value stored into the TPMMOD register.

the duty cycle is the ratio between TPMMOD + 1 and the TPMCxV values:

duty cycle = ((TPMMOD + 1)/TPMCxV) * 100

so for our case:

TPM1MOD = 511
TPM1C0V = 256
then the duty cycle would equal 50%

why do we want this?

with a PWM we are able to focus our IR receiver on a specific IR frequency so that a lot of other lights such as sunlight and flashlights will not affect the the sensor. also, when coded with a timer, the PWM can save power by turning on for a short period of time and then going back to sleep. the timer would then wake it up again a certain period of time later and the PWM would then do its job and go back to sleep.

to sum it up, the PWM:

helps focus received frequency
save power

what we are using

the following is a list of the code we are using and why its set to those values (please purchase a copy of 'HCS08 Unleashed' for full details):

TPM1SC = 0×08;

TPMSC takes a hex value. this code is used to set the TPM clock equal to the BUSCLK. the hex code states that bit 3 is high and for the coding of the TPMSC, only bit 3 high sets the TPM clock equal to the BUSCLK.

TPM1MOD = 511;

TPMMOD takes a positive integer. this code is used to set the PWM frequency equal to 40kHz. Based on the equation above, the TPMSC/TPMMOD equals PWM frequency.

TPM1C0SC = 0×28;

TPMCxSC takes a hex value. this code is used to set channel x to high-true PWM mode. the hex code states that bits 3 and 5 are high and for the coding of the TPMCxSC, only bits 3 and 5 high sets the channel to high-true PWM mode.

TPM1C0V = 256;

TPMCxV takes a positive integer. this code is used to determine the duty cycle. based on the equation above, (TPMMOD/TPMCxV) * 100 equals the duty cycle.