Intel Microcontroller Sample Calculations, Enabling the PWM Outputs, Generating Analog Outputs

PULSE-WIDTH MODULATOR

9.5.1Sample Calculations

For example, assume that the operating frequency equals 25 MHz, the desired period of the PWM output waveform is either 20.48 µs (512 state times) if the divide-by-two prescaler is disabled or

40.96µs (1,024 state times) if the prescaler is enabled. If PWMx_CONTROL equals 8AH (138 decimal), the pulsewidth is held high for 11.04 µs (and low for 9.44 µs) of the total 20.48 µs pe- riod, resulting in a duty cycle of approximately 54%. If the prescaler is enabled, the same values would produce a period of 40.96 µs with the pulsewidth being held high for 22.08 µs (and low for

18.88µs), for the same duty cycle, approximately 54%.

9.5.2Enabling the PWM Outputs

Each PWM output is multiplexed with a port pin, so you must configure it as a special-function output signal before using the PWM function. To do so, follow this sequence:

1.Clear the corresponding bit of P4_DIR (see Table 9-5).

2.Set the corresponding bit of P4_MODE (see Table 9-5).

3.Set or clear the corresponding bit of P4_REG (see Table 9-5).

Table 9-5 shows the alternate port function along with the register setting that selects the PWM output instead of the port function.

Table 9-5.PWM Output Alternate Functions

PWM Output	Alternate Port Function	PWM Output Enabled When:
PWM0	P4.0	P4_DIR.0 = 0, P4_MODE.0 = 1, P4_REG = X
PWM1	P4.1	P4_DIR.1 = 0, P4_MODE.1 = 1, P4_REG = X
PWM2	P4.2	P4_DIR.2 = 0, P4_MODE.2 = 1, P4_REG = X

9.5.3Generating Analog Outputs

The PWM modules can generate a rectangular pulse train that varies in duty cycle and period. Filtering this output will create a smooth analog signal. To make a signal swing over the desired analog range, first buffer the signal and then filter it with either a simple RC network or an active filter. Figure 9-6 is a block diagram of the type of circuit needed to create the smooth analog sig- nal.

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