AD822 datasheet(43/88 Pages) AD

43 / 88 page

ADuC841/ADuC842/ADuC843

Rev. 0 | Page 43 of 88

PWM Modes of Operation

Mode 0: PWM Disabled

The PWM is disabled allowing P2.6 and P2.7 to be used as

normal.

Mode 1: Single Variable Resolution PWM

In Mode 1, both the pulse length and the cycle time (period) are

programmable in user code, allowing the resolution of the

PWM to be variable.

PWM1H/L sets the period of the output waveform. Reducing

PWM1H/L reduces the resolution of the PWM output but

increases the maximum output rate of the PWM. For example,

setting PWM1H/L to 65536 gives a 16-bit PWM with a maxi-

mum output rate of 266 Hz (16.777 MHz/65536). Setting

PWM1H/L to 4096 gives a 12-bit PWM with a maximum

output rate of 4096 Hz (16.777 MHz/4096).

PWM0H/L sets the duty cycle of the PWM output waveform, as

shown in Figure 48.

P2.7

PWM COUNTER

PWM1H/L

PWM0H/L

Figure 48. PWM in Mode 1

Mode 2: Twin 8-Bit PWM

In Mode 2, the duty cycle of the PWM outputs and the resolu-

tion of the PWM outputs are both programmable. The maximum

resolution of the PWM output is 8 bits.

PWM1L sets the period for both PWM outputs. Typically, this is

set to 255 (FFH) to give an 8-bit PWM, although it is possible to

reduce this as necessary. A value of 100 could be loaded here to

give a percentage PWM, i.e., the PWM is accurate to 1%.

The outputs of the PWM at P2.6 and P2.7 are shown in Figure 49.

As can be seen, the output of PWM0 (P2.6) goes low when the

PWM counter equals PWM0L. The output of PWM1 (P2.7)

goes high when the PWM counter equals PWM1H and goes

low again when the PWM counter equals PWM0H. Setting

PWM1H to 0 ensures that both PWM outputs start simultaneously.

P2.7

P2.6

PWM COUNTER

PWM1H

PWM1L

PWM0H

PWM0L

Figure 49. PWM Mode 2

Mode 3: Twin 16-Bit PWM

In Mode 3, the PWM counter is fixed to count from 0 to 65536,

giving a fixed 16-bit PWM. Operating from the 16.777 MHz

core clock results in a PWM output rate of 256 Hz. The duty

cycle of the PWM outputs at P2.6 and P2.7 is independently

programmable.

As shown in Figure 50, while the PWM counter is less than

PWM0H/L, the output of PWM0 (P2.6) is high. Once the PWM

counter equals PWM0H/L, PWM0 (P2.6) goes low and remains

low until the PWM counter rolls over.

Similarly, while the PWM counter is less than PWM1H/L, the

output of PWM1 (P2.7) is high. Once the PWM counter equals

PWM1H/L, PWM1 (P2.7) goes low and remains low until the

PWM counter rolls over.

In this mode, both PWM outputs are synchronized, i.e., once

the PWM counter rolls over to 0, both PWM0 (P2.6) and

PWM1 go high.

P2.7

P2.6

PWM COUNTER

PWM1H/L

65536

PWM0H/L

Figure 50. PWM Mode 3

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