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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
484 of 808
NXP Semiconductors
UM10360
Chapter 24: LPC17xx Pulse Width Modulator (PWM)
4.
Sample waveform with rules for single and double edge control
A sample of how PWM values relate to waveform outputs is shown in
PWM output logic is shown in
that allows selection of either single or
double edge controlled PWM outputs via the muxes controlled by the PWMSELn bits. The
match register selections for various PWM outputs is shown in
. This
implementation supports up to N-1 single edge PWM outputs or (N-1)/2 double edge
PWM outputs, where N is the number of match registers that are implemented. PWM
types can be mixed if desired.
[1]
Identical to single edge mode in this case since Match 0 is the neighboring match register. Essentially,
PWM1 cannot be a double edged output.
[2]
It is generally not advantageous to use PWM channels 3 and 5 for double edge PWM outputs because it
would reduce the number of double edge PWM outputs that are possible. Using PWM 2, PWM4, and
PWM6 for double edge PWM outputs provides the most pairings.
The waveforms below show a single PWM cycle and demonstrate PWM outputs under the
following conditions:
The timer is configured for PWM mode (counter resets to 1).
Match 0 is configured to reset the timer/counter when a match event occurs.
All PWM related Match registers are configured for toggle on match.
Control bits PWMSEL2 and PWMSEL4 are set.
The Match register values are as follows:
MR0 = 100 (PWM rate)
MR1 = 41, MR2 = 78 (PWM2 output)
MR3 = 53, MR4 = 27 (PWM4 output)
MR5 = 65 (PWM5 output)
Fig 118. Sample PWM waveforms
Table 424. Set and reset inputs for PWM Flip-Flops
PWM Channel
Single Edge PWM (PWMSELn = 0)
Double Edge PWM (PWMSELn = 1)
Set by
Reset by
Set by
Reset by
1
Match 0
Match 1
Match 0
Match 1
2
Match 0
Match 2
Match 1
Match 2
3
Match 0
Match 3
Match 2
Match 3
4
Match 0
Match 4
Match 3
Match 4
5
Match 0
Match 5
Match 4
Match 5
6
Match 0
Match 6
Match 5
Match 6
PWM2
PWM4
PWM5
100
(counter is reset)
1
27
41
53
65
78