Cirrus Logic EP93xx - Manual (Page 734)

24-2 DS785UM1
Copyright 2007 Cirrus Logic
Pulse Width Modulator
EP93xx User’s Guide
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With those two counters specified, a fixed pulse is generated. The two channels are totally
independent. This is a DC-level PWM.
Either PWM channel can be utilized to create reoccurring pulses at the PWMx output pins.
Depending upon how a PWM is programmed, its output can vary from a continuous level
(100% duty-cycle), to a square wave (50% duty-cycle), to a narrow pulse approaching a 0%
duty-cycle. Both PWMs offer 16-bit resolution of the input clock signal.
The outputs of both PWM channels are programmed in terms of PWM input clock cycles.
Each PWM may be programmed statically (when it is halted) or dynamically (while it is
running). The output of either PWM may be programmed as normal or inverted. With the
exception of inversion, if a PWM is programmed statically, no change to the output will occur
until the PWM is enabled. If a PWM is reprogrammed while it is running (enabled), the output
is updated to the new programming (total period, total period asserted) at the beginning of the
next PWM cycle. The exception for inverted operation is explained below. Both PWMs are
reset to the halted condition.
The output of either PWM can be programmed for either normal or inverted operation.
Inversion affects the output pin when the PWM peripheral is halted and also when it is
running. Both outputs are reset to the normal (non-inverted) configuration, which places the
output pins in a LOW condition at reset. When the output is reprogrammed to be inverted (or
to be normal), the new programming does not become effective until the rising edge of the
PWM input clock signal.
Note: In the design, because of the use of clock gating on PCLK, the write enable and read
enable were altered to work correctly within the design.
24.2.1 PWM Programming Examples
The reference clock for PWM is XTALI.
24.2.1.1 Example
To produce a PWM output of 100 kHz (10
μ
sec) and 20% duty cycle with a system clock of
66 MHz (15 nsec):
1. Calculate PWMxTermCnt = (66 MHz / 0.1 MHz) – 1 = 659 (decimal).
2. Calculate PWMxDutyCycle = (0.2 × (659 + 1)) – 1 = 131 (decimal).
24.2.1.2 Static Programming (PWM is Not Running) Example
Table 24-1. Static Programming Steps
Step Register Value
Stop PWM PWMxEn 0x0000
Wait for PWM to finish current cycle -----------------
Program TC value with 659 (decimal) PWMxTermCnt 0x0293
Program DC value with 131 (decimal) PWMxDutyCycle 0x0083
Program PWM output to invert PWMxInvert 0x0001
Enable/Start PWM PWMxEn 0x0001