Silicon Laboratories C8051F347 - Manual (Page 262)

C8051F340/1/2/3/4/5/6/7/8/9/A/B/C/D
262 Rev. 1.3
22.2.5. 8-Bit Pulse Width Modulator Mode
Each module can be used independently to generate a pulse width modulated (PWM) output on its associ-
ated CEXn pin. The frequency of the output is dependent on the timebase for the PCA counter/timer. The
duty cycle of the PWM output signal is varied using the module's PCA0CPLn capture/compare register.
When the value in the low byte of the PCA counter/timer (PCA0L) is equal to the value in PCA0CPLn, the
output on the CEXn pin will be set. When the count value in PCA0L overflows, the CEXn output will be
reset (see Figure 22.8). Also, when the counter/timer low byte (PCA0L) overflows from 0xFF to 0x00,
PCA0CPLn is reloaded automatically with the value stored in the module’s capture/compare high byte
(PCA0CPHn) without software intervention. Setting the ECOMn and PWMn bits in the PCA0CPMn register
enables 8-Bit Pulse Width Modulator mode. The duty cycle for 8-Bit PWM Mode is given by Equation 22.2.
Important Note About Capture/Compare Registers : When writing a 16-bit value to the PCA0 Capture/
Compare registers, the low byte should always be written first. Writing to PCA0CPLn clears the ECOMn bit
to ‘0’; writing to PCA0CPHn sets ECOMn to ‘1’.
Equation 22.2. 8-Bit PWM Duty Cycle
Using Equation 22.2, the largest duty cycle is 100% (PCA0CPHn = 0), and the smallest duty cycle is
0.39% (PCA0CPHn = 0xFF). A 0% duty cycle may be generated by clearing the ECOMn bit to ‘0’.
Figure 22.8. PCA 8-Bit PWM Mode Diagram
DutyCycle
256 PCA0CPHn –
 
256
--------------------------------------------------- =
8-bit
Comparator
PCA0L
PCA0CPLn
PCA0CPHn
CEXn
Crossbar Port I/O
Enable
Overflow
PCA Timebase
0 0 x 0 x
Q
Q
SET
CLR
S
R
match
PCA0CPMn
P
W
M
1
6
n
E
C
O
M
n
E
C
C
F
n
T
O
G
n
P
W
M
n
C
A
P
P
n
C
A
P
N
n
M
A
T
n
0
ENB
ENB
0
1
Write to
PCA0CPLn
Write to
PCA0CPHn
Reset