Rainbow Electronics T89C51RD2 - Manual (Page 30)

Rev. F - 15 February, 2001 30
T89C51RD2
Figure 11. PCA High Speed Output Mode
Before enabling ECOM bit, CCAPnL and CCAPnH should be set with a non zero value, otherwise an unwanted
match could happen.
Once ECOM set, writing CCAPnL will clear ECOM so that an unwanted match doesn’t occur while modifying
the compare value. Writing to CCAPnH will set ECOM. For this reason, user software should write CCAPnL first,
and then CCAPnH. Of course, the ECOM bit can still be controlled by accessing to CCAPMn register.
6.5.4. Pulse Width Modulator Mode
All of the PCA modules can be used as PWM outputs. Figure 12 shows the PWM function. The frequency of the
output depends on the source for the PCA timer. All of the modules will have the same frequency of output
because they all share the PCA timer. The duty cycle of each module is independently variable using the module's
capture register CCAPLn. When the value of the PCA CL SFR is less than the value in the module's CCAPLn
CH CL
CCAPnH CCAPnL
ECOMn
CCAPMn, n = 0 to 4
0xDA to 0xDE
CAPNn MATn TOGn PWMn ECCFn CAPPn
16 bit comparator
Match
CF CR
CCON
0xD8
CCF4 CCF3 CCF2 CCF1 CCF0
PCA IT
Enable
CEXn
PCA counter/timer
Write to
CCAPnH
Reset
Write to
CCAPnL
1 0