Owner manual

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 ECCFnCAPPn

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