AT89C51ED2-RDTUM datasheet(45/137 Pages) ATMEL

45 / 137 page

4235K–8051–05/08

AT89C51RD2/ED2

Figure 13-5. PCA High Speed Output Mode

Before enabling ECOM bit, CCAPnL and CCAPnH should be set with a non zero value, other-

wise an unwanted match could happen.

Once ECOM is 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.

13.4

Pulse Width Modulator Mode

All of the PCA modules can be used as PWM outputs. Figure 13-6 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 modules capture register CCAPLn. When the value

of the PCA CL SFR is less than the value in the modules CCAPLn SFR the output will be low,

when it is equal to or greater than the output will be high. When CL overflows from FF to 00,

CCAPLn is reloaded with the value in CCAPHn. This allows updating the PWM without glitches.

The PWM and ECOM bits in the module's CCAPMn register must be set to enable the PWM

mode.

CCAPnH

CCAPnL

ECO Mn

CCAPMn, n = 0 to 4

0xDA to 0xDE

CAPNn MATn TOGn PWMn ECCFn

CAPPn

16 bit comparator

Match

CCON

0xD8

CCF4 CCF3 CCF2 CCF1 CCF0

PCA IT

Enable

CEXn

PCA counter/timer

Write to

CCAPnH

Reset

Write to

CCA PnL

AT89C51ED2-RDTUM Datasheet(PDF) 45 Page - ATMEL Corporation