AT89C51ED2-RDTIM datasheet(46/137 Pages) ATMEL

46 / 137 page

AT89C51RD2/ED2

4235E–8051–04/04

Figure 20. 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 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.

Pulse Width Modulator

Mode

All of the PCA modules can be used as PWM outputs. Figure 21 shows the PWM func-

tion. 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 reg-

ister 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 mod-

ule's CCAPMn register must be set to enable the PWM mode.

CCAPnH

CCAPnL

ECOMn

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-RDTIM 数据表(PDF) 46 Page - ATMEL Corporation

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