Datasheet

AT89C51RB2/RC2

4180D–8051–06/05

16-bit Software Timer/

Compare Mode

The PCA Modules can be used as software timers by setting both the ECOM and MAT

bits in the Modules CCAPMn register. The PCA timer will be compared to the Module's

capture registers and when a match occurs, an interrupt will occur if the CCFn (CCON

SFR) and the ECCFn (CCAPMn SFR) bits for the Module are both set (see Figure 14).

Figure 14. PCA Compare Mode and PCA Watchdog Timer

Note: 1. Only for Module 4

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

otherwise an unwanted match could occur. Writing to CCAPnH will set the ECOM bit.

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.

CH CL

CCAPnH CCAPnL

ECOMn

CCAPMn, n = 0 to 4

0xDA to 0xDE

CAPNn MATn TOGn PWMn ECCFnCAPPn

16 bit Comparator

Match

CCON

0xD8

PCA IT

Enable

PCA Counter/Timer

RESET

(1)

CIDL CPS1 CPS0 ECF

CMOD

0xD9

WDTE

Reset

Write to

CCAPnL

Write to

CCAPnH

CF CCF2 CCF1 CCF0

CCF3

CCF4