AT89C51ED2-RDTUM datasheet(35/137 Pages) ATMEL

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4235K–8051–05/08

AT89C51RD2/ED2

13. Programmable Counter Array (PCA)

The PCA provides more timing capabilities with less CPU intervention than the standard

timer/counters. Its advantages include reduced software overhead and improved accuracy. The

PCA consists of a dedicated timer/counter which serves as the time base for an array of five

compare/capture modules. Its clock input can be programmed to count any one of the following

signals:

• Peripheral clock frequency (FCLK PERIPH) ÷ 6

• Peripheral clock frequency (FCLK PERIPH) ÷ 2

• Timer 0 overflow

• External input on ECI (P1.2)

Each compare/capture module can be programmed in any one of the following modes:

• Rising and/or falling edge capture

• Software timer

• High-speed output

• Pulse width modulator

Module 4 can also be programmed as a watchdog timer (see Section "PCA Watchdog Timer",

page 46).

When the compare/capture modules are programmed in the capture mode, software timer, or

high speed output mode, an interrupt can be generated when the module executes its function.

All five modules plus the PCA timer overflow share one interrupt vector.

The PCA timer/counter and compare/capture modules share Port 1 for external I/O. These pins

are listed below. If one or several bits in the port are not used for the PCA, they can still be used

for standard I/O.

The PCA timer is a common time base for all five modules (see Figure 13-1). The timer count

source is determined from the CPS1 and CPS0 bits in the CMOD register (Table 13-1) and can

be programmed to run at:

• 1/6 the

peripheral clock frequency (FCLK PERIPH)

• 1/2 the

peripheral clock frequency (FCLK PERIPH)

• The Timer 0 overflow

• The input on the ECI pin (P1.2)

The CMOD register includes three additional bits associated with the PCA (See Figure 13-1 and

Table 13-1).

• The CIDL bit which allows the PCA to stop during idle mode.

• The WDTE bit which enables or disables the watchdog function on module 4.

PCA Component

External I/O Pin

16-bit Counter

P1.2/ECI

16-bit Module 0

P1.3/CEX0

16-bit Module 1

P1.4/CEX1

16-bit Module 2

P1.5/CEX2

16-bit Module 3

P1.6/CEX3

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