Rainbow Electronics T89C51RD2 - Manual (Page 22)

Rev. F - 15 February, 2001 22
T89C51RD2
6.5. 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:
•
Oscillator frequency
ч
12 (
ч
6 in X2 mode)
•
Oscillator frequency
ч
4 (
ч
2 in X2 mode)
•
Timer 0 overflow
•
External input on ECI (P1.2)
Each compare/capture modules can be programmed in any one of the following modes:
•
rising and/or falling edge capture,
•
software timer,
•
high-speed output, or
•
pulse width modulator.
Module 4 can also be programmed as a watchdog timer (See Section "PCA Watchdog Timer", page 31).
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 the port is not used for the PCA, it can still be used for standard I/O.
The PCA timer is a common time base for all five modules (See Figure 7). The timer count source is determined
from the CPS1 and CPS0 bits in the CMOD SFR (See Table 7) and can be programmed to run at:
•
1/12 the oscillator frequency. (Or 1/6 in X2 Mode)
•
1/4 the oscillator frequency. (Or 1/2 in X2 Mode)
•
The Timer 0 overflow
•
The input on the ECI pin (P1.2)
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
16-bit Module 4 P1.7 / CEX4