4.2Timer/Counter Operation

4.2.1Introduction

One 8254 programmable timer/counter chip is installed in 48H/96H series. There are three counters in one 8254 chip and 6 possible operation modes for each counter. The block diagram of the timer/counter system is shown in Figure 4.2.

P1C4

Trigger

8254 Chip

 

 

 

 

 

 

 

 

 

 

 

Edge

 

 

C

 

 

Event IRQ

 

 

 

 

 

 

 

 

 

Control

Counter #0

O

 

 

 

 

 

 

 

 

2 MHz Clock

 

 

'H'

G

 

 

 

 

 

 

 

C Timer #1

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

'H'

 

 

 

 

 

 

 

 

 

 

 

 

 

 

G

 

 

 

 

 

 

 

 

 

C

 

 

Timer IRQ

 

 

 

 

 

'H'

Timer #2

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

G

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.2 Timer/counter system of 48H/96H series.

The timer #1 and timer #2 of the 8254 chip are cascaded as a 32-bit programmable timer. In software library, the timer #1 and #2 are always set as mode 2 (rate generator).

In software library, the counter #0 is used as an event counter, that is, interrupt on terminal count of 8254 mode 0 . Please refer to chapter 5 for programming the timer/counter functions.

4.2.2 General Purpose Timer/Counter

The counter 0 is a general purpose timer/counter for users applications. It can be used as an event counter, or used for measuring frequency, or other functions. The following Mode are provided by the 82C54 chip.

•Mode 0: Interrupt on Terminal Count

•Mode 1: Programmable One-Shot.

•Mode 2: Rate Generator.

•Mode 3: Square Wave Rate Generator.

•Mode 4: Software Triggered Strobe.

•Mode 5: Hardware Triggered Strobe.

Operation Theorem ∙ 25