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Getting Started

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2.8 Clock System

The clock system of the cPCI/PCI-8554/R provides the internal clock

source for the 8254 chips. The clock for counter/timer 1 ~ 10 can be one

of 4 sources; an external clock source, a cascaded source from the ‘last’

channel, CK1 or COUT10. The next section will outline details on setting

the clock for each counter/timer and CK1. The clock of counter/timer 11 is

fixed at 8Mhz, and the clock of counter/timer 12 is connected to COUT11

2.9 Counters Architecture

There are four 8254 programmable timer/counter chips on the cPCI/PCI-

8554/R card. Each 8254 chip contains 3 counter/timer and are labeled

from 1 to 12. Counters 11 and 12 are default cascaded counters, but can

be set for independent operation through jumpers JP12 & JP13. Counters

1 ~ 10 can also be set as an independent or cascaded counters. Table 3

illustrates the relationship between the reference 8254-chip label and the

counter labels.

8254 Chip

Label

Reference

Number

Counter

Label

Type of Counter

Counter 1 Independent or Cascaded

Counter 2 Independent or Cascaded

Chip 1 U3

Counter 3 Independent or Cascaded

Counter 4 Independent or Cascaded

Counter 5 Independent or Cascaded

Chip 2 U4

Counter 6 Independent or Cascaded

Counter 7 Independent or Cascaded

Counter 8 Independent or Cascaded

Chip 3 U5

Counter 9 Independent or Cascaded

Counter 10 Independent or Cascaded

Counter 11 Independent or Cascaded

Chip 4 U6

Counter 12 Independent or Cascaded

Table 3. Counters label relationship

1 2 ... 20 21 22 23 24 25 26 27 28 29 30 ... 66 67

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