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Chapter 7 Programmable Pacer
7.1 The Intel 8254
The PCM-3718 cards use the Intel 8254 programmable interval timer/
counter version 2. The popular 8254 offers three independent
16-bit down counters. Each counter has a clock input, control gate and an
output. You can program each counter for maximum count values from 2
to 65535.
Version 2 of the 8254 has a maximum input clock frequency of 10 MHz.
The PCM-3718 provide 1 MHz and 10 MHz input frequencies to the
8254 from an onboard crystal oscillator. Jumper JP1 controls the input
frequency. See page 12 for more information.
Counters 1 and 2 on the 8254 are cascaded and operated in a fixed divider
configuration. Counter 1 input is connected to the 1 MHz or 10 MHz
clock frequency, and the output of Counter 1 is connected to the input of
Counter 2. The output of Counter 2 is internally configured to provide
trigger pulses to the A/D converter, as shown below:
1MHZ
OR
10MHZ
OSC.
Intel 8254 has six operational modes, from Mode 0 through Mode 5. To
generate a pacer clock, program both Counter 1 and Counter 2 for Mode 3
(square wave generation).
PCM-3718 Series User Manual
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
COUNTER 1
CLK IN
OUT
52
COUNTER 2
PACER
CLK IN
OUT
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