rtd DM6210, User Manual

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7-3
The 8254 programmable interval timer provides three 16-bit, 8-MHz timers for timing and counting
functions such as frequency measurement, event counting, and interrupts. All three of the Timer/Counters are
available for the user. Figure 7-1 shows the TC circuitry.
Each timer/counter has two inputs, CLK in and GATE in, and one output, timer/counter OUT. They can be
programmed as binary or BCD down counters by writing the appropriate data to the command word, as
described in the I/O map discussion in Chapter 4.
The output from from each Timer/Counter is available at the connector CN3.
The timers can be programmed to operate in one of six modes, depending on your application. The
following paragraphs briefly describe each mode.
Mode 0, Event Counter (Interrupt on Terminal Count). This mode is typically used for event counting.
While the timer/counter counts down, the output is low, and when the count is complete, it goes high. The
output stays high until a new Mode 0 control word is written to the timer/counter.
Mode 1, Hardware-Retriggerable One-Shot. The output is initially high and goes low on the clock pulse
following a trigger to begin the one-shot pulse. The output remains low until the count reaches 0, and then goes high
and remains high until the clock pulse after the next trigger.
Mode 2, Rate Generator. This mode functions like a divide-by-N counter and is typically used to generate a
real-time clock interrupt. The output is initially high, and when the count decrements to 1, the output goes low for
one clock pulse. The output then goes high again, the timer/counter reloads the initial count, and the process
is repeated. This sequence continues indefinitely.
Fig. 7-1 — User TC Circuitry
CLK0
6210
I/O CONNECTOR
CN3
PIN 46
PIN 43
PIN 42
8254
TIMER/
COUNTER
0
CLK
GATE
OUT
TIMER/
COUNTER
1
CLK
GATE
OUT
TIMER/
COUNTER
2
CLK
GATE
OUT
PIN 39
PIN 44
XTAL (8 MHz)
JP1
+5 V
+5 V
+5 V
PIN 19
PIN 40
PIN 41
PIN 45
T/C OUT 0
OUT0
OUT1
EXT CLK 0
T/C OUT 1
EXT CLK 1
EXT GATE 2
T/C OUT 2
EXT CLK 2
EXT GATE 0
EXT GATE 1
EC0
OSC
EC1
OSC
EC2