Hercules III User Manual Rev A.2
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78
20. COUNTER/TIMER OPERATION
Hercules III contains two counter/timers that provide various timing functions on the board for A/D timing and user
functions. These counters are controlled with registers in the on-board data acquisition controller FPGA.
20.1
Counter 0
– A/D Sample Control
Counter 0 is a 24-
bit, “divide-by-n” counter used for controlling A/D sampling. The counter has a clock input, a
gate input, and an output. The input is a 10MHz or 1MHz clock provided on the board and selected with bit
CKFRQ0 in register Base+4, bit 5. The gate is an optional signal that can be input on pin 21 of I/O header J14
when DIOCTR (Base+11, bit 7) is 1. If this signal is not used, the counter runs freely. The output is a positive
pulse whose frequency is equal to the input clock divided by the 24-bit divisor programmed into the counter. The
output appears on pin 24 of the I/O header when DIOCTR is 1.
The counter operates by counting down from the programmed divisor value. When the counter reaches zero, it
outputs a positive-going pulse equal to one input clock period (100ns or 1µs, depending on the input clock
selected by CKFRQ0). The counter then reloads to the initial load value and repeats the process, indefinitely.
The output frequency can range from 5MHz (10MHz clock, divisor = 2) to 0.06Hz (1MHz clock divided by
16,777,215, or 224-1). The output is fed into the A/D timing circuit and can be selected to trigger A/D conversions
when Base+4 register bits AINTE is 1 and ADCLK is 0. Using the control register at Base+15, the counter can be
loaded, cleared, enabled and disabled. The optional gate can be enabled and disabled, and the counter value
can be latched for reading.
20.2
Counter 1
– Counting/Totalizing Functions
Counter 1 is similar to Counter 0 except that it is a 16-bit counter. Counter 1 also has an input, a gate and an
output. These signals may be user-provided on the I/O header when DIOCTR is 0, or the input may come from
the on-board clock generator. When the on-board clock generator is used, the clock frequency is either 10MHz or
100KHz, as determined by control Base+4 register bit CKFRQ1.
The output is a positive-going pulse that appears on pin 26 of the I/O header. The output pulse occurs when the
counter reaches zero. When the counter reaches zero, it reloads and restarts on the next clock pulse. The
output stays high for the entire time the counter is at zero; i.e., from the input pulse that causes the counter to
reach zero until the input pulse that causes the counter to reload.
When DIOCTR is 0, Counter 1 operates as follows.
•
It counts positive edges of the signal on pin 23 on the I/O header.
•
The gate is provided on pin 22. If the signal is high, the counter counts. If the signal is low, the counter
holds its value and ignore input pulses. This pin has a pull-up so the counter can operate without any
external gate signal.
NOTE:
When counting external pulses, Counter 1 only updates its read register every fourth pulse. This
behavior is due to the synchronous design of the counter having to contend with the asynchronous input
pulses. The count register contents are correct on the fourth pulse but remain static until four additional
pulses occur on the input.
When DIOCTR is 1, Counter 1 operates as follows.
The counter takes its input from the on-board clock generator based on the value of the Base+4 register
CKFRQ1 bit. There is no gating and the counter runs continuously.
Counter 1 may be used as either a pulse generator or a totalizer/counter. In pulse generator mode, the output
signal on pin 26 is of interest. In totalizer/counter mode, the counter value is of interest and may be read by first
latching the value and then reading it. The width of the pulse is equal to the time period of the selected counters
clock source.