Manual PCI-A12-16A
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M0-M2: These bits set the operational mode of the selected counter.
Mode
M2
M1
M0
0 0
0
0
1 0
0
1
2 X
1
0
3 X
1
1
4 1
0
0
5 1
0
1
BCD: Set the selected counter to count in binary (B0 = 0) or BCD (B0 = 1).
Reading and Loading the Counters
If you attempt to read the counters on the fly when there is a high input frequency, you will most likely get
erroneous data. This is partly caused by carries rippling through the counter during the read operation. Also,
the low and high bytes are read sequentially rather than simultaneously and, thus, it is possible that carries
will be propagated from the low to the high byte during the read cycle.
To circumvent these problems, you can perform a counter-latch operation in advance of the read cycle. To do
this, load the RW1 and RW2 bits with zeroes. This instantly latches the count of the selected counter(selected
via the SC1 and SC0 bits) in a 16-bit hold register. (An alternative method of latching counter(s) which has an
additional advantage of operating simultaneously on several counters is by use of a readback command to be
discussed later.) A subsequent read operation on the selected counter returns the held value. Latching is the
best way to read a counter on the fly without disturbing the counting process. You can only rely on directly
read counter data if the counting process is suspended while reading, by bringing the gate low, or by halting
the input pulses.
For each counter you must specify in advance the type of read or write operation that you intend to perform.
You have a choice of loading/reading (a) the high byte of the count, or (b) the low byte of the count, or (c) the
low byte followed by the high byte. This last is of the most general use and is selected for each counter by
setting the RW1 and RW0 bits to ones. Of course, subsequent read/load operations must be performed in
pairs in this sequence or the sequencing flip-flop in the 8254 chip will get out of step.
Base + B Read:
Counter Control Byte
B7 B6 B5 B4 B3 B2 B1 B0
1 1 CNT
STA
C2
C1
C0
0
CNT:
When is 0, latches the counters selected by bits C0-C2.
STA:
When is 0, returns the status byte of counters selected by C0-C2.
C0, C1, C2:
When high, select a particular counter for readback. C0 selects Counter 0, C1 selects
Counter 1, and C2 selects Counter 2.
You can perform two types of operations with the Counter Control Byte. When CNT=0, the counters selected
by C0 through C2 are latched simultaneously. When STA=0, the counter status byte is read when the counter
