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 (BCD = 0) or BCD (BCD = 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) that has an additional advantage of operating simultaneously on several counters is through
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.
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 most generally used and is selected for
each counter by setting the RW1 and RW0 bits to ones. 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. The
readback command byte format is:
B7 B6 B5 B4 B3 B2 B1 B0
1 1 CNT
STA
C2
C1
C0
0
CNT:
When 0, latches the counters selected by bits C0-C2.
STA:
When 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 readback command. When CNT=0, the counters
selected by C2 through C0 are latched simultaneously. When STA=0, the counter status byte is read
when the counter I/O location is accessed. The counter status byte provides information about the
current output state of the selected counter and its configuration.
Manual 104-DIO-48E, 24E
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