Intel N2820 FH8065301616603 Data Sheet

1242

Datasheet

The Counter Latch command, written to Port 43h, latches the count of a specific 
counter at the time the command is received. This command is used to ensure that the 
count read from the counter is accurate, particularly when reading a two-byte count. 
The count value is then read from each counter’s Count register as was programmed by 
the Control register.

The count is held in the latch until it is read or the counter is reprogrammed. The count 
is then unlatched. This allows reading the contents of the counters on the fly without 
affecting counting in progress. Multiple Counter Latch Commands may be used to latch 
more than one counter. Counter Latch commands do not affect the programmed mode 
of the counter in any way.

If a counter is latched and then, some time later, latched again before the count is 
read, the second Counter Latch command is ignored. The count read is the count at the 
time the first Counter Latch command was issued.

The Read Back command, written to Port 43h, latches the count value, programmed 
mode, and current states of the OUT pin and Null Count flag of the selected counter or 
counters. The value of the counter and its status may then be read by I/O access to the 
counter address.

The Read Back command may be used to latch multiple counter outputs at one time. 
This single command is functionally equivalent to several counter latch commands, one 
for each counter latched. Each counter's latched count is held until it is read or 
reprogrammed. Once read, a counter is unlatched. The other counters remain latched 
until they are read. If multiple count Read Back commands are issued to the same 
counter without reading the count, all but the first are ignored.

The Read Back command may additionally be used to latch status information of 
selected counters. The status of a counter is accessed by a read from that counter's I/
O port address. If multiple counter status latch operations are performed without 
reading the status, all but the first are ignored.

Both count and status of the selected counters may be latched simultaneously. This is 
functionally the same as issuing two consecutive, separate Read Back commands. If 
multiple count and/or status Read Back commands are issued to the same counters 
without any intervening reads, all but the first are ignored.

If both count and status of a counter are latched, the first read operation from that 
counter returns the latched status, regardless of which was latched first. The next one 
or two reads, depending on whether the counter is programmed for one or two type 
counts, returns the latched count. Subsequent reads return unlatched count.