82374EB Datasheet, PDF (77/208 Pages) Intel Corporation – SYSTEM COMPONENT (ESC)

82374EB 82374SB
3 3 3 COUNTER LATCH COMMAND REGISTER
Register Location
Default Value
Attribute
Size
043h Timer 1
04Bh Timer 2
xxh
Write Only
8 Bits
The Counter Latch command latches the current count value at the time the command is received This
command is used to insure 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 One two or all three counters
may be latched with one counter latch command
If a Counter is latched once and then some time later latched again before the count is read the second
Counter Latch Command is ignored The count read will be the count at the time the first Counter Latch
Command was issued
The count must be read according to the programmed format Specifically if the Counter is programmed for
two byte counts two bytes must be read The two bytes do not have to be read one right after the other read
write or programming operations for other Counters may be inserted between them
One precaution is worth noting If a Counter is programmed to read write two-byte counts a program must not
transfer control between reading the first and second byte to another routine which also reads from that same
Counter Otherwise an incorrect count will be read Finish reading the latched two-byte count before transfer-
ring control to another routine
Note that bit definitions for a write to this port have changed when the read-back command was selected
when compared to a normal control word write to this same port
Bit
Description
7 6 Counter Selection Bits 6 and 7 are used to select the counter for latching
Bit 7 6
00
01
10
11
Function
Latch counter 0 select
Latch counter 1 select
Latch counter 2 select
Read Back Command select
5 4 Specifies Counter Latch Command When bits 5 4 are both 0 the Counter Latch command is
selected during a write to the control word The normal meanings (mode countdown r w select) of the
bits in the control register at I O address 043h change when the Counter Latch command is selected
Following the Counter Latch command I O reads from the selected counter’s I O addresses produce
the current latched count
3 0 Reserved Must be 0
77