Datasheet

Functional Description
Intel® Xeon® Processor D-1500 Product Family 75
Datasheet - Volume 1 of 4: Integrated Platform Controller Hub
March 2015
Only two conventions need to be observed when programming the counters. First, for
each counter, the control word must be written before the initial count is written.
Second, the initial count must follow the count format specified in the control word
(least significant byte only, most significant byte only, or least significant byte and then
most significant byte).
A new initial count may be written to a counter at any time without affecting the
counter's programmed mode. Counting is affected as described in the mode definitions.
The new count must follow the programmed count format.
If a counter is programmed to read/write two-byte counts, the following precaution
applies: A program must not transfer control between writing the first and second byte
to another routine which also writes into that same counter. Otherwise, the counter will
be loaded with an incorrect count.
The Control Word Register at port 43h controls the operation of all three counters.
Several commands are available:
• Control Word Command. Specifies which counter to read or write, the operating
mode, and the count format (binary or BCD).
• Counter Latch Command. Latches the current count so that it can be read by the
system. The countdown process continues.
• Read Back Command. Reads the count value, programmed mode, the current
state of the OUT pins, and the state of the Null Count Flag of the selected counter.
Ta b l e 3- 1 2 lists the six operating modes for the interval counters.
3.7.2 Reading from the Interval Timer
It is often desirable to read the value of a counter without disturbing the count in
progress. There are three methods for reading the counters: a simple read operation,
counter Latch command, and the Read-Back command. Each is explained below.
With the simple read and counter latch command methods, 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.
Table 3-12. Counter Operating Modes
Mode Function Description
0 Out signal on end of count (=0) Output is 0. When count goes to 0, output goes to 1 and
stays at 1 until counter is reprogrammed.
1 Hardware retriggerable one-shot Output is 0. When count goes to 0, output goes to 1 for
one clock time.
2 Rate generator (divide by n counter) Output is 1. Output goes to 0 for one clock time, then
back to 1 and counter is reloaded.
3 Square wave output Output is 1. Output goes to 0 when counter rolls over, and
counter is reloaded. Output goes to 1 when counter rolls
over, and counter is reloaded, and so on.
4 Software triggered strobe Output is 1. Output goes to 0 when count expires for one
clock time.
5 Hardware triggered strobe Output is 1. Output goes to 0 when count expires for one
clock time.