Intel C2338 FH8065501516761 データシート
製品コード
FH8065501516761
Volume 2—8254 Programmable Interval Timer (PIT)—C2000 Product Family
Reading from the Interval Timer
Intel
®
Atom™ Processor C2000 Product Family for Microserver
Datasheet, Vol. 2 of 3
September 2014
534
Order Number: 330061-002US
27.5
Reading from the Interval Timer
It is often desirable to read the value of a counter without disturbing the count in
progress. The three methods for reading the counters are as follows: a simple read
operation, the Counter Latch command, and the Read-back command. They are
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
2-byte counts, 2 bytes must be read. The 2 bytes do not have to be read one right after
the other. Read, write, or programming operations for other counters are inserted
between them.
27.5.1
Simple Read
The first method is to perform a simple read operation. The counter is selected through
port 40h (Counter 0), 41h (Counter 1), or 42h (Counter 2).
Note:
27.5.2
Counter Latch Command
The Counter Latch command, written to port 43h, latches the count of a specific
counter at the time the command is received. This command ensures that the count
read from the counter is accurate, particularly when reading a 2-byte count. The count
value is then read from each counter Count register that 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 and allows reading the contents of the counters quickly without
affecting the counting in progress. Multiple counter latch commands are used to latch
more than one counter. Counter latch commands do not affect the programmed mode
of the counter in any way.
When a counter is latched and later latched again before the count is read, the second
Counter Latch command is ignored. The first Counter Latch command issued is the first
count read.