Intel N2820 FH8065301616603 数据表
产品代码
FH8065301616603
Datasheet
1241
PCU – iLB – 8254 Timers
The Control Word Register at port 43h controls the operation of all three counters.
Several commands are available:
Several commands are available:
•
Control Word Command. Specifies which counter to read or write, the operating
mode, and the count format (binary or BCD).
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.
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.
state of the OUT pins, and the state of the Null Count Flag of the selected counter.
lists the six operating modes for the interval counters.
27.3.2
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.
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.
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.
27.3.2.1
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).
Port 40h (Counter 0), 41h (Counter 1), or 42h (Counter 2).
Note:
Performing a direct read from the counter does not return a determinate value,
because the counting process is asynchronous to read operations. However, in the case
of Counter 2, the count can be stopped by writing 0b to the NSC.TC2E register bit.
because the counting process is asynchronous to read operations. However, in the case
of Counter 2, the count can be stopped by writing 0b to the NSC.TC2E register bit.
Table 187. 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 re-triggerable 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, etc.
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.