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LPC Interface Bridge Registers (D31:F0)
8-30
82801AA and 82801AB Datasheet
8.3.1.2
LTCH_CMD—Counter Latch Command
The Counter Latch Command latches the current count value. This command is used to insure that
the count read from the counter is accurate. The count value is then read from each counter's count
register through the Counter Ports Access Ports Register (40h for counter 0, 41h for counter 1, and
42h for counter 2). The count must be read according to the programmed format, i.e. 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 the reads). If a counter is latched once and then latched again before the count is read, the
second Counter Latch Command is ignored.
8.3.2
SBYTE_FMT—Interval Timer Status Byte Format Register
I/O Address:
Counter 0 = 40h,
Counter 1 = 41h,
Counter 2 = 42h
Bits[6:0] undefined, Bit 7=0
Attribute:
Size:
RO
8 bits per counter
Default Value:
Each counter's status byte can be read following a Read Back Command. If latch status is chosen
(bit 4=0, Read Back Command) as a read back option for a given counter, the next read from the
counter's Counter Access Ports Register (40h for counter 0, 41h for counter 1, and 42h for counter
2) returns the status byte. The status byte returns the following:
Bit
Description
7:6
Counter Selection.
These bits select the counter for latching. If “11” is written, then the write is
interpreted as a read back command.
00 = Counter 0
01 = Counter 1
10 = Counter 2
5:4
Counter Latch Command.
00 = Selects the Counter Latch Command.
3:0
Reserved. Must be 0.
Bit
Description
7
Counter OUT Pin State.
1 =OUT pin of the counter is also a 1.
0 =OUT pin of the counter is also a 0.
6
Count Register Status.
This bit indicates when the last count written to the Count Register (CR) has
been loaded into the counting element (CE). The exact time this happens depends on the counter
mode, but until the count is loaded into the counting element (CE), the count value will be incorrect.
0 = Count has been transferred from CR to CE and is available for reading.
1 = Null Count. Count has not been transferred from CR to CE and is not yet available for reading.
5:4
Read/Write Selection Status.
These reflect the read/write selection made through bits[5:4] of the
control register. The binary codes returned during the status read match the codes used to program
the counter read/write selection.
00 = Counter Latch Command
01 = Read/Write Least Significant Byte (LSB)
10 = Read/Write Most Significant Byte (MSB)
11 = Read/Write LSB then MSB