CS5535 datasheet(102/555 Pages) NSC

102 / 555 page

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102

Revision 0.8

PIT Functional Description (Continued)

The read-back command present in the 8254 PIT is used

to determine the current counter value and its status like

counting format, the counting mode, the low-order or high-

order byte or both being read or written, and the status of

its output. Figure 4-16 shows the format of the read-back

command. The two most significant bits define the read-

back command with their value 11b. CT and ST indicate

that the value and the status of the counter are to be deter-

mined respectively. The C0-C2 bits define the counter

whose value or status is to be determined. With the read-

back command, several counter latch commands can be

issued in parallel by indicating several counters simulta-

neously with the C0-C2 bits. The 8254 then behaves as if

several counter latch commands have been issued individ-

ually, and transfers the individual count values into the out-

put latches of each counter. All successive counter latch

commands, whether issued by its own counter or a next

read-back command, are ignored if the counter concerned

has not been read by counter read instructions. To deter-

mine the programmed mode of a particular counter, set CT

= 1 and ST = 0.

The read-back command latches the current mode and

provides a status byte (see Figure 4-17) at the port address

of the counter concerned. This status byte is fetched by a

counter read instruction. The PIN bit indicates the current

status of the concerned counter’s output pin. If PIN = 1,

then the counter output is at logic 1, else at logic 0. Bit 0

shows whether the last written counter value has already

been transferred to the counter. Not before zero = 0 is it

meaningful to read back the counter value.

Figure 4-16. PIT Read-Back Command Format

Figure 4-17. PIT Status Byte Format

CT: Determine count value of selected counter.

0 = Determine count value.

1 = Do not determine count value.

ST: Determine status of selected counter.

0 = Determine count status.

1 = Do not determine count status.

C2, C1, C0: Counter selection.

0 = Counter not selected.

1 = Counter selected.

7654321

PIN: Status of counter output pin:

0 = Output pin low.

1 = Output pin high.

LOAD: Is counter loaded with a start value?

0 = Counter loaded, count value can be read.

1 = Counter not yet loaded, count value cannot be read.

LH: Corresponds to bits [5:4] of the Control Word register.

MODE: Corresponds to bits [3:1] of the Control Word register.

BCD: Corresponds to bit 0 of the Control Word register.

76543210

MODE

BCD

LOAD

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