Operational Description

Operational Description

AU : Dec.-11, 16

The complete functional definition of the 8253/54 is programmed by the system software. Once programmed, the 8253/54 is ready to perform whatever timing tasks it is assigned to accomplish.

Programming the 8253/54 : Each counter of the 8253/54 is individually programmed by writing a control word into the control word register (A₀ A₁ = 11). The Fig. 10.3.1 shows the control word format. Bits SC₁ and SC₀ select the counter, bits RW₁ and RW₀ select the read, write or latch command, bits M₂, M₁ and M₀ select the mode of operation and bit BCD decides whether it is a BCD counter or binary counter.

WRITE Operation :

1. Write a control word into control register.

2. Load the low-order byte of a count in the counter register.

3. Load the high-order byte of count in the counter register.

READ Operation : In some applications, especially in event counters, it is necessary to read the value of the count in process. This can be done by three possible methods :

1. Simple Read : It involves reading a count after inhibiting the counter by controlling the gate input or the clock input of the selected counter, and two I/O read operations are performed by the CPU. The first I/O operation reads the low-order byte, and the second I/O operation reads the high order byte.

2. Counter Latch Command : In the second method, an appropriate control word is written into the control register to latch a count in the output latch, and two I/O read operations are performed by the CPU. The first I/O operation reads the low-order byte, and the second 1/O operation reads the high order byte.

3. Read-Back Command (Available only for 8254) : The third method uses the Read-Back command. This command allows the user to check the count value, programmed mode, and current status of the OUT pin and Null count flag of the selected counter(s). Fig. 10.3.2 shows the format of the control word register for Read-Back command.

D₅ : 0 = Latch count of selected counters)

D₄ : 0 = Latch status of selected counters)

D₃ : 1 = Select counter 2

D₂ : 1 = Select counter 1

D₁ : 1 = Select counter 0

D₀ : Reserved for future expansion : must be 0.

Fig. 10.3.2 Control word register for read-back command

The Read-Back command may be used to latch multiple counter output latches by setting the  bit D₅ = 0 and selecting the desired counter(s). Each counter's latch count in held until it is read (or the counter is reprogrammed). That counter is automatically unlatched when read.

Other Features of Read - Back Command (Available only for 8254) :

The Read-Back command may also be used to latch status information of selected counter(s) by setting STATUS bit D4 = 0. The contents of the counter must be latched before reading. The status of a counter is then accessed by a read from that counter. The Fig. 10.3.3 shows the counter status format.

Bit D₅ – D₀ contains the counter's programmed mode exactly as written in the last mode control word. Bit D₇ contains the current status of the output pin. In 8254, it is not possible to read count from the counter, if the count is not loaded into the Counting Element (CE). The Bit D₆ indicates whether the counting element has count or not. If D₆ = 0, counting element has count otherwise null count.

Interleaved Read and Write : Another feature of the 8254 is that reads and writes of the same counter may be interleaved. For example, if the counter is programmed for the two byte counts, the following sequence is valid.

1. Read least significant byte

2. Write new least significant byte.

3. Read most significant byte.

4. Write new most significant byte.

Review Questions

1. Draw the control word of 8253 timer/counter and explain. AU : Dec.-11, Marks 8

2. Explain the working of 8254 timer with a neat block diagram and its command word format. AU : Dec.-16, Marks 8