Intel mcs-48 manual 8253/8253-5, Lsb, Msb

8253/8253-5

8253 READIWRITE PROCEDURE

Write Operations

MODE Control Word

The systems software must program each counter of the 8253 with the mode and quantity desired. The program- mer must write out to the 8253 a MODE contrel word and the programmed number of count register bytes (1 or 2) prior to actually using the selected counter.

The actual order of the programming is quite flexible. Writing out of the MODE control word can be in any sequence of counter selection, e.g., counter #0 does not have to be first or counter #2 last. Each counter'sMODE control word register has a separate address so that its loading is completely sequence independent. (SCO, SC1)

The loading of the Count Register with the actual count value, however, must be done in exactly the sequence programmed in the MODE control word (RLO, RL1). This loading of the counter'scount register is still sequence independent like the MODE control word loading, but when a selected count register is to be loaded it !!!..!!§! be loaded with the number of bytes programmed in the MODE control word (RLO, RL 1). The one or two bytes to be loaded in the count register do not have to follow the associated MODE control word. They can be programmed at any time following the MODE control word loading as long as the correct number of bytes is loaded in order.

All counters are down counters. Thus, the value loaded into the count register will actually be decremented. Loading all zeroes into a count register will result in the maximum count (2'6 for Binary or 10'forBCD). In MODE 0 the new count will not restart until the load has been completed. It will accept one of two bytes depending on how the MODE control words (RLO, HL 1) are program- med. Then proceed with the restart operation.

Counter·n·

Col,lnt Register byte

LSB

Counter n

Cou nt Register byte

MSB

Counter n

Note: Format shown is a simple example of loading the 8253 and does not imply that it is the only format that can be used.

Figure 6. Programming Format

No.1		MODE Control Word	I	I
		Counter 0
No.2		MODE Control Word	1	1
		Counter 1
No.3		MODE Control Word	1	1
		Counter 2
No.4	LSB	Count Register Byte	0	1
		Counter 1
		Count Register Byte	0	1
No.5	MSB	Counter 1
No.6	LSB	Count Register Byte	1	0
		Counter 2
No. 7	MSB	Count Register Byte	1	0
		Counter 2
No.8	LSB	Count Regist~r Byte	0	0
		Counter 0
No.9	MSB	Count Register Byte	0	0
		Counter 0

Note: The exclusive addresses of each counter'scount register make

the task of programming the 8253 a very simple matter. and

maximum effective use of the device will result if this feature

is fully utilized.

Figure 7. Alternate Programming Formats

9-12 AFN-00745A-G7