Control Word Register:
This register has the programmed information, which will be sent out (by the microprocessor) to the device. It describes how the PIT logically works.
In order to initialize the counters, the microprocessor has to write a control word (CW) in this register. It can be done by setting proper values for the pins of the Read/Write Logic block and then by sending the control word to the Data/Bus Buffer block.
The control word contains 8 bits, labelled as D7..D0 (D7 is the MSB).
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Bit#
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D7 D6
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D5 D4
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D3 D2 D1
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D0
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Name
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SC1 SC0
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RW1 RW0
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M2 M1 M0
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= 0, 16-b binary counter
|
|
Func.
|
Select Counter
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Read/Write
|
Select Mode
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= 1, 4-decade BCD counter
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The following table define how to use the Read/Write bits (RW1, RW0).
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RW 1
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RW 0
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Description
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0
|
0
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Counter Latch Command
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|
0
|
1
|
Read/Write the last significant byte (LSB) only
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|
1
|
0
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Read/Write the most significant byte (MSB)
only
|
|
1
|
1
|
Read/Write LSB first, followed by MSB
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While setting the PIT, the microprocessor initially sends a control message, afterwards a count message to the PIT. The counting procedure will start after the PIT has achieved these messages, and, in some of the cases, if it detects the rising edge from the GATE input signal. On most of PCs, the address for the Control Word Register is respectively 043 hex, and 040h, 041h, 042h for each of the counter.