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Control port of 8255

In 8255, there are two types of command words or control word are available. And they are −

  • Mode definition control word and

  • Port C bit set/reset control word.

Both these are written to the control port only. From the point of view of the microprocessor there is a single 8-bit control port, which is selected when CS* = 0, WR* = 0, A1 = 1 and A0 = 1. But if we consider the architecture of 8255, internally there are two control ports, one for mode definition control and another for Port C bit set/reset control. The contents of the control port get latched in mode definition control port if the MS bit of control port = 1. If the MS bit of control port = 0, the contents of the control port gets latched in Port C bit set/reset control port. This can be seen from the following fig.

Now let us discuss about the Mode Definition Control Word. The following fig depicts the same −

The following table depicts the meaning and purposes for all these bits −

Bit 0 (PClI)1 = Port C lower (PC3-0) as input
0 = Port C lower (PC3-0) as output
Bit 1 (PBI)1 Port B as input
0 = Port B as output
Bit 2 (MB)1 = Port B in mode 1
0 = Port B in mode 0
Bit 3 (PCuI)1 = Port C upper (PC7-4) as input
0 = Port C upper (PC7-4) as output
Bit 4 (PAI)1 = Port A as input
0 = Port A as output
Bits 6, 50 0 = Port A in mode 0
(MA2,MA1)0 1 = Port A in mode 1
1 0 = Port A in mode 2
1 1 = Port A in mode 2
Bit 7Must be 1 to indicate that it is mode definition control.

Now let us discuss about the Port C Bit Set/Reset C4 Word. The following fig depicts the same −

The following table depicts the meaning and purposes for all these bits −

Bit 0 (S/R*)1 = Set Port C bit selected by bits 3, 2, and 1
0 = Reset Port C bit selected by bits 3, 2, and 1
Bits 3, 2, 1 (SB2, 1, 0)000 = Select bit 0 of Port C
001 = Select bit 1 of Port C
010 = Select bit 2 of Port C
011 = Select bit 3 of Port C
100 = Select bit 4 of Port C
101 = Select bit 5 of Port C
110 = Select bit 6 of Port C
111 = Select bit 7 of Port C
Bits 6, 5, 4Are don't cares. Generally loaded with 000
Bit 70 to indicate it is Port C bit set/reset control
Updated on 30-Jul-2019 22:30:25

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