User`s manual

82C55 CONTROL REGISTER

BADR4 + 3

WRITE

Group BGroup A

CLBM1CUAM2M3MS

01234567

The 8255 can be programmed to operate in Input/ Output (mode 0), Strobed Input/ Output (mode 1) or

Bi-Directional Bus (mode 2).

When the PC is powered up or RESET, the 8255 is reset. This places all 24 lines in Input mode and no

further programming is needed to use the 24 lines as TTL inputs.

To program the 82C55 for other modes, assemble the following control code byte into an 8-bit byte.

MS = Mode Set. 1 = mode set active

Output0000

Input1111

INDEPENDENT FUNCTIONCHCLBA

Bi-Directional BusMode 2X1

Strobed Input / OutputMode 110

Input / OutputMode 010

GROUP A FUNCTIONM2M3

M1 = 0 is mode 0 for group B. Input / Output

M1 = 1 is mode 1 for group B. Strobed Input / Output

All four groups can be independently programmed in one of several modes. The most commonly used

mode is mode 0, input / output mode. The codes for programming the 82C55 in mode 0 are shown in

Table 6-4.

Table 6-4. Mode 0 Configuration Codes for 82C55

ININININ1559B1111

OUTINININ1549A0111

INOUTININ153991011

OUTOUTININ152980011

ININOUTIN147931101

OUTINOUTIN146920101

INOUTOUTIN145911001

OUTOUTOUTIN144900001

INININOUT1398B1110

OUTININOUT1388A0110

INOUTINOUT137891010

OUTOUTINOUT136880010

ININOUTOUT131831100

OUTINOUTOUT130820100

INOUTOUTOUT129811000

OUTOUTOUTOUT128800000

CLBCUADECHEXD0D1D3D4

NOTE: D7 is always 1; D6, D5, and D2 are always 0.