Intel 8255 - Bit Set/reset (BSR) Mode

Bit Set/reset (BSR) Mode

The Bit Set/Reset (BSR) mode is applicable to port C only. Each line of port C (PC0 - PC7) can be set/reset by suitably loading the control word register. BSR mode and I/O mode are independent and selection of BSR mode does not affect the operation of other ports in I/O mode.

  • D7 bit is always 0 for BSR mode.
  • Bits D6, D5 and D4 are don't care bits.
  • Bits D3, D2 and D1 are used to select the pin of Port C.
  • Bit D0 is used to set/reset the selected pin of Port C.

Selection of port C pin is determined as follows:

B3 B2 B1 Bit/pin of port C selected
0 0 0 PC0
0 0 1 PC1
0 1 0 PC2
0 1 1 PC3
1 0 0 PC4
1 0 1 PC5
1 1 0 PC6
1 1 1 PC7

As an example, if it is needed that PC5 be set, then in the control word,

  1. Since it is BSR mode, D7 = '0'.
  2. Since D4, D5, D6 are not used, assume them to be '0'.
  3. PC5 has to be selected, hence, D3 = '1', D2 = '0', D1 = '1'.
  4. PC5 has to be set, hence, D0 = '1'.

Thus, as per the above values, 0B (Hex) will be loaded into the Control Word Register (CWR).

D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 0 1 0 1 1

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