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,
- Since it is BSR mode, D7 = '0'.
- Since D4, D5, D6 are not used, assume them to be '0'.
- PC5 has to be selected, hence, D3 = '1', D2 = '0', D1 = '1'.
- 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 |
Read more about this topic: Intel 8255
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