BSR Mode of Programmable Peripheral Interface (8255A)
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These are two basic modes of operation of 8255.
- I/O mode
- Bit Set-Reset mode (BSR).
In I/O mode, the 8255 ports work as programmable I/O ports, while in BSR mode only port C (PC0-PC7) can be used to set or reset its individual port bits.
Under the I/O mode of operation, further, there are three modes of operation of 8255, so as to support different types of applications, mode 0, mode 1 and mode 2.
8255A: BSR(Bit Set/Reset) Mode
- In this mode, any of the 8-bits of port C can set or reset depending on D0 of the control word.
- The bit to set or reset selected by bit select flags D3, D2and D1 of the CWR ( Control Word Register).
- BSR Control Word affects one bit at a time.
- It does not affect the I/O mode
8255A Control Word and Control Register in BSR Mode
The content Control register is known as Control Word.
Control word specifies an I/O function for each port this register can be.
Moreover, Accessed to write a control word when A0 and A1 are at logic1, the register not accessible for a read operation.
Bit D7 of the control register either specifies the I/O function or the bitSet/Reset function, as classified in figure.
Also, If bit D7=0, bits D6-D0 determine I/O function in various mode, as shown in the figure.
If bit D7=0 port C operates in the bitSet/Reset (BSR) mode.
The BSR control word does not affect the function of port A and B.