Operation Modes

Operation Modes

AU : May-04, 07, 08, 17, 18, Dec.-04, 09, 11, 13, 17, 19

1. Bit Set-Reset (BSR) Mode

The individual bits of Port C can be set or reset by sending out a single OUT instruction to the control register. When Port C is used for control/status operation, this feature can be used to set or reset individual bits.

2. I/O Modes

Mode 0 : Simple input/output

In this mode, Ports A and B are used as two simple 8-bit I/O ports and Port C as two 4-bit ports. Each port (or half - port, in case of C) can be programmed to function as simply an input port or an output port. The input/output features in Mode 0 are as follows :

1. Outputs are latched. 2. Inputs are buffered, not latched.

3. Ports do not have handshake or interrupt capability.

Mode 1 : Input/Output with handshake

In this mode, input or output data transfer is controlled by handshaking signals. Handshaking signals are used to transfer data between devices whose data transfer speeds are not same. For example, computer can send data to the printer with large speed but printer can't accept data and print data with this rate. So computer has to send data with the speed with which printer can accept. This type of data transfer is achieved by using handshaking signals alongwith data signals. Fig. 8.4.1 shows data transfer between computer and printer using handshaking signals.

These handshaking signals are used to tell computer whether printer is ready to accept the data or not. If printer is ready to accept the data then after sending data on data bus, computer uses another handshaking signal  to tell printer that valid data is available on the data bus.

The 8255 mode 1 which supports handshaking has following features.

1. Two ports (A and B) function as 8-bit I/O ports. They can be configured either as input or output ports.

2. Each port uses three lines from Port C as handshake signals. The remaining two lines of Port C can be used for simple 1/O functions.

3. Input and output data are latched.

4. Interrupt logic is supported.

Mode 2 : Bi-directional I/O data transfer

This mode allows bi-directional data transfer (transmission and reception) over a single 8-bit data bus using handshaking signals. This feature is available only in Group A with Port A as the 8-bit bidirectional data bus; and PC₃ - PC₇ are used for handshaking purpose. In this mode, both inputs and outputs are latched. Due to use of a single 8-bit data bus for bi-directional data transfer, the data sent out by the CPU through Port A appears on the bus connecting it to the peripheral, only when the peripheral requests it. The remaining lines of Port C i.e. PC₀ - PC₂ can be used for simple I/O functions. The Port B can be programmed in mode 0 or in mode 1. When Port B is programmed in mode 1, PC₀ - PC₂ lines of Port C are used as handshaking signals.

Review Questions

1. Explain in detail about various operating modes of 8255 PPI.

AU : May-07, Marks 16, Dec.-ll,13,17,19, May-18, Marks 8

2. Explain the mode 0 of 8255 in detail.

AU : Dec.-04, Marks 16

3. Explain the functioning of 8255 programmable peripheral interface and its modes.

AU : May-17, Marks 16