Mumbai University > Computer Engineering > Sem 5 > Microprocessor

Marks: 5M

Year: Dec 2014

1. The control word format of the 8255 is shown in Fig. below. The contents of the control register are called the control word that specifies the input/ output functions of each port.
2. The control word register can be accessed to write the control word by selecting AD_1 and AD_0 in high condition i.e. both ON. This register is not available for read operation.
3. As indicated in Fig, the most significant bit (D_7) of the control word specifies either I/O function or the BIT set/ reset function.
4. With D_7= 1, the 8255 works in the I/O modes. Port-C operates in BIT SET/RESET (BSR) mode with D_7=0. The BSR Mode in no way affects the functioning of port-A and port-B.
5. Following steps are essential to communicate with the peripherals through the ports of the 8255
6. Determine address of ports-A, B, or C and of the control register according to the chip select logic and the address lines AD_1 and AD_0
7. Write the control word in the control word register.
8. Write input/output instructions in order to communicate with the peripherals through ports-A, B, and C of the 8255.