Explain control word register of 8255
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Sixth-Semester-BE-Degree-Examination-JuneJuly-2013-Compiler-Design-Question-paper-->View question

Explain control word register of 8255.

Related to microprocessor.


By:Purnima

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The control register (or the control logic, or the command word register) is an 8-bit register used to select the modes of operation and input/output designation of the ports.
Control Word Register (CWR) also tells 8255, the mode in which 8255 is going to be used .

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  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.

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