Control Word Formats

Control Word Formats

AU : May-04, 18, Dec.-05,07, 08, 16, 19

A high on the RESET pin causes all 24 lines of the three 8-bit ports to be in the input mode. All flip-flops are cleared and the interrupts are reset. This condition is maintained even after the RESET goes low. The ports of the 8255 can then be programmed for any other mode by writing a single control word into the control register, when required.

 

1. For Bit Set/Reset Mode

Fig. 8.5.1 shows bit set/reset control word format.

The eight possible combinations of the states of bits D₃ - D₁ (B₂ B₁ B₀ )in the Bit Set-Reset format (BSR) determine particular bit in PC₀ – PC₇ being set or reset as per the status of bit D₀. A BSR word is to be written for each bit that is to be set or reset. For example, if bit PC₃ is to be set and bit PC₄ is to be reset, the appropriate BSR words that will have to be loaded into the control register will be, 0×××0111 and 0×××100 0, respectively, where × is don't care.

The BSR word can also be used for enabling or disabling interrupt signals generated by Port C when the 8255 is programmed for Mode 1 or 2 operation. This is done by setting or resetting the associated bits of the interrupts. This is described in detail in next section.

 

2. For I/O Mode

The mode definition format for I/O mode is shown in Fig. 8.5.2. The control words for both, mode definition and Bit Set-Reset are loaded into the same control register, with bit D₇ used for specifying whether the word loaded into the control register is a mode definition word or Bit Set-Reset word. If D is high, the word is taken as a mode definition word, and if it is low, it is taken as a Bit Set-Reset word. The appropriate bits are set or reset depending on the type of operation desired, and loaded into the control register.

 

Example 8.5.1 Write a program to initialize 8255 in the configuration given below :

1. Port A : Simple input

2. Port B : Simple output

3. Port C_L : Output

4. Port C_U : Input

Assume address of the control word register of 8255 as 83H.

 

Example 8.5.2 Write a program to initialize 8255 in the configuration given below :

1. Port A : Output with handshake

2. Port B : Input with handshake

3. Port C_L : Output

4. Port C_U : Input

Assume address of the control word register of 8255 as 23H.

Solution :

Source program :

MVI A, AEH ; Load control word

OUT 23H   ; Send control word

 

Lab Experiment 8.5.1 : Blink port C bit o of 8255.

Statement : Write a program to blink Port C bit 0 of the 8255. Assume address of control word register of 8255 as 83H. Use Bit Set/Reset mode.

Solution : Control word to make bit 0 high.

 Source program :

BACK : MVI A, 01H ; Load bit pattern to make PC0 high

OUT 83H ; Send it to control word register

CALL DELAY ; Call Delay subroutine

MVI A, OOH ; Load bit pattern to make PC0 Low

OUT 83H ; Send it to control word register

CALL Delay ; Call Delay subroutine

JMP BACK ; Repeat

Review Questions

1. Show the control word format of 8255 and explain how each bit is programmed.

AU : May-04, 18, Dec.-19, Marks 4

2. Explain the bit set/reset mode of 8255.

3. Write the format cf control word for 8255 PPI.