Operating Modes

Operating Modes

1. Input Modes

The 8279 provides 3 basic input modes :

• Scanned keyboard

• Scanned sensor matrix

• Strobed input

1. Scanned keyboard

In this mode, keyboard can be scanned in two ways : Encoded scan and Decoded scan.

Encoded scan : In the encoded scan, scan lines (SL₂ - SL₀) are decoded externally to provide 8 scan lines. We know that 8279 provides 8 returns lines. Therefore, the maximum size of keyboard matrix is 8 × 8 = 64. When the key is pressed, 8279 stores the encoded status of scan lines and return lines along with the status of SHIFT and CNTL/STB keys into the FIFO RAM, as shown in the Fig. 12.6.1.

CNTL is the MSB of the character and SHIFT is the next most significant bit. The next three bits are from the scan counter. The last three bits indicate to which return line the key is connected. With this 8-bit key code 8279 can recognize 256 (2⁸) different characters.

Example 12.6.1 Find the key code for condition given below : CNTL/STB SHIFT keys are open.

The pressed key is connected to scan line 2 and return line 4.

Solution :

CNTL = 1

SHIFT = 1

Scan Code = 0 1 0 (scan line 2)

Return code = 1 0 0 (return line 4)

ஃ Key code  = D4H

Decoded scan : In the decoded mode, the internal decoder decodes the least significant 2 bits of scan lines internally to provide four possible combinations on the scan lines (SC₃ – SC₀) : 1110, 1101, 1011 and 0111. Therefore maximum size of keyboard matrix is 8 × 4 = 32. In this mode, keycode is generated in similar way as in the encoded mode, only bit 5 of keycode is always 0. Therefore, 8279 can recognize only 128 (2⁷) characters.

The scanned keyboard mode allows key depressions in 2-key lockout or N-key rollover mode with key debounce.

2-key lockout : In this mode, simultaneous key depression is not allowed. When any key is depressed, the debounce logic is set and 8279 checks for any other key depress during next two scans. Now we will see how this mode reacts with three possible conditions that can occur during debounce scanning.

Condition 1 : If other key depress not found during next two scans, it is a single key depression and the key code is entered into FIFO RAM along with the status of CNTL and SHIFT lines. If the FIFO was empty, IRQ will be set to signal the CPU that there is an entry in the FIFO RAM. If the FIFO RAM was full, the key will not be entered and the error flag will be set.

Condition 2 : If another key depress is encountered, no entry to the FIFO can occur. If all other keys are released before first one, then it will be entered to the FIFO. If first key is released before any other, it will be entirely ignored.

Condition 3 : If two keys are depressed within the debounce cycle, it is a simultaneous depression. Neither key will be recognized until one key remains depressed alone. The last key will be treated as a single key depression.

N-Key Rollover : In N-Key rollover, each key depression is treated independently from all others. When a key is depressed, the debounce logic is set and 8279 checks for key depress during next two scans. Now we will see how this mode reacts with three possible conditions that can occur during debounce scanning.

Condition 1         : If key is still pressed then key is entered into the FIFO.

Condition 2         : If other keys are pressed, they are recognized and entered into the FIFO.

Condition 3         : If a simultaneous depression occurs, the keys are recognized and entered according to the keyboard scan found them.

2. Scanned sensor matrix : In the sensor matrix mode, image of the sensor matrix is kept in the sensor RAM. The status of the sensor switches are input directly to the sensor RAM. 8279 scans rows one by one and stores the status of each row in the corresponding location in the sensor RAM. For example, when 8279 scans first row of sensor matrix it stores the status of first row in the location 0 of the sensor RAM. At the end of sensor matrix scan if any sensor value change is detected then 8279 sets 'S' bit in the status register and activates the IRQ signal. In the autoincrement mode, the IRQ line is cleared by issuing End of Interrupt command, otherwise it is cleared by the first data read operation. When multiple changes in the sensor matrix occurs, multiple interrupts are generated. In sensor matrix mode, the debounce logic is inhibited. Although it is inhibited, sensor matrix mode has the advantage that CPU knows how long the sensor was closed and when it was released. The scanned keyboard mode can only indicate validated key closure. In encoded mode, size of sensor matrix is 8 × 8 whereas in decoded mode size of sensor matrix is 8 × 4. In both the modes CNTL and SHIFT lines are ignored.

3. Strobed input mode

In the strobed input mode, data is entered to the FIFO RAM from the returned lines. The data is entered at the rising edge of the CNTL/STB signal.

2. Display Modes

The 8279 provides 2 basic output modes

• Left Entry (type writer type)

• Right Entry (calculator type)

a. • Left Entry (type writer type)

In the left entry mode, 8279 displays characters from left to right in the multiplexed displays like a typewriter. In this, each display position directly corresponds to a byte (or nibble) in the display RAM. Address 0 in the RAM is the left-most display character and address 15 (or address 7 in 8 character display) is the right most display.

The characters can be displayed on the specific digit by loading character code in the corresponding location in the display RAM.

Autoincrement in Left Entry

In left entry mode, if autoincrement flag is set to 1 after each write operation display RAM address is incremented by one so that it will point the next location in the display RAM. This autoincrement facility allows user to load display RAM in a sequential manner, and it is not necessary to specify display RAM address for each write operation.

b. Right Entry

In the right entry mode, 8279 displays characters from right to left in the multiplexed display like a calculator. The first entry is displayed on the right most display. The next entry is also displayed on the right most display after the display is shifted left one character.

When new character is entered, it shifts all previous characters left by one position and displays new entry on the right most display.

Autoincrement in Right Entry

In the right entry mode, autoincrementing and non autoincrementing have the same effect as in the left entry except if the address sequence is changed.

Review Questions

1. What do you mean by encoded scan and decoded scan ?

2. Explain the input modes provided by 8279.

3. Explain the terms 2-key lockout and N-key rollover.

4. Why  maximum size of keyboard matrix is 8 × 8 = 64, when interfaced with 8279 ?

5. Explain the display modes provided by 8279.

6. Explain the working of 8279 as a keyboard/display controller and explain its command registers

AU : Dec -16, Marks 16