Keyboard Scan

TM 11-5865-215-13

a l-of-4 code. Matrix row 1 was selected with code 1 at output port 0

as previously shown. Rows 2, 3, and 4 are selected with the following

codes:

l-of-4

Output port 0 line

Selected

code

OB00

OB01

OB02

OB03

row

The columns are read via input port 4 (IB40 thru IB43) and corres-

ponding I/O bus lines (I/O 0 thru I/O 3):

1-of-4

Input port 4 line

code

IB43

IB42

IB41

IB40

Key pressed

SQL

()

TEST TONE

Not used

As shown in figure 5-6, when the SQL key is pressed, code 1 appears

at input port 4. The microprocessor reads this code, interprets it

into a matrix character, and stores this character as a receiver com-

mand bit in the appropriate file of RAM memory. This, and other re-

ceiver command bits are later sorted and transferred to the receiver,

the SDU display, and the computer. The keyboard scan pass firmware

flowchart is shown in figure 5-7.

If no key of a scanned group (i.e., input port 4) is pressed, the

microprocessor advances to the next group by reading input port 5,

then input port 6. If no activated key is encountered, the micro-

processor generates code 2 via output port 0 to select the next row

(2) of the keyboard. Then input ports 4, 5, and 6 are again read

sequentially. The microprocessor continues this scan sequence until

the last group of keys (matrix positions 49 thru 4B) has been read.

Then the keyboard scan-pass flag is cleared, and the first group of

keys (matrix positions 11 thru 14) are again scanned. A complete

keyboard scan sequence takes less than 1 ms.

If an operator held one key down for an extended length of time,

the keyboard scan-pass flag would remain set. This causes the firm-

ware programs to loop repetitively from the KEYBOARD SCAN-PASS FLAG

CLEARED decision to the START KEYBOARD SCAN position and back (see

figure 5-7). If this loop was not activated, the microprocessor RAM

memory would overflow with data from the same pressed key. The key-

board scan-pass flag prevents this memory overflow by setting when

this activated key is first encountered. Then, if the key remains

down, the set keyboard scan-pass flag causes the firmware to loop

back to scan start, ignoring this key.

5-18