Quantcast Keyboard Scan 5865-01-076-9050">

 

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Figure 5-5. Keyboard Matrix and Scan Sequence
Up
TM-11-5865-215-13 Receiver Control C-10026/USQ NSN
Next
Figure 5-6. Keyboard Circuit Simplified Schematic Diagram
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
2
1
0
1
1
2
1
3
1
0
1
3
4
1
1
0
1
4
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
1
1
1
1
0
SQL
1
1.
()
1
TEST TONE
3
x
x
x
x
Not used
x
x
x
x
4
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

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