DEC VT220 - Page 134

The 8051 scans the 18 drive lines. Key closures are detected by
reading the eight data lines. The complete matrix is scanned every
8.33 ms.
When a key closure is detected, it is scanned again to verify that
it is really a key closure and not just electrical noise.
Once the key closure is verifi ,
position information into a key
Transmission is handled by the
Transmitter (UART) in 8@51.
8@51 firmware translates the
e and transmits it to the CPU.
Universal Asynchronous Receiver
A ghost key indication can occur ee of the four corners of
a matrix rectangle are closed (Figure 7-6), causing what is known
as a sneak path. The key positions in the matrix are arranged to
avoid sneak paths. However, if a sneak path does occur, the
firmware prevents keycode fo key that caused the sneak
path to be transmitted until one of the involved keys is released.
This prevents transmission of t entirely.
Table 7-1 shows the keyboard matrix on the LK201-AA (American)
keyboard. Keycap designations are lis for reference only; you
can compare them to Figure 7-7 (A and B).
DRiVE
LINES
DATA
LINES
1 CONDITIONS
ARE SWITCHES 82, 83, AND C3 CLOSED,
SWITCH
C2 OPEN, L,INE 215 BEING DRIVEN AND LINE C
IS BEING READ
INTERSECTION C2 BEING LOOKED AT.
IT SHOULD NOT
KEY CLOSURE BECAUSE SWITCH C2 IS OPEN
3 HOWEVER A SNEAK PATH is PRESENT FROM LINE 2
THROUGH SWiTCHES 82, 83,
AND C3 TO liNE
A GHOST KEY is READ AT INTERSECTiON C2,
Figure 7-6 Example of Ghost Key Generation
7-8