Z-World eDisplay OP7200, User Manual

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User’s Manual 35
3.8 Keypad
The OP7200 is equipped with a nine-position keypad. The keypad is attached to the front
bezel with an adhesive backing and is connected through J16 to the OP7200 printed-
circuit board with a flex cable. Only 7 of the 10 conductors in the cable are used at the
present time. The extra lines are reserved for an expanded keypad or LED indicators. The
interface to the keypad is through programmed I/O bits composed of four scan rows of
three keys each. Driving a particular scan line (/KB–S0:3) low will read back a zero on the
keypad data lines (/PB0:2–K0:2) associated with the three keys on the selected row.
Diodes D16–D19 prevent feedback, allowing the software to read the keypad even when
multiple keys are pressed simultaneously. Resistors R138–R141 and capacitors C79–C81
and C83 form a low-pass filter to protect against ESD damage. These same circuits help to
eliminate EMI from being radiated from the keypad or its flex cable. R149, R151–R152,
and C82–C84 perform a similar function for the keypad data lines. The tri-state receiver
chip U13 connects the key data to the microprocessor data bus at the appropriate time
when directed by the control signals /IORD and /PE5-IO-CS1. Note that only the low-
order three bits of the data bus are connected. The software must mask off the high 5 bits
since they are undefined.
Figure 18 shows how the keypad is encoded with respect the scan and data lines. For
example, if /KB–S2 is asserted low, then keys D, H, and E are read back on data lines K0,
K1, and K2 respectively. A zero read on the data lines indicates that the key is pressed and
a one indicates that it is not. Ones are always read back on data lines that are not assigned
to any particular key. Once the values read from the keypad remain constant for a length of
time, the read can be assumed to be valid.
Figure 18. OP7200 Keypad Encoding
A B C D E
F G H
J
/PB0–K0
B
C
D
A
/PB1–K1
F
G
H
/PB2–K2
J
E
/KB–S0
/KB–S1
/KB–S2
/KB–S3