
B-2
XT-Keyboard Interface
B.1
XT KEYBOARD ENABLE
The élanSC310 microcontroller’s XT keyboard circuitry is enabled by bit 3 of Index 0ADh.
Setting this bit
I
Allows the 8042CS and SYSCLK pins to become three-stated so they may be used as
inputs (all other requirements being met).
I
Qualifies an internal decode so that port 060h is read as an internal port.
I
Switches a multiplexer to vector IRQ1 from the external pin to the output of this cir-
cuitry.
KEYBOARD INTERFACE CONTROL
Two bits are provided for control of the XT keyboard interface. These bits are located at
port 061h. Bit 7 is used to clear the keyboard interrupt and shift register, a flip-flop that
enables the data line as an output (to act as a busy signal to the keyboard when Low).
Two writes are required for the proper operation of this bit—one to set, and another to
clear it. If bit 7 is not cleared, the shift register is held in a
High, makes the KBCLK line an output (driving it Low), which can also be used as a busy
signal to the keyboard.
B.2
clear
configuration. Bit 6, when
B.3
KEYBOARD DATA PORT
Once a serial keyboard byte has been assembled, it can be read at port 060h.
B.4
I/O MAP SUMMARY
Setting bit 3 of Index ADh enables the XT keyboard.
I
I
Port 060h contains the keyboard data.
I
Setting bit 7 of Port 061h clears the keyboard shift register and interrupt.
I
Setting bit 6 of Port 061h forces the keyboard clock Low.
PINS USED
The XT keyboard option uses the following pins:
B.5
I
SYSCLK
Becomes the keyboard clock (XTCLK).
I
8042CS
Becomes the keyboard data (XTDAT).
Because both pins can also act as outputs and the data line is driven as an output at the
end of each transferred byte, the pins must be driven by open-drain or open-collector
drivers with external pull-ups.