pc87317vul National Semiconductor Corporation, pc87317vul Datasheet - Page 51

• Manufacturer Part Number: pc87317vul
• Description: Pc87317vul/pc97317vul Superi/o Plug And Play Compatible With Acpi Compliant Controller/extender
• Manufacturer: National Semiconductor Corporation

P12, P16 and P17 are driven by quasi-bidirectional drivers.
(See FIGURE 3-6 "Quasi-Bidirectional Driver".) These sig-
nals are called quasi-bidirectional because the output buffer
cannot be turned off (even when the I/O signal is used for
input).
During output, a 1 written to output is strongly pulled up for
the duration of a (short) write pulse, and thereafter main-
tained by a high impedance “weak” active pull-up (imple-
mented by a degenerated transistor employed as a
switchable pull-up resistor). A series resistor to those port
3.5 INTERNAL KBC - PC87317VUL INTERFACE
The KBC interfaces internally with the PC87317VUL via
three registers: an input (DBBIN), output (DBBOUT) and
status (STATUS) register. See FIGURE 3-1 "KBC System
Functional Block Diagram" on page 47 and TABLE 3-1
"System Interface Operations".
TABLE 3-1 "System Interface Operations" illustrates the
use of address line A2 to differentiate between data and
commands. The device is selected by chip identification of
default address 60h (when A2 is 0) or 64h (when A2 is 1).
After reset, these addresses can be changed by software.
PC87317VUL
Write
MR
Port
Internal Bus
Keyboard (and Mouse) Controller (KBC) (Logical Devices 0 and 1)
Port Pin
Port Pin
FIGURE 3-6. Quasi-Bidirectional Driver
FIGURE 3-7. Current Limiting Resistor
D
100 500
100 500
ORL, ANL
PORT
F/F
–
–
R
P
R
IN
Q
Q
51
lines used for input is recommended to limit the surge cur-
rent during the strong pull-up. See FIGURE 3-7 "Current
Limiting Resistor".
If a 1 is asserted, an externally applied signal may pull down
the output. Therefore, input from this quasi-bidirectional cir-
cuit can be correctly read if preceded by a 1 written to out-
put.
P20 and P21 are driven by open-drain drivers.
When the KBC is reset, all port data bits are initialized to 1.
RD WR
0
1
0
1
1
0
1
0
TABLE 3-1. System Interface Operations
Addresses
R: current limiting resistor
A small-value series current limiting
resistor is recommended when
port pins are used for input.
Default
60h
60h
64h
64h
Read DBBOUT
Write DBBIN, F1 Clear (Data)
Read STATUS
Write DBBIN, F1 Set (Command)
Q1
Q2
+VCC
Q3
Operation
PAD
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