Keyboard Command Write
This is an 8 bit write only register. When written, the C/D status bit of the status register is set to one and the IBF bit is
set.
Keyboard Status Read
This is an 8 bit read only register. Refer to the description of the Status Register for more information.
CPU-to-Host Communication
The LPC47S45x CPU can write to the Output Data register via register DBB. A write to this register automatically
sets Bit 0 (OBF) in the Status register. See Table 53.
Table 53 − Host Interface Flags
8042 INSTRUCTION
FLAG
OUT DBB
Set OBF, and, if enabled, the KIRQ output signal goes high
Host-to-CPU Communication
The host system can send both commands and data to the Input Data register. The CPU differentiates between
commands and data by reading the value of Bit 3 of the Status register. When bit 3 is "1", the CPU interprets the register
contents as a command. When bit 3 is "0", the CPU interprets the register contents as data. During a host write
operation, bit 3 is set to "1" if SA2 = 1 or reset to "0" if SA2 = 0.
KIRQ
If "EN FLAGS" has been executed and P24 is set to a one: the OBF flag is gated onto KIRQ. The KIRQ signal can be
connected to system interrupt to signify that the LPC47S45x CPU has written to the output data register via "OUT
DBB,A". If P24 is set to a zero, KIRQ is forced low. On power-up, after a valid RST pulse has been delivered to the
device, KIRQ is reset to 0. KIRQ will normally reflects the status of writes "DBB". (KIRQ is normally selected as IRQ1 for
keyboard support.)
If "EN FLAGS” has not been executed: KIRQ can be controlled by writing to P24. Writing a zero to P24 forces KIRQ
low; a high forces KIRQ high.
MIRQ
If "EN FLAGS" has been executed and P25 is set to a one:; IBF is inverted and gated onto MIRQ. The MIRQ signal can
be connected to system interrupt to signify that the LPC47S45x CPU has read the DBB register. If "EN FLAGS” has not
been executed, MIRQ is controlled by P25, Writing a zero to P25 forces MIRQ low, a high forces MIRQ high. (MIRQ is
normally selected as IRQ12 for mouse support).
Gate A20
A general purpose P21 is used as a software controlled Gate A20 or user defined output.
8042 PINS
The 8042 functions P17, P16 and P12 are implemented as in a true 8042 part. Reference the 8042 spec for all timing.
A port signal of 0 drives the output to 0. A port signal of 1 causes the port enable signal to drive the output to 1 within 20-
30nsec. After 500nsec (six 8042 clocks) the port enable goes away and the external pull-up maintains the output signal
as 1.
In 8042 mode, the pins can be programmed as open drain. When programmed in open drain mode, the port enables
do not come into play. If the port signal is 0 the output will be 0. If the port signal is 1, the output tristates: an external
pull-up can pull the pin high, and the pin can be shared. In 8042 mode, the pins cannot be programmed as input nor
inverted through the GP configuration registers.
6.11.2 EXTERNAL KEYBOARD AND MOUSE INTERFACE
Industry-standard PC-AT-compatible keyboards employ a two-wire, bidirectional TTL interface for data transmission.
Several sources also supply PS/2 mouse products that employ the same type of interface. To facilitate system
SMSC LPC47S45x
Page 109 of 259
Rev. 06-01-06
DATASHEET
SMSC [ SMSC CORPORATION ]
