Rev. 3.0, 03/01, page 388 of 390
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Application note for PS/2 interface
Problem:
Cannot detect change in CLOCK line when host sends data to device.
Description:
The KBCS (bit 10) of the KBCSR (address 0x1000DC00) always reads as one when the
KBCIE (bit 15) of the KBCSR is set to zero.
When using the method to transferring data to the keyboard described on the HD64465
data sheet the driver never see the KBCS change from one to zero.
A software work around exists for this problem.
Work around:
When the KBCIE bit is set to one then the CLOCK line state is available on the KBCS bit.
With the KBCIE bit set to one the driver can send the required data to the keyboard. The
side affect is that the hardware shift register is also receiving the data.
To prevent this data from causing an interrupt the KBCIE bit is cleared at the stop bit time
and the KBRDF (bit 0) of the KBISR (address 0x1000DC04) is set to one.
Problem:
Cannot poll for input data when device interrupts are enabled.
Description:
There is no independent interrupt enable bit for the keyboard input shift register.
This is a problem because the host must send commands to the keyboard and receive an
acknowledge byte from the keyboard. Some commands require the host to receive
additional data bytes.
Without a means to disable the keyboard interrupt request and poll the hardware for a
response from keyboard the driver code becomes much more complicated.
Work around:
Restructure the keyboard driver to allow the interrupt service task to differentiate the
keyboard scan code stream from the host to keyboard command and response stream.
Problem:
Driver looses synchronization with the message stream.
Description:
With a current production version of the Logitech or Microsoft PS/2 mouse connected the
HD64465 will set the CLOCK line low before the falling edge of the tenth clock.
Summary of Contents for HD64465
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