Software Commands
There are special software commands which can be
executed by reading or writing to the 82C37A.
These commands
do not depend on the specific data pattern on the
data bus, but are activated by the I/O operation itself. On
read type commands, the data value is not guaranteed.
These commands are:
Clear First/Last Flip-Flop - This command is executed
prior to writing or reading new address or word count information
to the 82C37A. This command initializes the flip-flop
to a known state (low byte first) so that subsequent accesses
to register contents by the microprocessor will address
upper and lower bytes in the correct sequence.
Set First/Last Flip-Flop - This command will set the flip-flop
to select the high byte first on read and write operations to
address and word count registers.
Master Clear - This software instruction has the same effect
as the hardware Reset. The Command, Status, Request,
and Temporary registers, and Internal First/Last Flip-Flop
and mode register counter are cleared and the Mask register
is set. The 82C37A will enter the idle cycle.
Clear Mask Register - This command clears the mask bits
of all four channels, enabling them to accept DMA requests.
Clear Mode Register Counter - Since only one address
location is available for reading the Mode registers, an internal
two-bit counter has been included to select Mode registers
during read operation. To read the Mode registers, first
execute the Clear Mode Register Counter command, then
do consecutive reads until the desired channel is read. Read
order is channel 0 first, channel 3 last. The lower two bits on
all Mode registers will read as ones.