a8237 Programmable DMA Controller Data Sheet
Clear Byte Pointer Command
The byte pointer is a single-bit internal register that selects either the least
significant or most significant byte of the 16-bit registers in the a8237. The
byte pointer allows microprocessor write and read operations via the 8-bit
data bus. The clear byte pointer command is a write command that resets
the byte pointer, allowing subsequent access to the least significant byte
of any 16-bit register. The data bus value for the clear byte pointer
command is ignored. The resetinput and the master clear command
resets the byte pointer.
Set Byte Pointer Command
The set byte pointer command is a read command that sets the byte
pointer, allowing subsequent access to the most significant byte of any
16-bit register. The data bus value for the set byte pointer command is
unknown. The byte pointer is reset by the resetinput and the master
clear command.
Master Clear Command
The master clear command performs the same function as the reset
input. This command resets the command, status, request, temporary and
byte pointer registers, mode register counter, state machine, and also sets
the mask register.
Clear Mask Register Command
The clear mask register command is a write command that resets the 4-bit
mask register, enabling DMA requests from the dreqinputs. The data bus
value for the clear mask register command is ignored.
Clear Mode Register Counter Command
The clear mode register counter command is a read command that resets
the 2-bit mode register counter. This counter is incremented after each
subsequent read of the mode register, allowing the user to cycle through
all four mode registers. The data bus value for the clear mode register
counter command is unknown.
This section describes the following a8237MegaCore function
operations:
Operation
■
■
■
State machine
Transfer modes
Other operations
Altera Corporation
17
ALTERA [ ALTERA CORPORATION ]
