82C237 Datasheet, PDF (9/25 Pages) Intersil Corporation – CMOS High Performance Programmable DMA Controller

82C237
Current Word Count Register - Each channel has a 16-Bit
Current Word Count register. This register determines the
number of transfers to be performed. The actual number of
transfers will be one more than the number programmed in
the Current Word Count register (i.e., programming a count
of 100 will result in 101 transfers). The word count is
decremented after each transfer. When the value in the
register goes from zero to FFFFH, a TC will be generated.
This register is loaded or read in successive 8-bit bytes by
the microprocessor in the Program Condition. See Figure 6
for programming information. Following the end of a DMA
service it may also be reinitialized by an Autoinitialization
back to its original value. Autoinitialization can occur only
when an EOP occurs. If it is not Autoinitialized, this register
will have a count of FFFFH after TC.
Base Address and Base Word Count Registers - Each
Channel has a pair of Base Address and Base Word Count
registers. These 16-bit registers store the original value of
their associated current registers. During Autoinitialize these
values are used to restore the current registers to their
original values. The base registers are written simulta-
neously with their corresponding current register in 8-bit
bytes in the Program Condition by the microprocessor. See
Figure 6 for programming information. These registers can-
not be read by the microprocessor.
Command Register - This 8-bit register controls the opera-
tion of the 82C237. It is programmed by the microprocessor
and is cleared by RESET or a Master Clear instruction. The
following diagram lists the function of the Command register
bits. See Figure 4 for Read and Write addresses.
Command Register
76543210
BIT NUMBER
0 Memory-to-memory disable
1 Memory-to-memory enable
0 Channel 0 address hold disable
1 Channel 0 address hold enable
X If bit 0 = 0
0 Controller enable
1 Controller disable
0 Normal timing
1 Compressed timing
X If bit 0 = 1
0 Fixed priority
1 Rotating priority
0 Late write selection
1 Extended write selection
X If bit 3 = 1
0 DREQ sense active high
1 DREQ sense active low
0 DACK sense active low
1 DACK sense active high
Mode Register - Each channel has a 6-bit Mode register
associated with it. When the register is being written to by
the microprocessor in the Program condition, bits 0 and 1
determine which channel Mode register is to be written.
When the processor reads a Mode register, bits 0 and 1 will
both be ones. See the following diagram and Figure 4 for
Mode register functions and addresses.
Mode Register
76543210
BIT NUMBER
00 Channel 0 select
01 Channel 1 select
10 Channel 2 select
11 Channel 3 select
XX Readback
00 Verify transfer
01 Write transfer
10 Read transfer
11 Illegal
XX If bits 6 and 7 = 11
0 Autoinitialization disable
1 Autoinitialization enable
0 Address increment select
1 Address decrement select
00 Demand mode select
01 Single mode select
10 Block mode select
11 Cascade mode select
Request Register - The 82C237 can respond to requests
for DMA service which are initiated by software as well as by
a DREQ. Each channel has a request bit associated with it in
the 4-bit Request register. These are non-maskable and
subject to prioritization by the Priority Encoder network.
Each register bit is set or reset separately under software
control. The entire register is cleared by a RESET or Master
Clear instruction. To set or reset a bit, the software loads the
proper form of the data word. See Figure 4 for register
address coding, and the following diagram for Request
register format. A software request for DMA operation can
be made in block or single modes. For memory-to-memory
transfers, the software request for channel 0 should be set.
When reading the Request register, bits 4-7 will always read
as ones, and bits 0-3 will display the request bits of channels
0-3 respectively.
Mode Register
76543210
BIT NUMBER
Don’t Care,
Write
Bits 4-7
All Ones,
Read
00 Select Channel 0
01 Select Channel 1
10 Select Channel 2
11 Select Channel 3
0 Reset request bit
1 Set request bit
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