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82C237 Datasheet(PDF) 5 Page - Intersil Corporation

Part No. 82C237
Description  CMOS High Performance Programmable DMA Controller
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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4-152
82C237
Functional Description
The 82C237 is an improved version of the Intersil 82C37A
DMA controller and is fully software and pin for pin compati-
ble with the 82C37A. All operational and pin descriptions of
the 82C37A apply to the 82C237 with additional features
noted in the section titled 82C237 Operation.
The 82C237 direct memory access controller is designed to
improve the data transfer rate in systems which must
transfer data from an I/O device to memory, or move a block
of memory to an I/O device. It will also perform memory-to-
memory block moves, or fill a block of memory with data
from a single location. Operating modes are provided to
handle single byte transfers as well as discontinuous data
streams, which allows the 82C237 to control data movement
with software transparency.
The DMA controller is a state-driven address and control
signal generator, which permits data to be transferred
directly from an I/O device to memory or vice versa without
ever being stored in a temporary register. This can greatly
increase the data transfer rate for sequential operations,
compared
with
processor
move
or
repeated
string
instructions.
Memory-to-memory
operations
require
temporary internal storage of the data byte between
generation of the source and destination addresses, so
memory-to-memory transfers take place at less than half the
rate of I/O operations, but still much faster than with central
processor techniques. The maximum data transfer rates
obtainable with the 82C237 are shown in Figure 1.
The block diagram of the 82C237 is shown on page 2. The
timing and control block, priority block, and internal registers
are the main components. Figure 2 lists the name and size
of the internal registers. The timing and control block derives
internal timing from CLK input, and generates external
control signals. The Priority Encoder block resolves priority
contention between DMA channels requesting service
simultaneously.
DMA Operation
In a system, the 82C237 address and control outputs and
data bus pins are basically connected in parallel with the
system busses. An external latch is required for the upper
address byte. While inactive, the controller’s outputs are in a
high impedance state. When activated by a DMA request
and bus control is relinquished by the host, the 82C237
drives the busses and generates the control signals to
perform the data transfer. The operation performed by
activating one of the four DMA request inputs has previously
been programmed into the controller via the Command,
Mode, Address, and Word Count registers.
For example, if a block of data is to be transferred from RAM
to an I/O device, the starting address of the data is loaded
into the 82C237 Current and Base Address registers for a
particular channel, and the length of the block is loaded into
the channel’s Word Count register. The corresponding Mode
register is programmed for a memory-to-I/O operation (read
transfer), and various options are selected by the Command
register and the other Mode register bits. The channel’s
mask bit is cleared to enable recognition of a DMA request
(DREQ). The DREQ can either be a hardware signal or a
software command.
Once initiated, the block DMA transfer will proceed as the
controller outputs the data address, simultaneous MEMR
and IOW pulses, and selects an I/O device via the DMA
acknowledge (DACK) outputs. The data byte flows directly
from the RAM to the I/O device. After each byte is
transferred, the address is automatically incremented (or
decremented) and the word count is decremented. The
operation is then repeated for the next byte. The controller
stops transferring data when the Word Count register
underflows, or an external EOP is applied.
To
further
understand
82C237
operation,
the
states
generated by each CLK cycle must be considered. The DMA
controller operates in two major cycles, active and idle. After
being programmed, the controller is normally idle until a
DMA request occurs on an unmasked channel, or a software
request is given. The 82C237 will then request control of the
system busses and enter the active cycle. The active cycle is
composed of several internal states, depending on what
options have been selected and what type of operation has
been requested.
82C237
TRANSFER
TYPE
8MHz
12.5MHz
UNIT
8-BIT
16-BIT
8-BIT
16-BIT
Compressed
4.00
8.00
6.25
12.5
MByte/sec
Normal I/O
2.67
5.34
4.17
8.34
MByte/sec
Memory-to-
Memory
1.00
2.00
1.56
3.12
MByte/sec
FIGURE 1. DMA TRANSFER RATES
NAME
SIZE
NUMBER
Base Address Registers
16-Bits
4
Base Word Count Registers
16-Bits
4
Current Address Registers
16-Bits
4
Current Word Count Registers
16-Bits
4
Temporary Address Register
16-Bits
1
Temporary Word Count Register
16-Bits
1
Status Register
8-Bits
1
Command Register
8-Bits
1
Temporary Register
8-Bits
1
Mode Registers
6-Bits
4
Mask Register
4-Bits
1
Request Register
4-Bits
1
Data-Width Register (See Note)
4-Bits
1
NOTE: 82C237 only
FIGURE 2. 82C237 INTERNAL REGISTERS




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