4-151

82C237

EOP

36

I/O

END OF PROCESS: End of Process (EOP) is an active low bidirectional signal. Information

concerning the completion of DMA services is available at the bidirectional EOP pin.

The 82C237 allows an external signal to terminate an active DMA service by pulling the EOP pin

low. A pulse is generated by the 82C237 when terminal count (TC) for any channel is reached,

except for channel 0 in memory-to-memory mode. During memory-to-memory transfers, EOP will

be output when the TC for channel 1 occurs.

The EOP pin is driven by an open drain transistor on-chip, and requires an external pull-up resistor

When an EOP pulse occurs, whether internally or externally generated, the 82C237 will terminate

the service, and if autoinitialize is enabled, the base registers will be written to the current registers

of that channel. The mask bit and TC bit in the status word will be set for the currently active channel

by EOP unless the channel is programmed for autoinitialize. In that case, the mask bit remains clear.

A0-A3

32-35

I/O

ADDRESS: The four least significant address lines are bidirectional three-state signals. In the Idle

cycle, they are inputs and are used by the 82C237 to address the control register to be loaded or

read. In the Active cycle, they are outputs and provide the lower 4-bits of the output address. When

in 16-bit mode (82C237 only), and the active channel is a 16-bit channel (as defined by the Data-

Width register), then A0 will remain low during the entire transfer (i.e. an even word address will al-

ways be generated).

A4-A7

37-40

O

ADDRESS: The four most significant address lines are three-state outputs and provide 4-bits of

address. These lines are enabled only during the DMA service.

HRQ

10

O

HOLD REQUEST: The Hold Request (HRQ) output is used to request control of the system bus.

When a DREQ occurs and the corresponding mask bit is clear, or a software DMA request is made,

the 82C237 issues HRQ. The HLDA signal then informs the controller when access to the system

busses is permitted. For stand-alone operation where the 82C237 always controls the busses, HRQ

may be tied to HLDA. This will result in one S0 state before the transfer.

DACK0-

DACK3

14, 15

24, 25

O

DMA ACKNOWLEDGE: DMA acknowledge is used to notify the individual peripherals when one

has been granted a DMA cycle. The sense of these lines is programmable. RESET initializes them

to active low.

AEN

9

O

ADDRESS ENABLE: Address Enable enables the 8-bit latch containing the upper 8 address bits

onto the system address bus. AEN can also be used to disable other system bus drivers during DMA

transfers. AEN is active HIGH.

ADSTB

8

O

ADDRESS STROBE: This is an active high signal used to control latching of the upper address

byte. It will drive directly the strobe input of external transparent octal latches, such as the 82C82.

During block operations, ADSTB will only be issued when the upper address byte must be updated,

thus speeding operation through elimination of S1 states. ADSTB timing is referenced to the falling

edge of the 82C237 clock.

MEMR

3

O

MEMORY READ: The Memory Read signal is an active low three-state output used to access data

from the selected memory location during a DMA Read or a memory-to-memory transfer.

MEMW

4

O

MEMORY WRITE: The Memory Write is an active low three-state output used to write data to the

selected memory location during a DMA Write or a memory-to-memory transfer.

DWLE

5

O

DATA-WIDTH, LATCH ENABLE: In normal 8-bit transfer mode (16-bit transfer mode not enabled),

this output is always high impedance three-stated. In 16-bit transfer mode (82C237 only), this output

serves a dual purpose. During S1 cycles, the DWLE output indicates the data width (0 = 16-bit, 1 =

8-bit) of the active channel. During memory-to-memory transfers, the DWLE output is used to enable

an external latch which temporarily stores the 8 most significant bits of data during the read-from-

memory transfer. DWLE enables this byte of data onto the data bus during the write-to-memory

transfer of a memory-to-memory operation.

SYMBOL

PIN

NUMBER

TYPE

DESCRIPTION