82C237
Mask Register - Each channel has associated with it a mask Status Register - The Status register is available to be read
bit which can be set to disable an incoming DREQ. Each out of the 82C237 by the microprocessor. It contains
mask bit is set when its associated channel produces an EOP information about the status of the devices at this point. This
if the channel is not programmed to Autoinitialize. Each bit of information includes which channels have reached a terminal
the 4-bit Mask register may also be set or cleared separately count and which channels have pending DMA requests. Bits
or simultaneously under software control. The entire register 0-3 are set every time a TC is reached by that channel or an
is also set by a RESET or Master clear. This disables all hard- external EOP is applied. These bits are cleared upon RESET,
ware DMA requests until a Clear Mask Register instruction Master Clear, and on each Status Read. Bits 4-7 are set
allows them to occur. The instruction to separately set or clear whenever their corresponding channel is requesting service,
the mask bits is similar in form to that used with the Request regardless of the mask bit state. If the mask bits are set, soft-
register. Refer to the following diagram and Figure 4 for ware can poll the Status register to determine which channels
details. When reading the Mask register, bits 4-7 will always have DREQs, and selectively clear a mask bit, thus allowing
read as logical ones, and bits 0-3 will display the mask bits of user defined service priority. Status bits 4-7 are updated while
channels 0-3, respectively. The 4-bits of the mask register the clock is high, and latched on the falling edge. Status Bits
may be cleared simultaneously by using the Clear Mask Reg- 4-7 are cleared upon RESET or Master Clear.
ister command (see software commands section).
Status Register
Mask Register
7
6
5
4
3
2
1
0
BIT NUMBER
7
6
5
4
3
2
1
0
BIT NUMBER
1
1
1
1
1
1
1
1
Channel 0 has reached TC
Channel 1 has reached TC
Channel 2 has reached TC
Channel 3 has reached TC
Channel 0 request
00 Select channel 0 mask bit
01 Select channel 1 mask bit
10 Select channel 2 mask bit
11 Select channel 3 mask bit
Don’t Care,
0
1
Clear mask bit
Set mask bit
All four bits of the Mask register may also be written with a
single command.
Channel 1 request
Channel 2 request
7
6
5
4
3
2
1
0
BIT NUMBER
Channel 3 request
0
1
Clear channel 0 mask bit
Set channel 0 mask bit
Don’t Care,
Write
All Ones,
Read
Temporary Register - The Temporary register is used to hold
data during memory-to-memory transfers. Following the com-
pletion of the transfers, the last byte moved can be read by the
microprocessor. The Temporary register always contains the
last byte transferred in the previous memory-to-memory oper-
ation, unless cleared by a RESET or Master Clear.
0
1
Clear channel 1 mask bit
Set channel 1 mask bit
0
1
Clear channel 2 mask bit
Set channel 2 mask bit
0
1
Clear channel 3 mask bit
Set channel 3 mask bit
OPERATION
A3
A2
A1
A0
IOR
IOW
Read Status Register
Write Command Register
Read Request Register
Write Request Register
Read Command Register
Write Single Mask Bit
Read Mode Register
Write Mode Register
Set First/Last F/F
Clear First/Last F/F
Read Temporary Register
Master Clear
Clear Mode Reg. Counter
Clear Mask Register
Read All Mask Bits
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
Write All Mask Bits
FIGURE 4. SOFTWARE COMMAND CODES AND REGISTER CODES
4-157
INTERSIL [ Intersil ]
