XR88C681
18
Rev. 2.11
Bit 7
Bit 6
Bit 5
Bit 4
Description
1
0
Set Standby Mode (Channel A).
When this com-
mand is invoked via the Channel A Command Regis-
ter, power is removed from each of the transmitters,
receivers, counter/timer and additional circuits to
place the DUART in the standby (or lower power)
mode.
Please note that this command effects the
operation of the entire chip. Normal operation is re-
stored by a hardware reset or by invoking the “SET
ACTIVE MODE” command.
Reset IUS Latch (Channel B).
When this command
is invoked via the Channel B Command Register,
and the DUART is operating in Z-mode, it causes
the Interrupt-Under-Service (IUS) latch to be reset.
This, in turn, will cause the IEO output to toggle
“high”.
1
0
1
Set Active Mode (Channel A).
When this com-
mand is invoked via the Channel A Command Regis-
ter, the DUART is removed from the Standby Mode
and resumes normal operation.
Set Z-Mode (Channel B).
When this command is
invoked via the Channel B Command Register, the
DUART is conditioned to operate in the Z-Mode. For
a detailed discussion of the DUART’s operation while
in the Z-Mode, Please see
Section C.6.2. (Not avail-
able for the 28 pin DIP packaged devices)
1
0
Reserved.
1
Reserved.
Table 3. Miscellaneous Commands, Upper Nibble of all Command Registers,
Unless Otherwise Specified (Cont’d)
In addition to the commands which are available through
the command registers, the DUART also offers
“Address-Triggered” commands. These commands are
listed in
Table 1, “DUART PORT AND REGISTER
ADDRESSING”; and are further identified by being
“shaded” in
Table 1. Specifically, these commands are:
D
START COUNTER/TIMER COMMAND
D
STOP COUNTER/TIMER COMMAND
D
SET OUTPUT PORT BITS COMMAND
D
CLEAR OUTPUT PORT BITS COMMAND
Each of these commands are invoked by either reading or
writing data to their corresponding DUART addresses as
specified in
Table 1.
For Example:
The START COUNTER/TIMER COMMAND is invoked by
the procedure of reading DUART address 0E16. Please
note that this “Read Operation” will not result in placing
the contents of a DUART register on the data bus. The
only thing that will happen, in response to this procedure
is the Counter/Timer will initiate counting. For a detailed
discussion into the operation of the Counter/Timer, please
see
Section D.2.
Another example of an Address-Triggered commands is
the “SET OUTPUT PORT BITS” Command.
This
command is invoked by performing a write of data to
DUART address 0E16. When the user invokes this
command, he/she is setting certain bits (to “1”) within the
OPR (Output Port Register). All other bits, within the OPR
(not specified to be set), are not changed. The state of the
output port pins are complements of the individual bits
within the OPR. Hence, if OPR[0] is set to “1”, the state of
the corresponding output port pin, OP0, is now set to a
logic “0”. Consequently, one can think of the “SET
OUTPUT PORT BITS” command as the “CLEAR
OUTPUT PORT PINS” command. For a more detailed
discussion into the operation of the Output Ports, please
see
Section F.