UPI-C42/UPI-L42
thereby providing additional user programmable
memory space. This feature is enabled by the
A20EN instruction and remains enabled until the de-
vice is reset. It is important to note that the execu-
tion of the A20EN instruction redefines Port 2, bit 1
as a pure output pin with read only characteristics.
The state of this pin can be modified only through a
valid ‘‘D1’’ command sequence (see Table 1). Once
enabled, the A20 logic will process a ‘‘D1’’ com-
mand sequence (write to output port) by setting/re-
setting the A20 bit on port 2, bit 1 (P2.1) without
requiring service from the internal CPU. The host
can directly control the status of the A20 bit. At no
time during this host interface transaction will the
IBF flag in the status register be activated. Table 1
gives several possible GATEA20 command/data se-
quences and UPI-C42 responses.
Table1. D1 Command Sequences
A0 R/W DBPins IBF A20 Comments
1 W D1h 0 n(1) SetA20 Sequence
0 W DFh 0 1 Only DB1 Is Processed
1 W FFH(2) 0n
1 W D1h 0 n ClearA20 Sequence
0 W DDh 0 0
1 W FFh 0 n
1 W D1h 0 n DoubleTrigger Set
1 W D1h 0 n Sequence
0 W DFh 0 1
1 W FFh 0 n
1 W D1h 0 n InvalidSequence
1 W XXh(3) 1 n No Change in State
0 W DDh 1 n ofA20 Bit
NOTES:
1. Indicates that P2.1 remains at the previous logic level.
2. Only FFh commands in a valid A20 sequence have no
effect on IBF. An FFh issued at any other time will activate
IBF.
3. Any command except D1.
The above sequences assume that the GATEA20
logic has been enabled via the A20EN instruction.
As noted, only the value on DB 1 (data bus, bit 1) is
processed. This bit will be directly passed through to
P2.1 (port 2, bit 1).
SUSPEND
The execution of the suspend instruction (82h or
E2h) causes the UPI-C42 to enter the suspend
mode. In this mode of operation the oscillator is not
running and the internal CPU operation is stopped.
The UPI-C42 consumes s40 mA in the suspend
mode. This mode can only be exited by RESET.
CPU operation will begin from PC e000h when the
UPI-C42 exits from the suspend power down mode.
Suspend Mode Summary#Oscillator Not Running
#CPU Operation Stopped
#Ports Tristated with Weak (E2–10mA) Pull-Up
#Micropower Mode (ICC s40 mA)
#This mode is exited by RESET
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