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UPI-C42UPI-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 settingre-
setting the A20 bit on port 2 bit 1 (P21) 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 commanddata se-
quences and UPI-C42 responses
Table 1 D1 Command Sequences
A0 RW DB Pins IBF A20 Comments
1 W D1h 0 n
(1)
Set A20 Sequence
0 W DFh 0 1 Only DB1 Is Processed
1 W FFH
(2)
0n
1 W D1h 0 n Clear A20 Sequence
0 W DDh 0 0
1 W FFh 0 n
1 W D1h 0 n Double Trigger Set
1 W D1h 0 n Sequence
0 W DFh 0 1
1 W FFh 0 n
1 W D1h 0 n Invalid Sequence
1 W XXh
(3)
1 n No Change in State
0 W DDh 1 n of A20 Bit
NOTES
1 Indicates that P21 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
P21 (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
s
40 mA in the suspend
mode This mode can only be exited by RESET
CPU operation will begin from PC
e
000h when the
UPI-C42 exits from the suspend power down mode
Suspend Mode Summary
Oscillator Not Running
CPU Operation Stopped
Ports Tristated with Weak (
E
2–10 mA) Pull-Up
Micropower Mode (I
CC
s
40 mA)
This mode is exited by RESET
7