Intel UPI-C42 Computer Hardware User Manual


 
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
Table 1 D1 Command Sequences
A0 RW 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 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
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