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P87L42 Datasheet(PDF) 7 Page - Intel Corporation |
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P87L42 Datasheet(HTML) 7 Page - Intel Corporation |
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7 / 25 page  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 s40 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 (E2–10 mA) Pull-Up Micropower Mode (ICC s 40 mA) This mode is exited by RESET 7 |
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