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8406901RA Datasheet(PDF) 4 Page - Intersil Corporation |
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8406901RA Datasheet(HTML) 4 Page - Intersil Corporation |
4 / 11 page 4 FN2979.2 August 25, 2005 Control Outputs The control outputs of the 82C88 are Data Enable (DEN), Data Transmit/Receive (DT/R) and Master Cascade Enable/ Peripheral Data Enable (MCE/PDEN). The DEN signal determines when the external bus should be enabled onto the local bus and the DT/R determines the direction of data transfer. These two signals usually go to the chip select and direction pins of a transceiver. The MCE/PDEN pin changes function with the two modes of the 82C88. When the 82C88 is in the IOB mode (IOB HIGH), the PDEN signal serves as a dedicated data enable signal for the I/O or Peripheral System bus. Interrupt Acknowledge and MCE The MCE signal is used during an interrupt acknowledge cycle if the 82C88 is in the System Bus mode (IOB LOW). During any interrupt sequence, there are two interrupt acknowledge cycles that occur back to back. During the first interrupt cycle no data or address transfers take place. Logic should be provided to mask off MCE during this cycle. Just before the second cycle begins the MCE signal gates a master Priority Interrupt Controller’s (PIC) cascade address onto the processor’s local bus where ALE (Address Latch Enable) strobes it into the address latches. On the leading edge of the second interrupt cycle, the addressed slave PIC gates an interrupt vector onto the system data bus where it is read by the processor. If the system contains only one PIC, the MCE signal is not used. In this case, the second Interrupt Acknowledge signal gates the interrupt vector onto the processor bus. Address Latch Enable and Halt Address Latch Enable (ALE) occurs during each machine cycle and serves to strobe the current address into the 82C82/82C83H address latches. ALE also serves to strobe the status (S0, S1, S2) into a latch for halt state decoding. Command Enable The Command Enable (CEN) input acts as a command qualifier for the 82C88. If the CEN pin is high, the 82C88 functions normally. If the CEN pin is pulled LOW, all command lines are held in their inactive state (not three- state). This feature can be used to implement memory partitioning and to eliminate address conflicts between system bus devices and resident bus devices. 82C88 82C88 |
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