82C88_05 Datasheet, PDF (4/11 Pages) Intersil Corporation – CMOS Bus Controller

82C88
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.
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FN2979.2
August 25, 2005