[Contents] [Index] [Help] [Retrace] [Browse <] [Browse >]

The signals in this group are available for various types of system
control; most of these have an immediate or near immediate effect on
expansion cards and/or the system CPU itself.

Hardware Bus Error/Interrupt (/BERR)
   This is a general indicator of a bus fault or special condition of
   some kind. Any expansion card capable of detecting a hardware error
   relating directly to that card can assert /BERR when that bus error
   condition is detected, especially any sort of harmful hardware error
   condition.  This signal is the strongest possible indicator of a bad
   situation, as it causes all PICs to get off the bus, and will usually
   generate a level 2 exception on the host CPU. For any condition that
   can be handled in software and doesn't pose an immediate threat to
   hardware, notification via a standard processor interrupt is the
   better choice.  The bus controller will drive /BERR in the event of a
   detected bus collision or DMA error (an attempt by a bus master to
   access local bus resources it doesn't have valid access permission
   for). All cards must monitor /BERR and be prepared to tri-state all
   of their on-bus output buffers whenever this signal is asserted. An
   expansion bus master will attempt to retry a cycle aborted by a
   single /BERR and notify system software in the case of two subsequent
   /BERR results.   Since any number of devices may assert /BERR, and
   all bus cards must monitor it, any device that drives /BERR must
   drive with an  open collector  or similar device, and any device that
   monitors /BERR should place a minimal load on it.  This signal is
   pulled high by a passive backplane resistor.

Note that, especially for the slave device being addressed, that /BERR
alone is not always necessarily an indication of a bus failure in the pure
sense, but may indicate some other kind of unusual condition.  Therefore,
a device should still respond to the bus address, if otherwise
appropriate, when a /BERR condition is indicated.  It simply tri-states is
bus buffers and other outputs, and waits for a change in the bus state.
If the /BERR signal is negated with the cycle unterminated, the special
condition has been resolved and the slave responds to the rest of the
cycle as it normally would have. If the cycle is terminated by the bus
master, the resolution of the special condition has indicated that the
addressed slave is not needed, and so the cycle terminates without the
slave being used.

System Reset (/RESET, /IORST)
   The bus supplies two versions of the system reset signal.  The /RESET
   signal is bi-directional and unbuffered, allowing an expansion card
   to hard reset the system.  It should only be used by boards that need
   this reset capability, and is driven only by an  open collector  or
   similar device.  The /IORST signal is a buffered output-only version
   of the reset signal that should be used as the normal reset input to
   boards not concerned with resetting the system on their own.  All
   expansion devices are required to reset their  autoconfiguration  logic
   when /IORST is asserted.  These signals are pulled high by passive
   backplane resistors.

System Halt (/HLT)
   This signal is driven, along with /RESET, to assert a full-system
   reset.  A full-system reset is asserted on a powerup reset or a
   keyboard reset; any PIC that needs to differentiate between full
   system and I/O reset should monitor /HLT and /IORST unless it also
   needs to drive a reset condition.  This is driven with an
    open-collector  output, or the equivalent, and pulled up by a
   backplane resistor.

System Interrupts
   Two of the decoded, level-sensitive 680x0 interrupt inputs are
   available on the expansion bus, and these are labeled as /INT2 and
   /INT6. Each of these interrupt lines is shared by wired ORing, thus
   each line must be driven by an  open-collector  or equivalent output
   type.  Zorro III interrupts can be handled Zorro II style, via
   autovectors and daisy-chained polling, or they can be vectored using
   the  quick interrupt  protocol described in the Bus Architecture
   section.   Zorro II and Zorro III systems originally provided /INT1,
   /INT4, /INT5, and /INT7 lines as well, but as these were never
   properly supportable by system software, they have been eliminated.
   Those lines are considered reserved for future use in a Zorro III

[Back to Amiga Developer Docs]