
VXIbus Interface
In the soft reset state, a device is inactive, interrupts which are pending are
unasserted, and all pending bus requests are removed. While in this state, the device's
VMEbus slave interface is active. To achieve this functionality without resetting
everything on the board, the EPC-7 performs a sequence of events when RSTP bit is
set or when VME SYSRESET is asserted and SYSREST INPUT ENABLE bit is
clear (bit 7 of VSC, register 8144). This disables the SYRESET - PCRESET logic.
The following sequence occurs:
Bit VXR of the VET register (VXI Reset, bit 3 of register 815B) is asynchronously
cleared as long as RSTP is set. The VXR bit, when asserted, causes an interrupt if bit
5 of the BEE register (8155) is set. An asserted VXR bit places several other register
bits in the reset state. Bits 2-0 of VMOL (8158) are held asynchronously reset by the
VXR bit. This removes any pending interrupt request from the bus. The VXR bit also
disables VME/VXI accesses from occurring by masking off the VMEEN bit (bit 1 of
8102). This steers all VME/VXI memory references onto the AT bus (EOX, EOI).
SBER, bit 2 of the VET register, is also cleared by VXR, but bit BERRR (bit 1 of
BES, register 8154) is not cleared. In addition, the TTD (TTL Trigger Drive register,
815A) the BMA (8130/8132/8134/8136), BWA (8159), BWM (8151), and the ETR
(8163) registers are cleared by the VXT bit.
Finally, TTLTRIG0 drive is disabled while the VXR is asserted (This bit is treated
differently than the other Trigger drive bits since clearing the ETR may cause the
external trigger to drive TTLTRIG0).
Software can recognize the safe/soft reset states in the following ways:
7
7
1)
Enable VXR interrupt.
When a VXR interrupt is handled, check the VSC register to see if this is the safe or
soft reset state. If the RSTP bit is set, then software must wait to be reset from the soft
reset state. When this has been done by another VME/VXI agent, software must still
reset the VXR bit in the VET register before VME activity can again commence. This
is accomplished by writing a 1 into bit position 3 of register 815B. Also, SBER
should be reset at this time. If the RSTP bit is clear and the PASSED bit is clear, then
an external VXI agent has both set the board into the soft or safe states and then reset
the RSTP bit, before the interrupt handler had a chance to handle the safe/soft reset
state. The VXR and SBER bit must again be reset by software. If the RSTP bit is
clear and the PASSED bit is set, the VME SYSREST was asserted and software is
free to perform whatever cleanup actions it desires. Again, the VXR and SBER bits
should be reset before attempting any VME bus accesses.
2)
Poll SBER after VME/VXI bus accesses.
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