Bus Timer; I/O Utilities; System Control Registers; System Control Register 1 - Performance Computer PT-VME161 User Manual

I/O UTILITIES

System Control Registers

The System Control Registers provide the PT-VME161 with miscellaneous control functions not
provided in other registers. They are all 8-bit, read/write registers.
These registers can be read back. This allows the use of AND or OR instructions to clear and set indi-
vidual bits.
!
CAUTION: These registers should only be referenced using byte operations! Reading any of them as a
word or longword will be translated into multiple byte reads by the sizing logic, with the same data byte
being repeated on each reference. Writing them as a word or longword can cause problems because the
bus sizing forces multiple byte writes to occur. This will leave only the last (most significant) byte in the
selected register. The other bytes will have previously been pumped through the register and if they are
not identical to the MSB the respective bits of the control register will toggle.

System Control Register 1

System Control Register 1 is an 8-bit, read/write register. All bits are cleared to zero (0) by Local
Reset. It resides at location 0D000000h.

VMEbus Page Select

User LEDs

Bus Timer

The SCV64 as System Controller provides VMEbus Data Transfer Time-out functionality.
VMEbus data transfers are terminated with the error signal BERR*, if a slave does not
respond within a programmed time. The time-out period may be 16, 32, or 64 us, or
"never". The default value after reset is 64 us. The VXL1 and VXL0 control bits in the
SCV64 VMEbus Arbiter Register determine the time-out period.
Access to the VMEbus address space is through the local addresses 80000000h-FFFFFFFFh. In
order to Access the full VMEbus address space, the VMEbus Page Select bit (VPG) in the
System Control Register is used to define VMEbus address bit 31. When the VMEbus Page
Select bit is 0, PT-VME161 addresses 80000000h-FFFFFFFFh are mapped to VMEbus
addresses 00000000h - 7FFFFFFFh. When the VMEbus Page Select bit is 1, PT-VME161
addresses 80000000h-FFFFFFFFh are mapped to VMEbus addresses 80000000h-FFFFFFFFh.
Local Reset clears (0) this bit.
There are four LEDs on the front panel (D3, D2, D1, and D0) that are under program control.
Writing a one to the LED3-LED0 bits in the System Control Register will light the respective
LED. When Local Reset is asserted all the four LEDs will be turned off.
Extensible Single Board Computer/Controller User's Manual 45
Performance Computer