
MOTOROLA
2-4
CTM
REFERENCE
BUS INTERFACE UNIT SUBMODULE (BIUSM)
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2
IARB[2:0] — Interrupt arbitration identification bits
The interrupt arbitration bit field (IARB), composed of IARB[2:0] in the BIUMCR and the IARB3 bit
within each submodule, provides fifteen different arbitration identification numbers that can be
used to arbitrate between interrupt requests occurring on the IMB with the same interrupt priority
level.
The IARB field defaults to zero on reset, thus preventing the module from arbitrating during an
interrupt arbitration acknowledge cycle (IACK). If no IMB arbitration takes place during the IACK
cycle the spurious interrupt vector is generated by the SIM (system integration module). This tells
the system that the interrupt arbitration number has not been initialized. The seven levels of
interrupt are the primary means by which interrupt priority is established. The 4-bit interrupt
arbitration number is the secondary priority, allowing up to 15 requests at each primary level.
During the IACK cycle the request with the highest arbitration number gets serviced (binary 1111
is the highest priority and binary 0001 is the lowest).
Many IMB modules have one software assignable arbitration number for the whole module. The
CTM allows two different arbitration numbers to be used by providing each submodule with its own
IARB3 bit (which can be set or cleared in software). Once IARB[2:0] are assigned in the BIUSM,
they apply to all CTM interrupt requests. Therefore, CTM submodule interrupts can be interleaved
in priority with requests from other modules at the same interrupt level.
IARB[2:0] are all cleared by reset.
TBRS1, TBRS0 — Time base register bus select bits
These bits specify which time base bus is accessed when the time base register (BIUTBR) is read.
2.5.2
BIUTEST — BIUSM test configuration register
The BIUTEST register is located at CTM base address offsets $02 and $03 and is reserved for
factory testing of the CTM.
TBRS1 TBRS0
0
0
1
1
Time base bus
TBB1
TBB2
TBB3
TBB4
0
1
0
1
F
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