MC68F375
CONFIGURABLE TIMER MODULE (CTM9)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
13-52
13.8.4.2 BIUTBR — BIUSM Time Base Register
In normal operation, the BIUTBR is a read-only register used to read the value present
on one of the time base buses. The time base bus being accessed is determined by
TBRS1 and TBRS0 in the BIUMCR. Writing to the BIUTBR has no effect, except in
certain test modes.
13.9 Counter Prescaler Submodule (CPSM)
The counter prescaler submodule (CPSM) generates six different clock frequencies
which can be used by any counter submodule. Five of these frequencies are derived
from a fixed divider. The divide ratio of the last clock frequency is software selectable
from a choice of four divide ratios. Note that this submodule is contained within the
BIUSM. A block diagram of the CPSM is given in Figure 13-14. The clock division
any or all CTM submodules.
10:8
IARG[2:0]
Interrupt arbitration identification. 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 occur-
ring 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.
7:6
—
Reserved
5,0
TBRS1,
TBRS0
Time base register bus select. These bits specify which time base bus is accessed when the
time base register (BIUTBR) is read.
00 = Time base bus TBB1
01 = Time base bus TBB2
10 = Time base bus TBB3
11 = Time base bus TBB4
4:1
—
Reserved
BIUTBR — BIUSM Time Base Register
0xYF F204
MSB
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
LSB
0
MSB
LSB
RESET:
0
Table 13-19 BIUMCR Bit Settings (Continued)
Bit(s)
Name
Description
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