MPC555
/
MPC556
SYSTEM CONFIGURATION AND PROTECTION
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
6-12
6.5 Hardware Bus Monitor
The bus monitor ensures that each bus cycle is terminated within a reasonable period
of time. The USIU provides a bus monitor option to monitor internal to external bus ac-
cesses on the external bus. The monitor counts from transfer start to transfer acknowl-
edge and from transfer acknowledge to transfer acknowledge within bursts. If the
monitor times out, transfer error acknowledge (TEA) is asserted internally.
The bus monitor timing bit in the system protection control register (SYPCR) defines
the bus monitor time-out period. The programmability of the time-out allows for varia-
tion in system peripheral response time. The timing mechanism is clocked by the sys-
tem clock divided by eight. The maximum value is 2040 system clock cycles.
The bus monitor enable (BME) bit in the SYPCR enables or disables the bus monitor.
The bus monitor is always enabled, however, when freeze is asserted or when a de-
bug mode request is pending, regardless of the state of this bit.
6.6 MPC555
/ MPC556 Decrementer
The decrementer (DEC) is a 32-bit decrementing counter defined by the MPC555 /
MPC556 architecture to provide a decrementer interrupt. This binary counter is
clocked by the same frequency as the time base (also defined by the MPC555 /
MPC556 architecture). The operation of the time base and decrementer are therefore
coherent. In the MPC555 / MPC556, the DEC is clocked by the TMBCLK clock. The
decrementer period is computed as follows:
Table 6-3 Priority of Interrupt Sources
Priority
Interrupt Source
Interrupt Code
0 (Highest)
IRQ0
00000000
1 Level
0
00000100
2
IRQ1
00001000
3
Level 1
00001100
4
IRQ2
00010000
5
Level 2
00010100
6
IRQ3
00011000
7
Level 3
00011100
8
IRQ4
00100000
9
Level 4
00100100
10
IRQ5
00101000
11
Level 5
00101100
12
IRQ6
00110000
13
Level 6
00110100
14
IRQ7
00111000
15 (Lowest)
Level 7
00111100
16-31
Reserved
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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