Freescale Semiconductor PowerQUICC MPC870, Reference Manual

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System Interface Unit
MPC885 PowerQUICC Family Reference Manual, Rev. 2
10-10 Freescale Semiconductor
Each register has a key register that can be in an open or locked state. At power-on reset, all key registers
are open. Each key register has an associated address in the internal memory map, as shown in
Table 10-6 .
A write of 0x55CC_AA33 to a key register unlocks its associated SIU register; any other access (including
reads or writes of any other value) to a key register locks its associated SIU register. For example, writing
a 0x55CC_AA33 to the TBK key register allows the Timebase/decrementer register to be written. The key
registers are write-only; a read of the key register does not return the last value written.
When a register is locked, an attempt to write to it will result in a machine check exception, and will not
change the value in the register. One exception to this is the timebase register (TBU and TBL), locked with
TBK. A write to the timebase register when it is locked results in a software emulation exception.
Reads are allowed at all times to any of the SIU registers, regardless of whether they are locked or
unlocked.
0x308 TBREFBK—Timebase reference register B key 32 bits
0x30C TBK—Timebase/decrementer register key 32 bits
0x310–31F Reserved 16 bytes
0x320 Reserved 32 bits
0x324 Reserved 32 bits
0x328 Reserved 32 bits
0x32C Reserved 32 bits
0x330–33F Reserved 16 bytes
0x340 PISCRK—Periodic interrupt status and control register key 32 bits
0x344 PITCK—Periodic interrupt count register key 32 bits
0x348–37F Reserved 56 bytes
Clocks and Reset Keys
0x380 SCCRK—System clock control key 32 bits
0x384 PLPRCRK—PLL and reset control register key 32 bits
0x388 RSRK—Reset status register key 32 bits
0x38C–7FF Reserved 1140 bytes
Table 10-6. Key Registers (continued)
Offset Name Size