
6-12
Debug Interface
Rev. A
Copyright 2000 by LSI Logic Corporation. All rights reserved.
Bit 1 denotes whether the Comms Data Write Register is available (from
the viewpoint of the processor). If, from the point of view of the processor,
the Comms Data Write Register is free (W=0), new data can be written.
If the register is not free (W = 1), the processor must poll until W = 0.
From the point of view of the debugger, when W = 1, some new data has
been written that then can be scanned out.
Bit 0 denotes whether there is new data in the Comms Data Read
Register.
If, from the point of view of the processor, R = 1, there is some new data
that can be read using an MRC instruction.
From the point of view of the debugger, if R = 0, the Comms Data Read
Register is free, and new data can be placed there through the scan
chain. R = 1 indicates that data previously placed there through the scan
chain has not been collected by the processor, and so the debugger
must wait.
From the point of view of the debugger, the registers are accessed via
the scan chain in the usual way. From the point of view of the processor,
these registers are accessed via coprocessor register transfer
instructions.
Use the following instructions when accessing the Debug Comms
Channel Registers:
MRC CP14, 0, Rd, C0, C0
Returns the Debug Comms Control Register into Rd.
MCR CP14, 0, Rn, C1, C0
Writes the value in Rn to the Comms Data Write Register.
MRC CP14, 0, Rd, C1, C0
Returns the debug data read register into Rd.
Because the Thumb instruction set does not contain coprocessor
instructions, you are advised to access this data via SWI instructions
when in Thumb state.
6.7.2 Communications via the Comms Channel
Messages can be sent and received via the comms channel.