Debug Interface
ARM DDI 0210C Copyright © 2001, 2004 ARM Limited. All rights reserved. 5-19
• Interrupt-driven use of the DCC on page 5-20.
Sending a message to the debugger
When the processor has to send a message to the debugger, it must check that the
communications data write register is free for use by finding out if the W bit of the
debug communications control register is clear.
The processor reads the debug communications control register to check the status of
bit [1] (the W bit):
• If the W bit is clear then the communications data write register is clear.
• If the W bit is set, previously written data has not been read by the debugger. The
processor must continue to poll the control register until the W bit is clear.
As the data transfer occurs from the processor to the DCC data write register, the W bit
is set in the DCC control register. When the debugger polls this register it sees a
synchronized version of both the R and W bit. When the debugger sees that the W bit
is set, it can read the DCC data write register and scan the data out. The action of reading
this data register clears the W bit of the DCC control register. At this point, the
communications process can begin again.
Receiving a message from the debugger
Transferring a message from the debugger to the processor is similar to sending a
message to the debugger. In this case, the debugger polls the R bit of the DCC control
register:
• if the R bit is clear, the DCC data read register is free and data can be placed there
for the processor to read
• if the R bit is set, previously deposited data has not yet been collected, so the
debugger must wait.
When the DCC data read register is free, data is written there using the JTAG interface.
The action of this write sets the R bit in the DCC control register.
The processor polls the DCC control register. If the R bit is set, there is data that can be
read using an
MRC
instruction to coprocessor 14. When the debugger polls this register
and sees that the R bit is clear, the data has been taken and the process can now be
repeated.
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