Debug Interface and EmbeddedICE-RT
ARM DDI 0165B
Copyright 2000 ARM Limited. All rights reserved.
7-19
A typical use of this bit is by a monitor mode debug aware abort handler. This examines
the DbgAbt bit to determine whether the abort was externally or internally generated. If
the DbgAbt bit is set, the abort handler initiates communication with the debugger over
the comms channel.
7.8.4
Communications using the comms channel
You can send and receive messages using the comms channel. These are described in:
Sending a message to the debugger
Before the processor can send a message to the debugger, it must check that the comms
data write register is free for use by finding out if the W bit of the debug comms control
register is clear.
The processor reads the debug comms control register to check the status of the W bit:
If the W bit is clear, the comms 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.
When the W bit is clear, a message is written by a register transfer to coprocessor 14.
As the data transfer occurs from the processor to the comms data write register, the W
bit is set in the debug comms control register.
The debugger has two options available for reading data from the comms data write
register:
Poll the debug comms channel control register before reading the comms data
written. If the W bit is set, there is valid data present in the debug comms data
write register. The debugger can then read this data and scan the data out. The
action of reading the data clears the debug comms channel control register W bit.
Then the communications process can begin again.
Poll the comms data write register, obtaining data and valid status. The data
scanned out consists of the contents of the comms data write register (which
might or might not be valid), and a flag that indicates whether the data read is
valid or not. The status flag is present in the Addr[0] bit position of scan chain 2
scan chain 2.