JTAG problem

On my board with C5501, I can download the program
and modify the memory and registers. But when I want
to run the program(single step or run), an error
message like the following appeared:
"Error 0x00000020/-1151 Error during: Execution, 
Processor communication timeout."
Could anybody tell me what the possible cause of 
this problem?
thx!!

Chao Xiang

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