Maxim Integrated DS4830 Optical Microcontroller - Manual (Page 164)

DS4830 User’s Guide
164
debug engine. Also, note that the interrupt handler is an essential part of the CPU and a pending interrupt could be
granted during single step operation since the IGE bit state present on debug mode entry is restored for the single step.
21.2.4
– Return
To terminate the debug mode and return the debug engine to background mode, the host must issue a Return command
to the debug engine. This command causes the following actions:
1) Pop the return address from the stack
2) Set the IGE bit to logic 1 if debug mode was activated when IGE=1.
3) Supply the CPU with an instruction addressed by the return address
4) Allow the CPU to execute the normal user program
5) Set the status to 00b (non-debug)
To prevent a possible endless breakpoint matching loop, no break will occur for a breakpoint match on the first instruction
after returning from debug mode to background mode. Returning to background mode also enables all internal timer
functions.
21.2.5
– Debug Mode Special Considerations
The following are special considerations when using Debug Mode.

Special caution should be exercised when using the Write Register command on register bits that globally affect
system operation (e.g., IGE, STOP). If the write register command is used to invoke stop mode (setting STOP =
1), the /RST\ pin may be asserted to reset the debug engine and return to the background mode of operation.

Single stepping ('Trace') through any IGE bit change operation results in the debug engine overriding the bit
change since it retains the IGE bit setting captured when active debug mode was entered.

Single stepping ('Trace') into an operation that sets STOP = 1 when IGE = 1 effectively allows enabled interrupts
normally capable of causing exit from stop mode to do so.

Single stepping ('Trace') through any memory read instruction that reads from the utility ROM (such as 'move
Acc,' @DP[0] with DP[0] set to 8000h) will cause the memory read to return an incorrect value.

Single stepping ('Trace') cannot be used when executing code from the utility ROM.

Data memory allocation is important during system development if in-circuit debug is planned. The top 32-byte
memory location may be used by the debug service routine during debug mode. The data contents in these
locations may be altered and cannot be recovered.

One available stack location is needed for debug mode. If the stack is full when entering debug mode, the oldest
data in the stack will be overwritten.

Any signal sampling that relies upon the internal system clock (e.g., counter inputs) can be unreliable since the
system clock is turned off inside active debug mode between debug mode commands.