Chapter 5. Debug Support
5-17
Background Debug Mode (BDM)
2. A hardware breakpoint can be congured to generate a pending halt condition
similar to the assertion of BKPT. This type of halt is always rst made pending in
the processor. Next, the processor samples for pending halt and interrupt conditions
once per instruction. When a pending condition is asserted, the processor halts
3. The execution of a HALT instruction immediately suspends execution. Attempting
to execute HALT in user mode while CSR[UHE] = 0 generates a privilege violation
exception. If CSR[UHE] = 1, HALT can be executed in user mode. After HALT
executes, the processor can be restarted by serial shifting a GO command into the
debug module. Execution continues at the instruction after HALT.
4. The assertion of the BKPT input is treated as a pseudo-interrupt; that is, the halt
condition is postponed until the processor core samples for halts/interrupts. The
processor samples for these conditions once during the execution of each
instruction. If there is a pending halt condition at the sample time, the processor
suspends execution and enters the halted state.
The assertion of BKPT should be considered in the following two special cases:
After the system reset signal is negated, the processor waits for 16 processor clock
cycles before beginning reset exception processing. If the BKPT input is asserted
within eight cycles after RSTI is negated, the processor enters the halt state,
signaling halt status (0xF) on the PST outputs. While the processor is in this state,
all resources accessible through the debug module can be referenced. This is the
only chance to force the processor into emulation mode through CSR[EMU].
After system initialization, the processor’s response to the GO command depends on
the set of BDM commands performed while it is halted for a breakpoint.
Specically, if the PC register was loaded, the GO command causes the processor to
exit halted state and pass control to the instruction address in the PC, bypassing
normal reset exception processing. If the PC was not loaded, the GO command
causes the processor to exit halted state and continue reset exception processing.
The ColdFire architecture also handles a special case of BKPT being asserted while
the processor is stopped by execution of the STOP instruction. For this case, the
processor exits the stopped mode and enters the halted state, at which point, all BDM
commands may be exercised. When restarted, the processor continues by executing
the next sequential instruction, that is, the instruction following the STOP opcode.
CSR[27–24] indicates the halt source, showing the highest priority source for multiple halt
conditions.
5.5.2 BDM Serial Interface
When the CPU is halted and PST reects the halt status, the development system can send
unrestricted commands to the debug module. The debug module implements a synchronous
protocol using two inputs (DSCLK and DSI) and one output (DSO), where DSCLK and
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.