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ColdFire CF4e Core User’s Manual
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Real-Time Trace Support
into two consecutive 4-bit nibbles. Each nibble can either transmit information concerning
the processor’s execution status (PST) or debug data (DDATA). The processor status may
not be related to the current bus transfer, due to the decoupling FIFOs.
External development systems can use PSTDDATA outputs with an external image of the
program to completely track the dynamic execution path. This tracking is complicated by
any change in flow, especially when branch target address calculation is based on the
contents of a program-visible register (variant addressing). PSTDDATA outputs can be
configured to display the target address of such instructions in sequential nibble
increments across multiple processor clock cycles, as described in Section 11.3.1, “Begin
Execution of Taken Branch (PST = 0x5).” Four 32-bit storage elements form a FIFO
buffer connecting the processor’s high-speed local bus to the external development system
through PSTDDATA[7:0]. The buffer captures branch target addresses and certain data
values for eventual display on the PSTDDATA port, two nibbles at a time starting with the
least significant bit (lsb).
Execution speed is affected only when three storage elements contain valid data
to be
dumped to the PSTDDATA port. This occurs only when two values are captured
simultaneously in a read-modify-write operation. The core stalls until two FIFO entries
are available.
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