HCS08 datasheet(279/316 Pages) NXP

279 / 316 page

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

279

17.4.3.5

Debug FIFO High Register (DBGFH)

This register provides read-only access to the high-order eight bits of the FIFO. Writes to this register have

no meaning or effect. In the event-only trigger modes, the FIFO only stores data into the low-order byte

of each FIFO word, so this register is not used and will read 0x00.

Reading DBGFH does not cause the FIFO to shift to the next word. When reading 16-bit words out of the

FIFO, read DBGFH before reading DBGFL because reading DBGFL causes the FIFO to advance to the

next word of information.

17.4.3.6

Debug FIFO Low Register (DBGFL)

This register provides read-only access to the low-order eight bits of the FIFO. Writes to this register have

no meaning or effect.

Reading DBGFL causes the FIFO to shift to the next available word of information. When the debug

module is operating in event-only modes, only 8-bit data is stored into the FIFO (high-order half of each

FIFO word is unused). When reading 8-bit words out of the FIFO, simply read DBGFL repeatedly to get

successive bytes of data from the FIFO. It isn’t necessary to read DBGFH in this case.

Do not attempt to read data from the FIFO while it is still armed (after arming but before the FIFO is filled

or ARMF is cleared) because the FIFO is prevented from advancing during reads of DBGFL. This can

interfere with normal sequencing of reads from the FIFO.

Reading DBGFL while the debugger is not armed causes the address of the most-recently fetched opcode

to be stored to the last location in the FIFO. By reading DBGFH then DBGFL periodically, external host

software can develop a profile of program execution. After eight reads from the FIFO, the ninth read will

return the information that was stored as a result of the first read. To use the profiling feature, read the FIFO

eight times without using the data to prime the sequence and then begin using the data to get a delayed

picture of what addresses were being executed. The information stored into the FIFO on reads of DBGFL

(while the FIFO is not armed) is the address of the most-recently fetched opcode.

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