Block Guide — S12BDM V4

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4.4 Standard BDM Firmware Commands

Firmware commands are used to access and manipulate CPU resources. The system must be in active

BDM to execute standard BDM firmware commands, see 4.2 Enabling and Activating BDM. Normal

instruction execution is suspended while the CPU executes the firmware located in the standard BDM

firmware lookup table. The hardware command BACKGROUND is the usual way to activate BDM.

As the system enters active BDM, the standard BDM firmware lookup table and BDM registers become

visible in the on-chip memory map at $FF00–$FFFF, and the CPU begins executing the standard BDM

firmware. The standard BDM firmware watches for serial commands and executes them as they are

received.

The firmware commands are shown in Table 4-2.

Table 4-2 Firmware Commands

NOTES:

1. If enabled, ACK will occur when data is ready for transmission for all BDM READ commands and will occur after the write

is complete for all BDM WRITE commands.

Opcode

(hex)

Data

Description

READ_NEXT

62

16-bit data out

Increment X by 2 (X = X + 2), then read word X points to.

READ_PC

63

16-bit data out

Read program counter.

READ_D

64

16-bit data out

Read D accumulator.

READ_X

65

16-bit data out

Read X index register.

READ_Y

66

16-bit data out

Read Y index register.

READ_SP

67

16-bit data out

Read stack pointer.

WRITE_NEXT

42

16-bit data in

Increment X by 2 (X = X + 2), then write word to location

pointed to by X.

WRITE_PC

43

16-bit data in

Write program counter.

WRITE_D

44

16-bit data in

Write D accumulator.

WRITE_X

45

16-bit data in

Write X index register.

WRITE_Y

46

16-bit data in

Write Y index register.

WRITE_SP

47

16-bit data in

Write stack pointer.

GO

08

none

Go to user program. If enabled, ACK will occur when

leaving active background mode.

2. Both WAIT (with clocks to the S12 CPU core disabled) and STOP disable the ACK function. The GO_UNTIL command

will not get an Acknowledge if one of these two CPU instructions occurs before the "UNTIL" instruction. This can be a prob-

lem for any instruction that uses ACK, but GO_UNTIL is a lot more difficult for the development tool to time-out.

0C

none

Go to user program. If enabled, ACK will occur upon

returning to active background mode.

TRACE1

10

none

Execute one user instruction then return to active BDM. If

enabled, ACK will occur upon returning to active

background mode.

TAGGO

18

none

Enable tagging and go to user program. There is no ACK

pulse related to this command.