
Background-Debug Mode (BDM)
MOTOROLA
Debug Support
19-11
19.3.3.3
Command Sequence Diagram
A command sequence diagram (see
Figure 19-4) shows the serial bus traffic for each command. Each
bubble in the diagram represents a single 17-bit transfer across the bus. The top half in each bubble
corresponds to the data transmitted by the development system to the debug module; the bottom half
Table 19-8 BDM Command Format
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
OPERATION
0
R/W
OP SIZE
0
A/D
REGISTER
EXTENSION WORD(S)
Table 19-9 BDM Bit Descriptions
BIT NAME
DESCRIPTION
Operation Field
The operation field specifies the command.
R/W Field
The R/W field specifies the direction of operand transfer. When the bit
is set, the transfer is from the CPU to the development system. When
the bit is cleared, data is written to the CPU or to memory from the
development system.
Operand Size
For sized operations, this field specifies the operand data size. All
addresses are expressed as 32-bit absolute values. The size field is
Address / Data (A/D)
Field
The A/D field is used in commands that operate on address and data
registers in the processor. It determines whether the register field
specifies a data or address register. A one indicates an address
register; zero, a data register.
Register Field
In commands that operate on processor registers, this field specifies
which register is selected. The field value contains the register
number.
Extension Word(s)
(as required)
Certain commands require extension words for addresses and/or
immediate data. Addresses require two extension words because only
absolute long addressing is permitted. Immediate data can be either
one or two words in length—byte and word data each require a single
extension word; longword data requires two words. Both operands and
addresses are transferred most significant word first. In the following
descriptions of the BDM command set, the optional set of extension
words is defined as “Address,” “Data,” or “Operand Data.”
Table 19-10 BDM Size Field Encoding
ENCODING
OPERAND SIZE
BIT VALUES
00
Byte
8 bits
01
Word
16 bits
10
Longword
32 bits
11
Reserved
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