AN2104 Freescale Semiconductor, AN2104 Datasheet - Page 3

• Manufacturer Part Number: AN2104
• Description: Using Background Debug Mode
• Manufacturer: Freescale Semiconductor

AN2104
To communicate with the BDM on the part, two pins are used: BKGD and
GND. This method of serial interface is used to both send and receive
data. A special communications protocol is used that resynchronizes at
the beginning of each bit. By doing this, a greater frequency tolerance
for synchronization is allowed.
All bits are started with a falling edge signal that is initiated by the
external host. After the MCU sees this falling edge, it waits nine E-clock
cycles and then samples the level on the BKGD pin. The data is
transferred MSB (most significant bit) first at the rate of 16 E-clock cycles
per bit. The E-clock is defined as the SYSCLK divided by two.
The two types of BDM commands are:
When using hardware commands, the BDM is enabled, but not active,
and the user’s code is running. See
internal and external memory, which is accessible to the microcontroller,
to be read or written. This also includes on-chip I/O (input/output) and
control registers.
The control logic watches the bus for any free bus cycles that it can use
to execute the hardware command. By using the free bus cycles, the
CPU is not disturbed. If, however, a free cycle is not found within a
specified time, it will use a bus cycle, which temporarily freezes the CPU.
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Figure 2. BDM Tool Connector
BKGD
V
NC
FP
1
3
5
Table
2
4
6
GND
RESET
V
DD
1. These commands allow all
Theory of Operation
Application Note
3