Block Guide — S12BDM V4

22

Figure 4-1 represents the BDM command structure. The command blocks illustrate a series of eight bit

times starting with a falling edge. The bar across the top of the blocks indicates that the BKGD line idles

in the high state. The time for an 8-bit command is 8

Figure 4-1 BDM Command Structure

4.6 BDM Serial Interface

The BDM communicates with external devices serially via the BKGD pin. During reset, this pin is a mode

select input which selects between normal and special modes of operation. After reset, this pin becomes

the dedicated serial interface pin for the BDM.

The BDM serial interface is timed using the clock selected by the CLKSW bit in the status register see

3.1 BDM Status Register. This clock will be referred to as the target clock in the following explanation.

The BDM serial interface uses a clocking scheme in which the external host generates a falling edge on

the BKGD pin to indicate the start of each bit time. This falling edge is sent for every bit whether data is

transmitted or received. Data is transferred most significant bit (MSB) first at 16 target clock cycles per

bit. The interface times out if 512 clock cycles occur between falling edges from the host.

The BKGD pin is a pseudo open-drain pin and has an weak on-chip active pull-up that is enabled at all

times. It is assumed that there is an external pull-up and that drivers connected to BKGD do not typically

drive the high level. Since R-C rise time could be unacceptably long, the target system and host provide

NOTES:

1. Target clock cycles are cycles measured using the target MCU’s serial clock rate. See 4.6 BDM Serial Interface and

3.1 BDM Status Register for information on how serial clock rate is selected.

HARDWARE

HARDWARE

FIRMWARE

FIRMWARE

GO,

44-BC

BC = BUS CLOCK CYCLES

COMMAND

ADDRESS

150-BC

DELAY

NEXT

DELAY

8 BITS

AT

∼16 TC/BIT

16 BITS

AT

∼16 TC/BIT

16 BITS

AT

∼16 TC/BIT

COMMAND

ADDRESS

DATA

NEXT

DATA

READ

WRITE

READ

WRITE

TRACE

COMMAND

NEXT

COMMAND

DATA

64-BC

DELAY

NEXT

COMMAND

150-BC

DELAY

32-BC

DELAY

COMMAND

COMMAND

COMMAND

COMMAND

DATA

NEXT

COMMAND

TC = TARGET CLOCK CYCLES