MC9RS08KA2 Datasheet, PDF (98/132 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 12 Development Support
12.3.2 Communication Details
The BDC serial interface requires the host to generate a falling edge on the BKGD pin to indicate the start
of each bit time. The host provides this falling edge whether data is transmitted or received.
The BDC serial communication protocol requires the host to know the target BDC clock speed.
Commands and data are sent most significant bit first (MSB-first) at 16 BDC clock cycles per bit. The
interface times out if 512 BDC clock cycles occur between falling edges from the host. Any BDC
command that was in progress when this timeout occurs is aborted without affecting the memory or
operating mode of the target MCU system.
Figure 12-3 shows an external host transmitting a logic 1 or 0 to the BKGD pin of a target MCU. The host
is asynchronous to the target so there is a 0-to-1 cycle delay from the host-generated falling edge to where
the target perceives the beginning of the bit time. Ten target BDC clock cycles later, the target senses the
bit level on the BKGD pin. Typically, the host actively drives the pseudo-open-drain BKGD pin during
host-to-target transmissions to speed up rising edges. Because the target does not drive the BKGD pin
during the host-to-target period, there is no need to treat the line as an open-drain signal during this period.
BDC CLOCK
(TARGET MCU)
HOST
TRANSMIT 1
HOST
TRANSMIT 0
SYNCHRONIZATION
UNCERTAINTY
PERCEIVED START
OF BIT TIME
10 CYCLES
TARGET SENSES BIT LEVEL
EARLIEST START
OF NEXT BIT
Figure 12-3. BDC Host-to-Target Serial Bit Timing
MC9RS08KA2 Series Data Sheet, Rev. 2
98
Freescale Semiconductor