MC9S08SH32 Datasheet, PDF (269/328 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 17 Development Support
Figure 17-2 shows an external host transmitting a logic 1 or 0 to the BKGD pin of a target HCS08 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 transmission 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 17-2. BDC Host-to-Target Serial Bit Timing
MC9S08SH32 Series Data Sheet, Rev. 2
Freescale Semiconductor
269
PRELIMINARY