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MC1321x Reference Manual, Rev. 1.6
21-4
Freescale Semiconductor
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.
Figure 21-2. BDC Host-to-Target Serial Bit Timing
shows the host receiving a logic 1 from the target HCS08 MCU. Because the host is
asynchronous to the target MCU, there is a 0-to-1 cycle delay from the host-generated falling edge on
BKGD to the perceived start of the bit time in the target MCU. The host holds the BKGD pin low long
enough for the target to recognize it (at least two target BDC cycles). The host must release the low drive
before the target MCU drives a brief active-high speed-up pulse seven cycles after the perceived start of
the bit time. The host should sample the bit level about 10 cycles after it started the bit time.
Figure 21-3. BDC Target-to-Host Serial Bit Timing (Logic 1)
EARLIEST START
TARGET SENSES BIT LEVEL
10 CYCLES
SYNCHRONIZATION
UNCERTAINTY
BDC CLOCK
(TARGET MCU)
HOST
TRANSMIT 1
HOST
TRANSMIT 0
PERCEIVED START
OF BIT TIME
OF NEXT BIT
HOST SAMPLES BKGD PIN
10 CYCLES
BDC CLOCK
(TARGET MCU)
HOST DRIVE
TO BKGD PIN
TARGET MCU
SPEEDUP PULSE
PERCEIVED START
OF BIT TIME
HIGH-IMPEDANCE
HIGH-IMPEDANCE
HIGH-IMPEDANCE
BKGD PIN
R-C RISE
10 CYCLES
EARLIEST START
OF NEXT BIT
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