XC912BC32CFU8 Motorola Semiconductor Products, XC912BC32CFU8 Datasheet - Page 317

• Manufacturer Part Number: XC912BC32CFU8
• Description: M68HC12B Family Data Sheet
• Manufacturer: Motorola Semiconductor Products

M68HC12B Family — Rev. 8.0
MOTOROLA
TARGET MCU
PERCEIVED
TRANSMIT 1
TRANSMIT 0
OF BIT TIME
E CLOCK
START
HOST
HOST
SYNCHRONIZATION
UNCERTAINTY
The BKGD pin can receive a high or low level or transmit a high or low level.
Figure
Interface timing is synchronous to MCU clocks but asynchronous to the external
host. The internal clock signal is shown for reference in counting cycles.
Figure 18-1
a target M68HC12 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 E 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. Since the target
does not drive the BKGD pin during this period, there is no need to treat the line as
an open-drain signal during host-to-target transmissions.
Figure 18-2
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 E 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 18-1. BDM Host to Target Serial Bit Timing
18-1,
shows the host receiving a logic 1 from the target MCU. Since the host
shows an external host transmitting a logic 1 or 0 to the BKGD pin of
Figure
10 CYCLES
Development Support
18-2, and
TARGET SENSES BIT
Figure 18-3
show timing for each of these cases.
Background Debug Mode (BDM)
Development Support
EARLIEST
START OF
NEXT BIT
Data Sheet
317