Freescale Semiconductor MC9S08PT60, Справочное руководство

Страница: 633 / 679

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.
E A R LIE S T S TA R T
T A R G E T S E N S E S B IT LE V E L
10 C Y C LE S
S Y N C H R O N IZ A T IO N
U N C E R T A IN TY
B D C C LO C K
(TA R G E T M C U )
H O S T
T R A N S M IT 1
H O S T
T R A N S M IT 0
P E R C E IV E D S T A R T
O F B IT TIM E
O F N E X T B IT
Figure 24-2. BDC host-to-target serial bit timing
The next figure 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 speedup 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.
H O S T S A M P LE S B K G D P IN
10 C Y C LE S
B D C C LO C K
(T A R G E T M C U )
H O S T D R IV E
T O B K G D P IN
T A R G E T M C U
S P E E D U P P U LS E
P E R C E IV E D S TA R T
O F B IT T IM E
H IG H -IM P E D A N C E
H IG H -IM P E D A N C E
H IG H -IM P E D A N C E
B K G D P IN
R -C R IS E
10 C Y C LE S
E A R LIE S T S T A R T
O F N E X T B IT
Figure 24-3. BDC target-to-host serial bit timing (logic 1)
Chapter 24 Development support
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
Freescale Semiconductor, Inc. 633