MC9S12P128 Datasheet, PDF (145/564 Pages) Freescale Semiconductor, Inc – S12 Microcontrollers

Background Debug Module (S12SBDMV1)
BDM Clock
(Target MCU)
Host
Drive to
BKGD Pin
Target System
Speedup
Pulse
Perceived
Start of Bit Time
BKGD Pin
High-Impedance
R-C Rise
High-Impedance
High-Impedance
10 Cycles
10 Cycles
Host Samples
BKGD Pin
Figure 5-8. BDM Target-to-Host Serial Bit Timing (Logic 1)
Earliest
Start of
Next Bit
Figure 5-9 shows the host receiving a logic 0 from the target. Since the host is asynchronous to the target,
there is up to a one clock-cycle delay from the host-generated falling edge on BKGD to the start of the bit
time as perceived by the target. The host initiates the bit time but the target finishes it. Since the target
wants the host to receive a logic 0, it drives the BKGD pin low for 13 target clock cycles then briefly drives
it high to speed up the rising edge. The host samples the bit level about 10 target clock cycles after starting
the bit time.
BDM Clock
(Target MCU)
Host
Drive to
BKGD Pin
Target System
Drive and
Speedup Pulse
Perceived
Start of Bit Time
BKGD Pin
High-Impedance
Speedup Pulse
10 Cycles
10 Cycles
Host Samples
BKGD Pin
Figure 5-9. BDM Target-to-Host Serial Bit Timing (Logic 0)
Earliest
Start of
Next Bit
S12P-Family Reference Manual, Rev. 1.12
Freescale Semiconductor
145