MC9S12VR-Family - Data Sheet - Freescale Semiconductor

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
10 Cycles
10 Cycles
Host Samples
BKGD Pin
Figure 5-8. BDM Target-to-Host Serial Bit Timing (Logic 1)
MC9S12VR Family Reference Manual, Rev. 2.8
Background Debug Module (S12SBDMV1)
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
10 Cycles
10 Cycles
High-Impedance
High-Impedance
Host Samples
BKGD Pin
Figure 5-9. BDM Target-to-Host Serial Bit Timing (Logic 0)
High-Impedance
Speedup Pulse
Earliest
Start of
Next Bit
Earliest
Start of
Next Bit
Freescale Semiconductor 189