mc9s12xd256 Freescale Semiconductor, Inc, mc9s12xd256 Datasheet - Page 586

• Manufacturer Part Number: mc9s12xd256
• Description: Hcs12x Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 15 Background Debug Module (S12XBDMV2)
Figure 15-10
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.
15.4.7
BDM commands that require CPU execution are ultimately treated at the MCU bus rate. Since the BDM
clock source can be asynchronously related to the bus frequency, when CLKSW = 0, it is very helpful to
provide a handshake protocol in which the host could determine when an issued command is executed by
the CPU. The alternative is to always wait the amount of time equal to the appropriate number of cycles at
the slowest possible rate the clock could be running. This sub-section will describe the hardware
handshake protocol.
The hardware handshake protocol signals to the host controller when an issued command was successfully
executed by the target. This protocol is implemented by a 16 serial clock cycle low pulse followed by a
brief speedup pulse in the BKGD pin. This pulse is generated by the target MCU when a command, issued
by the host, has been successfully executed (see
After the ACK pulse has finished: the host can start the bit retrieval if the last issued command was a read
command, or start a new command if the last command was a write command or a control command
(BACKGROUND, GO, GO_UNTIL or TRACE1). The ACK pulse is not issued earlier than 32 serial clock
cycles after the BDM command was issued. The end of the BDM command is assumed to be the 16th tick
of the last bit. This minimum delay assures enough time for the host to perceive the ACK pulse. Note also
that, there is no upper limit for the delay between the command and the related ACK pulse, since the
command execution depends upon the CPU bus frequency, which in some cases could be very slow
586
Start of Bit Time
Speedup Pulse
Target System
(Target MCU)
BDM Clock
BKGD Pin
BKGD Pin
Perceived
Drive and
Drive to
Host
Serial Interface Hardware Handshake Protocol
shows the host receiving a logic 0 from the target. Since the host is asynchronous to the
Figure 15-10. BDM Target-to-Host Serial Bit Timing (Logic 0)
MC9S12XDP512 Data Sheet, Rev. 2.17
10 Cycles
10 Cycles
Figure
15-11). This pulse is referred to as the ACK pulse.
High-Impedance
Host Samples
BKGD Pin
Speedup Pulse
Freescale Semiconductor
Next Bit
Earliest
Start of