SC9S12XS256J1MAA FREESCALE [Freescale Semiconductor, Inc], SC9S12XS256J1MAA Datasheet - Page 183

• Manufacturer Part Number: SC9S12XS256J1MAA
• Description: HCS12 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

compared to the serial communication rate. This protocol allows a great flexibility for the POD designers,
since it does not rely on any accurate time measurement or short response time to any event in the serial
communication.
Figure 5-12
instruction is used as an example. First, the 8-bit instruction opcode is sent by the host, followed by the
address of the memory location to be read. The target BDM decodes the instruction. A bus cycle is grabbed
(free or stolen) by the BDM and it executes the READ_BYTE operation. Having retrieved the data, the
BDM issues an ACK pulse to the host controller, indicating that the addressed byte is ready to be retrieved.
After detecting the ACK pulse, the host initiates the byte retrieval process. Note that data is sent in the form
of a word and the host needs to determine which is the appropriate byte based on whether the address was
odd or even.
Freescale Semiconductor
BKGD Pin
(Target MCU)
ACK Pulse
BDM Clock
BKGD Pin
Transmits
Last Command Bit
16th Tick of the
Target
shows the ACK handshake protocol in a command level timing diagram. The READ_BYTE
READ_BYTE
If the ACK pulse was issued by the target, the host assumes the previous
command was executed. If the CPU enters wait or stop prior to executing a
hardware command, the ACK pulse will not be issued meaning that the
BDM command was not executed. After entering wait or stop mode, the
BDM command is no longer pending.
Host
Figure 5-12. Handshake Protocol at Command Level
High-Impedance
Byte Address
Target
32 Cycles
Figure 5-11. Target Acknowledge Pulse (ACK)
S12XS Family Reference Manual, Rev. 1.09
BDM Decodes
the Command
Minimum Delay
From the BDM Command
NOTE
16 Cycles
BDM Executes the
READ_BYTE Command
Speedup Pulse
Target
BDM Issues the
ACK Pulse (out of scale)
(2) Bytes are
Retrieved
Background Debug Module (S12XBDMV2)
Host
Next Bit
Earliest
Start of
High-Impedance
Host
Command
New BDM
Target
183