Chapter 15 Background Debug Module (S12XBDMV2)
MC9S12XDP512 Data Sheet, Rev. 2.17
582
Freescale Semiconductor
15.4.5
BDM Command Structure
Hardware and rmware BDM commands start with an 8-bit opcode followed by a 16-bit address and/or a
16-bit data word depending on the command. All the read commands return 16 bits of data despite the byte
or word implication in the command name.
8-bit reads return 16-bits of data, of which, only one byte will contain valid
data. If reading an even address, the valid data will appear in the MSB. If
reading an odd address, the valid data will appear in the LSB.
16-bit misaligned reads and writes are generally not allowed. If attempted
by BDM hardware command, the BDM will ignore the least signicant bit
of the address and will assume an even address from the remaining bits.
The following cycle count information is only valid when the external wait
function is not used (see wait bit of EBI sub-block). During an external wait
the BDM can not steal a cycle. Hence be careful with the external wait
function if the BDM serial interface is much faster than the bus, because of
For hardware data read commands, the external host must wait at least 150 bus clock cycles after sending
the address before attempting to obtain the read data. This is to be certain that valid data is available in the
BDM shift register, ready to be shifted out. For hardware write commands, the external host must wait
150 bus clock cycles after sending the data to be written before attempting to send a new command. This
is to avoid disturbing the BDM shift register before the write has been completed. The 150 bus clock cycle
delay in both cases includes the maximum 128 cycle delay that can be incurred as the BDM waits for a
free cycle before stealing a cycle.
For rmware read commands, the external host should wait at least 48 bus clock cycles after sending the
command opcode and before attempting to obtain the read data. This includes the potential of extra cycles
when the access is external and stretched (+1 to maximum +7 cycles) or to registers of the PRU (port
replacement unit) in emulation mode. The 48 cycle wait allows enough time for the requested data to be
made available in the BDM shift register, ready to be shifted out.
NOTE
This timing has increased from previous BDM modules due to the new
capability in which the BDM serial interface can potentially run faster than
the bus. On previous BDM modules this extra time could be hidden within
the serial time.
For rmware write commands, the external host must wait 36 bus clock cycles after sending the data to be
written before attempting to send a new command. This is to avoid disturbing the BDM shift register
before the write has been completed.