AN2153 Freescale Semiconductor / Motorola, AN2153 Datasheet - Page 18

• Manufacturer Part Number: AN2153
• Description: A Serial Bootloader for Reprogramming the MC9S12DP256 FLASH Memory
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Bootloader
S-Record Format
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memory and requires just six CPU clock cycles to execute. For systems
operating at the maximum bus speed of 25.0 MHz, six bus cycles adds
only 240 ns to the interrupt latency. In most applications, this small
amount of additional time will not affect the overall performance of the
system.
The S-record object file format was designed to allow binary object code
and/or data to be represented in printable ASCII hexadecimal format
allowing easy transportation between computer systems and
development tools. For M68HC12 Family members supporting less than
64 Kbytes of address space, S1 records, which contain a 16-bit address,
are sufficient to specify the location in the device’s memory space where
code and/or data are to be loaded. The load address contained in the S1
record generally corresponds directly to the address of on-chip or
off-chip memory device. For M68HC12 devices that support an address
space greater than 64 Kbytes, S1 records are not sufficient.
Because the M68HC12 Family is a 16-bit microcontroller with a 16-bit
program counter, it cannot directly address a total of more than
64 Kbytes of memory. To enable the M68HC12 Family to address more
than 64 Kbytes of program memory, a paging mechanism was designed
into the architecture. Program memory space expansion provides a
window of 16-Kbyte pages that are located from $8000–$BFFF. An 8-bit
paging register, called the PPAGE register, provides access to a
maximum of 256, 16-Kbyte pages or 4 megabytes of program memory.
While there may never be any devices that contain this much on-chip
memory, the MC68HC812A4 is capable of addressing this much
external memory. In addition, the MC9S12DP256 contains 256 Kbytes
of on-chip FLASH residing in a 1MB address space.
While many high-level debuggers are capable of directly loading linked,
absolute binary object files into a target system’s memory, the
bootloader does not have that ability. The bootloader is only capable of
loading object files that are represented in the S-record format. Because
S1 records only contain a 16-bit address, they are inadequate to specify
a load address for a memory space greater than 64 Kbytes. S2 records,
which contain a 24-bit load address, were originally defined for loading
object files into the memory space of the M68000 Family. It would seem
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AN2153