AN2007 Motorola / Freescale Semiconductor, AN2007 Datasheet - Page 2

• Manufacturer Part Number: AN2007
• Description: Evaluating ColdFire in a 68K Target System: MC68340 Gateway Reference Design
• Manufacturer: Motorola / Freescale Semiconductor

Design Considerations
The ColdFire architectures' foundation in Motorola's 68000 architecture allows designers to take advantage
of the established tool support, code evolution, and engineering expertise. It uses a variable length RISC
instruction set to optimize both code density and allow one instruction to be issued every clock cycle where
possible. The ColdFire instruction set is a subset of the 68K instruction set, that is compatible at both
assembler and binary levels. The programming model is also identical to the 68K, with the exception that it
has a simplified stack pointer and exception stack frame.
The Gateway reference design which will be discussed is an integrated circuit board which will bridge an
existing MC68340 system to the ColdFire MCF5206 microprocessor, to evaluate the ease of upgrading to a
higher performance architecture. It can be used to evaluate system enhancements such as on-chip instruction
and/or data cache and bursting to external memory. It can also be used to port a customer’s system code to
the ColdFire architecture in situ as opposed to the traditional method of initially porting code to an
evaluation platform. This paper is intended to describe the use and operation of the Gateway board.
In using the Gateway solution, the 68K processor initialisation code has to be configured to run on the
internal register map of the ColdFire processor. Although the ColdFire architecture is derived from the
680x0 family with a simplified set of instructions and addressing modes, assembler programs while
straightforward to port to ColdFire may require some modification before they will run. To help customers
with this translation process, Motorola has funded the development of an assembler code converter -
PortASM/68K, written by MicroAPL Ltd. in the U.K.
(Consult http://www.mot.com/SPS/HPESD/tools/companion.html for more information on this tool.)
This utility will run on either PC’s (under DOS, Windows 3.x, Windows95/98 & Windows NT) or Sun
Workstations (under SunOS or Solaris) and is available free of charge via download from the web.
This converter not only converts the 68K assembler code to ColdFire assembler but analyses how the
original code operates allowing the analyser to produce optimized ColdFire code in two passes, rather than
just a straight translation.
1.1 Design Considerations
There are two important differences between the ColdFire MCF5206 Microprocessor and MCF68340. The
first is the absence of DMA functionality and the second is the lack of an extra 8-bit GPIO port on the
ColdFire device. MCF5206 only has one eight bit port that is multiplexed with the PST and DDATA debug
signals used for the background debug mode port (BDM). The MC68340 has two dedicated I/O ports, Port
A and Port B. Hence, the Gateway hardware must create an additional I/O port and 2 channel DMA module
to emulate the MC68340 functionality.
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MC68340 Gateway Reference Design