The Motorola HCS12 family of micro-controllers was designed primarily for operation in Single-Chip mode. In Single-Chip mode all the peripherals are found either internally in the HCS12 silicon, or can be connected to the HCS12 through one of its serial interfaces like the SPI, SCI or IIC. In practice over 95% of HCS12 based designs use Single-Chip mode.

Despite the above, an increasing number of HCS12 applications need to access an external memory from the HCS12 CPU, by using the HCS12 in Expanded mode. Since Motorola did not target the HCS12 family for extensive use in expanded modes, operation in this mode is not very easy to meet. In fact, the expanded bus of the HCS12 poses some significant obstacles from the hardware-design perspective. These obstacles need to be resolved in the board design stage (rather than in the board debugging stage) in order to allow proper operation of the HCS12 micro-controller with the expanded mode external components.

Chapter 1 of this guide explains the difficulties of designing reliable HCS12 expanded mode applications, and suggests how to resolve these difficulties. This chapter is general and can benefit all HCS12 Expanded Mode designers, including ones that don’t use the Nohau HCS12 emulator or BDM.
Chapter 2 explains how to connect an expanded-mode target to a Nohau HCS12 full- Emulator for debugging. Some aspects need to be considered to allow such connection, and are discussed in details in this chapter.

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