Internal Memory Configuration and External Expansion - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 734

Microcontroller External memory

Address the memory

1. Set R/W to write
2. Place address on bus
3. Assert LSTRB if address is odd
4. Assert ECS and XCS if in range
5. Place data on data bus

Input the data

1. Decode the address
2. Store data from data bus

Terminate write data
1. Negate LSTRB if asserted
2. Negate ECS and XCS if asserted
3. Remove data from data bus

Terminate the write cycle

1. Release data to HI-Z

Start the next cycle

Figure 14.34 ■ Sequence of events occurring in an HCS12 write bus cycle

14.8.1 Memory Space Assignment
Any space unoccupied by the on-chip flash memory can be assigned to external memory
devices. When making a memory space assignment, the designer has two options to choose from.
• Equal size assignment . In this method, the available memory space is divided into
blocks of equal size and then each block is assigned to a memory device without
regard for the actual size of each memory-mapped device. A memory-mapped
device could be a memory chip or a peripheral device. Memory space tends to be
wasted using this approach because most memory-mapped peripheral chips need
only a few bytes to be assigned to their internal registers.
• Demand assignment . In this approach, the designer assigns the memory space
according to the size of the memory devices.