Internal Memory Configuration and External Expansion - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 733

Microcontroller External memory

Address the memory

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

Output the data

1. Decode the address
2. Place data on data bus

Acquire the data
1. Latch the data
2. Negate LSTRB if asserted
3. Negate ECS and XCS if asserted

Terminate the read cycle

1. Remove data from data
bus and release to HI-Z

Start the next cycle

Figure 14.33 ■ Sequence of events occurring in an HCS12 read bus cycle

14.8 Issues Related to Adding External Memory

Adding external memory becomes necessary when the application gets larger than the on-
chip memory. When external memory is added, the designer should also consider treating ex-
ternal peripheral chips (when possible) as memory devices because it makes the programming
of the peripheral chips easier. When adding external memory, there are three issues that need to
be considered.
• Memory space assignment
• Address decoder and control circuitry design
• Timing verification