June 2000, Issue 119

The Chips are Alive with the Sound of Music
Imitatiing the Dead Melody IC

SECOND COMPARISON

Many programs require table lookups. Microchip’s low- and mid-range processors have a single 8-bit indirect register for table lookups. This requires special handling when the table gets larger than 255 entries or it crosses a page boundary. A smaller word means smaller die sizes, and that spells savings for the customer. However, special handling can be confusing when working with larger tables. Because Atmel uses two 8-bit registers (16-bits) for indirect addressing, table lookups are easier to manage.

Atmel states program memory in 8-bit bytes. Because instructions are in words, you must adjust the program size accordingly (divide by two). However, table values are stacked two to a word. Microchip states program memory by instruction width (word), such that an instruction requires one program word. But, each table value also requires a full word. There are pros and cons to each methodology, therefore it is important to know about each processor so you can choose the best one for your design.