Designing Embedded Hardware, 2nd Edition

A Bigger AVR

So far, we have looked at a small AVR with very limited capabilities. In later chapters, we will look at various forms of input and output commonly found in embedded systems. For this, we will need processors with more functionality. We have exhausted the ATtiny15, so now we need to move on to processors with a bit more "grunt." Before getting into the details of I/O in the later chapters, you'll be introduced to these processors and learn what you need to do to include them in your design.

The first processor is the Atmel AT90S8535 . This is a mid-range AVR with lots of inbuilt I/O, such as a UART, SPI, timers, eight channels of analog input, an analog comparator, and internal EEPROM for parameter storage. The processor has 512 bytes of internal RAM and 8K of flash memory for program storage. Its smaller sibling, the AT90S4434, is identical in every other way except that it has smaller memory spaces of 4K for program storage and 256 bytes of RAM. But from a hardware point of view, the AT90S8535 and the AT90S4434 are the same.

The basic schematic for an AT90S8535-based computer, without any extras, is shown in Figure 15-10. It is not that different from the ATtiny15, save that it has a lot more pins. has an external pull-up 10k resistor. The processor has an external crystal (X1), and this requires two small decoupling capacitors, C1 and C2. There are four power ...