So far, we have looked at a small AVR with very limited capabilities. In Part III of this book, 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 and so now need to move on to processors with a bit more “grunt.” Before getting into the detail of I/O in the later chapters, I’ll introduce these processors to you and show you what you need to do to include them in your design.
The first processor is the ATMEL AT90S8535. This is a midrange AVR with lots of built-in I/O. As well as digital I/O, it has a variety of interfaces such as a serial port, SPI, analog input, timers, and counters. We’ll talk about some of these interfaces in detail in later chapters, but for the moment, we’ll concentrate on the processor itself.
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 way except that it has smaller memory spaces of 4K for program storage and 256 bytes of RAM. But from an electronics 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 6-10. It is not that different from the ATtiny15, save that it has a lot more pins. RESET has an external 10k pull-up resistor. The processor has an external crystal (X1), and this requires two small bypass capacitors, C1 and C2. There are four power pins for this processor, and each is decoupled with a 100nF ceramic capacitor. One of the power inputs (AVCC) is the power supply for the analog section of the chip, and this is isolated from the digital power supply by a 100Ω resistor, R2. This is to provide a small barrier between the analog section and any switching noise that may be present from the digital circuits. The remaining pins are general-purpose digital I/O, as with the ATtiny15. However, unlike the ATtiny15, these pins have dual functionality. They may be configured, under software control, for alternative I/O functions. The processor’s datasheet gives full details for configuring the functionality of the processor under software control.
That basic AVR design is applicable to most AVRs that you will find. The pinouts may be different, but the basic support required is the same. As with everything, grab the appropriate datasheet, and it will tell you the specifics for the particular processor that you are using.