This schematic shows several ATtiny85 minimal circuit configurations. The ATtiny85 is shown using its internal 8 MHz RC
oscillator, which divided by 8 gives a 1 MHz system clock, and with an external crystal and ADC buffer. The ATtiny85 has
8kB flash, 512 bytes of SRAM, and 512 bytes of EEPROM.
The internal oscillator has a base accuracy of +/-10%, which is great for most applications, but if you are
doing any critical timing, a crystal can get the accuracy to +/- 0.001%. Hooking a crystal to an ATtiny85 will
use two of the four total ADC inputs. The reset line takes another, leaving no lines to select an SPI
device. This circuit takes advantage of the programmer's assertion of reset to disable the SPI EEPROM during
programming, and enable it when the programmer lets reset go. Of course this only works with a single
SPI device, but sometimes one is all you need.
