Designing Embedded Hardware, 2nd Edition

Downloading Code

The AVR processors use internal flash memory for program storage, and this may be programmed in-circuit or, in the case of socketed components, out of circuit as well. The AVR processors are reprogrammed via a SPI port on the chip. Even AVR processors such as the ATtiny15, which do not have a SPI interface for their own use, still incorporate a SPI port for reprogramming. The pins PB0, PB1, and PB2 take on SPI functions (MOSI, MISO, and SCK) during programming.

VCC can be supplied by the external programmer downloading the code. For programming, VCC must be 5 V. If the embedded system's local supply will provide 5 V, then the connection to the programmer's VCC may be left unmade. However, if the embedded system's supply voltage is something other than 5 V, the programmer's VCC must be used, and any local power source within the embedded system should be disabled. plays an important role in downloading code. Programming begins with being asserted (driven low). This disables the CPU within the processor and thus allows access to the internal memory. It also changes the functionality of PB0, PB1, and PB2 to a SPI interface. The development software then sends, via the SPI interface, a sequence of codes to "unlock" the program memory and enable software to be downloaded. ...