32.9 Serial Downloading

Both the Flash and EEPROM memory arrays can be programmed using the serial SPI bus while RESET is pulled to GND. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After RESET is set low, the Programming Enable instruction needs to be executed first before the program/erase operations can be executed.

When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation (in the Serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip Erase operation turns the content of every memory location in both the Program and EEPROM arrays into 0xFF.

Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the serial clock (SCK) input are defined as follows:

• Low: > 2 CPU clock cycles for f_ck < 12 MHz, 3 CPU clock cycles for f_ck ≥ 12 MHz
• High: > 2 CPU clock cycles for f_ck < 12 MHz, 3 CPU clock cycles for f_ck ≥ 12 MHz