Microcontroller and flash memory integration

Electronic News, Feb 3, 1997 by John Bryant

A disadvantage of using external flash memory is that the microcontroller or DSP cannot execute out of the flash device that is being reprogrammed. And since external flash requires processor control for reprogramming, another external memory device may be required unless the processor can temporarily execute out of its internal RAM.

When a microcontroller or DSP has integrated flash, it may be reprogrammed in one of two ways depending on the processor. The most common method requires use of the processor I/O port and may require special circuitry to isolate the processor while it receives mode select commands, addresses, and data for the flash.

Alternatively, processors such as Atmel's AT89x series offer SPI programming capability. SPI allows the updating of integrated flash through a 4-pin connector. Because the AT89x-S's in-system-programming protocol is implemented in hardware, the CPU is held in reset while downloading new code. This eliminates the need to use an additional memory device.

John Bryant is microcontroller marketing director for Atmel.