In many designs, the system requirements call for programmable non-volatile memory for data storage; a bootstrap device for a microcontroller or DSP; or a configuration memory device for a FPGA. Depending on your application, the type of memory you select can vary from design to design, although multiple memory solutions are usually possible.
The system design usually specifies a serial or parallel interface, dependent upon the application requirements and your overall system costs.
For serial memory, a serial EEPROM or serial Flash could be selected, while a parallel EEPROM or parallel Flash is available to meet the parallel memory requirements.
For space-limited applications, a serial device uses fewer pins and is usually the best choice; however, if you are already using an external parallel memory device in the system design for program memory, you have the option to use the same memory device for both program and data memory. Each memory type has its own pros and cons, but the final selection is usually based on the importance of the overall system costs, memory density, battery life and the system's functional requirements.
Serial bootstrap PROM
For applications requiring an external boot PROM memory device that is programmable, using serial or parallel memory depends on the microcontroller or DSP selected. If the interface is serial, your memory selection could be either a serial EEPROM or a serial Flash device. Microcontrollers or DSPs with high-speed requirements can then boot from a serial interface and download the firmware into a separate high-speed volatile memory device such as SRAM or DRAM, for system operation. Using this technique, the operating speed or bus width of the processor is not limited by the non-volatile serial memory device.
Memory density and cost can be the deciding factor between using a serial EEPROM or a serial Flash device. If the memory density requirement is below 512 Mbits, a serial EEPROM can be selected, since there are no serial Flash devices available at these lower densities. At densities above 1 Mbit, there are no serial EEPROMs available, so you would select a serial Flash device. At densities of 512 Kbits and 1 Mbit, both memory types are available, and your decision would be based upon the functionality you require and your final system costs.
In FPGA designs, serial interface memory is used for configuration on power-up, since various densities are used. Depending upon the specific FPGA you select, the type of memory available could be limited by the serial interface. Today, FPGAs are designed with either an SPI interface or an interface that is specifically designed for FPGAs. In either case, your decision to use a serial EEPROM or serial Flash device is still based upon the density required and device costs.
Serial or parallel data memory
When programmable non-volatile data memory is needed in your design, you could select serial or parallel memory. Of the various types of memory available, your design could use a serial EEPROM, serial Flash, parallel EEPROM or a parallel Flash device. Whether you specify serial or parallel memory depends on the types of serial ports or spare I/O ports available in the microcontroller, the availability of an external parallel Flash device for program memory and the overall system design costs.
Consider whether the program memory should reside internal or external to the microcontroller in your system design. If it resides in an external parallel memory device, are there spare memory sectors available in the device for data memory? If there are not, an external serial memory device would be necessary. If sectors are available for data memory, the system requirements should be reviewed to determine if it is an option to use this memory. These requirements include memory endurance, battery life and the amount of available SRAM in the design. The temperature profile of the system must also be taken into account, since it affects these requirements.
* Typical endurance levels for parallel Flash devices are 10 000 cycles, serial Flash devices can range from 10 000 to 100 000 cycles and serial EEPROMs can exceed 1 million cycles. These levels can decrease substantially as temperature increases, so you might want to be conservative when making a decision based upon endurance.
