As MCU performance levels increase, demand for external memory expansion increases.
If you learn the first design implementation, it will be more accessible.

The development of the Internet of Things, lighting, building automation, capacitive touch sensing, smart mirroring, etc. has increased the use of microcontroller units (MCUs) and expanded new markets. As a result, the demand for MCU memory has also increased, and the density of internal memory has continuously increased, and the demand for external memory expansion has also increased.

The traditional role of microcontrollers is to assist the main processor and control peripheral devices, and the focus is on structural simplicity, easy control, and low power. The current trend is to develop performance to the point where it can replace the design that the main processor handles while maintaining the traditional role as much as possible.

The greatest demand for MCUs occurs when they are driven in a GUI environment. In addition, external memory implementation is necessary when a network node that collects a large amount of data and uploads it periodically, or a temporary data storage facility in the form of FIFOs is required to cope with high-speed network loads. Accordingly, the MCU market is moving toward a direction that demands high storage capacity and low price.

MCU external memory expansion is divided into Serial Interface Memory and Parallel Interface Memory. Serial memory is represented by Serial Flash and EEPROM, which use conventional general-purpose serial interfaces such as SPI or I2C. Quad-SPI and SD card memory are also extensions of serial communication from a circuit perspective.


Parallel memory is a typical form of extended memory, in which both address lines and data lines for accessing the basic memory unit are exposed to an external interface. In order to implement this type of memory in a form in which all exposed address lines are mapped to a part of the processor memory map, it must be designed by considering the entire memory map inside and outside the processor. In addition, address decoding is required to connect or expand multiple parallel memory devices externally.

STMicroelectronics MCU FAE Deputy Manager Taejong Jang explained the difficulty of memory expansion, saying, “In order to support various functions, Quad-SPI and SD card memory among serial memories require understanding of complex protocols, as well as analysis and understanding of the software stack for dedicated hardware control related to this.”

Parallel memory is also difficult to expand because it “requires characteristics and design considerations for connecting to various memories, and the many control lines and address/data lines on the circuit make debugging difficult,” he added.

The MCU called FMC (Flecible Memory Controller) of the STM32 line provides functions by dividing parallel memory into Static Memory, Dynamic Memory, and NAND Flash Memory. It supports SRAM, NOR flash, PSRAM, etc., which have traditional SRAM interface forms as static memory, and SDRAM, referred to as DRAM, as dynamic memory. In particular, it supports connection of NAND flash memory, which is widely used for storage purposes.

ST's MCUs can reduce bottlenecks between the high-speed MCU core and the slow-speed built-in flash memory with the built-in flash acceleration technology called ART accelerator. In addition, they have excellent low-power and high-temperature characteristics and are highly versatile because they use the ARM Cortex-M architecture.

“Most types of memory have clear specifications described in industry standards organizations. Like other technologies such as RTOS, USB, and TCP/IP that are based on basic theories and standards, once you gain experience and learn from the initial design implementation, it will be easier to do it later,” said Vice President Jang, trying to ease the difficulties of external memory expansion.

Accordingly, in the online seminar (eeWebinar) to be held on the 11th, we will cover the overview and usage of Quad-SPI and FMC included in STM32 MCU, as well as details on external memory expansion.