Reliable High-Density Flash Memory Enables Fast Boot for Embedded Industrial and Automotive Systems

Embedded systems in industrial, automotive, and consumer markets need non-volatile memory that stores firmware, configuration data, user preferences, and security credentials, even when power is removed. The memory must also be configured to allow an embedded controller reliable and fast access to the data while fitting in the available printed circuit board (PCB) space. Once deployed, a memory package must function securely over a wide range of temperatures and hundreds of thousands of read/write cycles over decades.

NOR Flash memory has emerged as a flexible and reliable non-volatile memory format that can meet the challenges of embedded systems. This article will review non-volatile memory types, communications configurations, PCB layouts, and reliability considerations for implementing non-volatile memory in embedded systems.

Memory matters

Two of the most common formats in modern embedded systems are NOR Flash and NAND Flash, each named for its structural resemblance to a logic gate. NAND Flash memory cells are arranged in series. This arrangement lends itself to greater volumes of data and higher storage densities for a lower cost per bit. NAND Flash is often found in vehicle infotainment systems, where it can store music, navigation, graphics, and other data-intensive files in a space- and cost-efficient way.

With its parallel arrangement of memory cells, NOR Flash allows faster random access to stored data. It is physically larger than NAND Flash, and more costly per bit, so it is usually chosen for applications like firmware that require less storage but need greater random-access speed.

That fast random access means NOR-Flash-based embedded systems are quick to start up and react to the operating environment. NOR Flash supports execute-in-place (XIP) operations, which allow processors to access embedded code directly from non-volatile Flash memory.

NOR Flash also supports code shadowing, in which code and data are copied into volatile random access memory (RAM). However, XIP allows designers to save board space and reduce costs by allocating RAM only to frequently accessed routines and working data, such as variables, buffers, and stacks.

Serial for speed?

The fastest NOR Flash memory communicates via a parallel bus. For example, 32-bit parallel NOR Flash with a 100 MHz clock speed can achieve up to 3.2 Gb/s of data throughput. However, the parallel construction requires significant PCB space and complexity, and 80 MB/s is sufficient throughput for most embedded systems.

High-density NOR Flash memory from Integrated Silicon Solution, Inc. (ISSI) is based on the serial peripheral interface (SPI) instead. SPI, common in chip-to-chip communication, relies on four signals—Clock, Chip Select, Data In, and Data Out—for a simplified interface that reduces board complexity and pin count.

SPI NOR Flash memory from ISSI supports an 80 MHz normal read speed, a dual transfer rate (DTR) up to 104 MHz in which data is transmitted at the beginning and end of each clock, and a fast read performance up to 166 MHz. That translates to a throughput of 166 Mb/s, one bit per clock, for single SPI. Dual SPI turns the Data In and Data Out pins bidirectional, doubling the available throughput to 332 Mb/s, while quad SPI delivers 664 Mb/s, or 83 MB/s.

Quad peripheral interface (QPI), another capability available in some NOR Flash products, turns two additional pins into bidirectional channels to transmit protocol information and data, reducing instruction overhead. In QPI mode, NOR Flash supports XIP and the smaller, less complex PCBs that XIP enables.

PCB planning

The space efficiency of SPI NOR Flash chips on PCBs is not just a matter of package size or footprint. Instead of the 40-plus pins needed for parallel NOR Flash, each with a separate, creatively-routed PCB trace, many embedded applications can use eight- to 16-pin SPI NOR Flash, simplifying PCB layouts and minimizing board layers.

SPI NOR Flash chips in ISSI’s IS25xP series are available in three configurations, allowing engineers to choose the one that best fits their PCB layout, manufacturing process, and application requirements. A small-outline package (SOP), also known as a small-outline integrated chip (SOIC), is a common configuration for SPI NOR Flash in embedded systems from industrial controls to automotive engine controllers and body-control modules. The external leads extending from two sides of the package (Figure 1) can be soldered by hand and easily inspected.

Figure 1: ISSI’s NOR Flash is available as a 16-pin SOP with 512 Mbit and 1 Gbit memory densities. (Image source: ISSI)

ISSI’s SOP NOR Flash chips are 2.65 mm tall and 10.31 mm square, including leads, with a 7.50 mm wide body. Engineers can choose a chip that runs on 1.8 V or 3 V and has the desired memory density, 512 Mbit or 1 Gbit. In addition to the ease of assembly that makes it useful for prototypes and development boards, SOP NOR Flash is chosen for critical embedded systems because of its relatively low cost and long track record of reliability.

Very thin, small outline, no-lead (WSON) packages have a lower profile and are more compact than SOP chips. WSON chips in the IS25xP series (Figure 2) are 0.75 mm tall x 6 mm x 8 mm.

Figure 2: IS25xP series NOR Flash is available in a 0.75 mm tall WSON package that supports QPI and 512 Mbit memory density at 1.8 V or 3 V. (Image source: ISSI)

Compact WSON NOR Flash packages have low parasitic inductance because their electrical contacts match up with contacts on the PCB just under the chip’s edges. Engineers balance this against the challenge of hidden solder joints, which require an automated production line and make inspection and rework challenging.

WSON packages store up to 512 Mbits of data. Common in IoT and portable applications, WSON SPI NOR Flash packages are often chosen for applications like sensor and camera modules or for electronic control units (ECUs) where space is tight.

For applications with complex, memory-intensive firmware, high-density NOR Flash memory in a thin fine-pitch ball grid array (TFBGA) package may be required. TFBGA packages in the series come in 6 mm x 8 mm rectangles that are a maximum of 1.2 mm tall (Figure 3).

Figure 3: The TFBGA packages in ISSI’s IS25xP series of NOR Flash chips are 6 mm x 8 mm x 1.2 mm. They support up to 2 Gbit of memory in 3 V operation. (Image source: ISSI)

TFBGA packages allow many connections to be made in a compact space while keeping parasitic inductance low. The IS25xP series also supports memory densities up to 2 Gbit. Improved electrical and memory performance has led engineers to choose TFBGA NOR Flash for automotive infotainment systems, digital instrument clusters, domain controllers, and networking equipment, despite the more expensive automated soldering and X-ray inspection requirements.

Implementing reliability

Whether an application needs an easy-to-implement SOP NOR Flash or a high-density TFBGA, embedded systems need a high degree of reliability. NOR Flash provides a known technology with excellent data retention. For example, ISSI’s NOR Flash chips can handle over 100,000 Erase/Program cycles and retain data for more than 20 years. The chips also have security features such as software and hardware write protection and advanced sector and block protection to ensure embedded systems are tamper-proof and can run as designed.

High-density NOR Flash chips also need physical reliability. All IS25xP series chips are rated for temperatures from -40°C to +105°C, and A3 chips can withstand temperatures up to +125°C. IS25xP series chips also support the low-power operation preferred for automotive and mobile applications. They draw 16 mA during active reading, 24 μA during standby, and as little as 2 μA in Deep-Power-Down mode.

Conclusion

Reliability, high-density capability, random-access speed, and space- and cost-efficiency have made SPI NOR Flash a top choice for engineers designing firmware- and software-intensive embedded systems. The scalability, security, and fast read performance of this non-volatile memory make it suitable for industrial, automotive, IoT, and mobile applications where users rely on fast system booting and data persistence.