Dealing with one small bottleneck

64Kb Grade-1 F-RAM

One subject that will probably never make the front page of newspapers is the speed of automotive grade nonvolatile memory. The sentence alone is enough to compel 99.9% of the world’s population to choose to watch paint dry rather than read on. However, for those of us working in automotive electronics that strive for faster, cheaper, and better designs, the speed of nonvolatile memory becomes a serious concern worthy of attention.

Nonvolatile memory for automotive applications is usually an SPI device running at a bus speed of 1- to 5-MHz. So why does an automotive memory run slower than an SPI memory in the industrial world, especially when you consider that the typical automotive grade memory is made from the same silicon as an industrial device? In a nutshell, it comes down to higher operating temperature. Operating temperature effects both speed and power consumption. Since the two effects are related, higher temperatures slow down the operating speed of silicon and increase the current consumption. The end results are slower speed and higher current consumption specifications than a comparable industrial device. We’ll talk about the increased current another time so let’s just focus on the slower operating speed of nonvolatile memory in automotive applications for now.

I have recently come across a couple of instances where my customers needed more speed from their nonvolatile memory. One design had an array of devices on an SPI bus. Unfortunately, due to the slowness of the nonvolatile memory, the speed of the SPI bus needed to be reduced for nonvolatile memory reads and writes and then switched back to a higher speed for all other peripherals. The time needed to switch speeds made the whole design dillema worse. The design would be much easier if the nonvolatile memory was as fast the rest of the parts on the SPI bus, like an F-RAM for example.

Another design had a control algorithm that needed to execute in one millisecond to achieve stable control. Another  requirement was the need to store data in nonvolatile memory on each pass trough the control algorithm. EEPROM was not up to the task since it is not able to deliver writes in under one millisecond. F-RAM, on the other hand, is capable of writing as fast as the SPI bus can deliver data, making it an ideal solution for the design.

Simply, F-RAM reads and writes at the full SPI bus speed of 16MHz (for the 64K Grade 1 part) allowing engineers to make a small step towards their holy grail – faster, cheaper, better designs.

What does every security system need? A switch.

F-RAM State Saver driving a relay

Sounds obvious but every security system needs a switch. Somewhere, at the output of the security system will be a switch that either turns the alarm on or turns some component off, disabling whatever it is that the security system is protecting. In a home alarm, the security system turns on the bell-box (the thing that makes a loud noise and flashes), and in an automotive security system, the ignition system is usually disabled in some way.

Now, as every good thief will tell you, if you cut the power to the security system and silence it soon after it starts to make a noise, then everyone will ignore it. Home security systems get round this obvious attack by having a backup battery in the bell-box and mounting it out of reach. Achieving the same in an automotive security system takes a little ingenuity. Autos usually only have one battery and everyone knows how to access it. If you are not familiar with this process just google it before you set out to steal cars.

One solution is to have a switch that remembers its previous state. That way once the alarm is triggered, cycling of the power supply will have no effect. Ramtron’s State Savers offer a simple, cheap electronic switch that remembers its previous state after a power cycle, which is ideal for security systems. You can learn more about our State Savers here.

EEPROM can be a right and royal pain

EEPROM with Micro and Capacitor

The circuit shown above is the memory sub-system that you would need to store data on power fail. The key components are a microcontroller for processing, an EEPROM for nonvolatile data storage, and a capacitor to provide enough power to give the micro enough time to finish writing as the power fails.

F-RAM and Microcontroller

Above is an equivalent circuit design using F-RAM. As you can see, the capacitor is no longer required for two simple reasons:

1. F-RAM writes 50 to 100 times faster than EEPROM, allowing the data to be written as the main power supply fails without support from capacitors
2. F-RAM uses about 1/100th the power of an EEPPROM so, in effect, the power that is dying lasts much longer

So why is this important? F-RAM allows for simpler, cost effective, and more reliable designs in automotive applications. Although F-RAM compoments are more expensive than EEPROMs, when you add up the solution cost including the EEPROM, extra capacitor, assembly issues, and extra board area, F-RAM becomes a compelling high-performance design alternative.

Also, the more data you need to write, the larger the capacitor needs to be in both size and Farads. One recent application that comes to mind needed capacitance of around 0.1F. This amount of capacitance does not come cheap. Another spec that engineers need to look at is the capacitance at max temperature. Capacitors leak and the leakage increases with temperature. Low-leakage capacitors are readily available but, again,  they are more expensive.

I recommend taking the time tocalculate the amount of capacitance you actually need with your EEPROM at the maximum operating temperature and ask yourself if F-RAM actually allows a cheaper and more effective solution.