It’s all very interesting, you say, but this doesn’t apply to automotive electronics. A car has only one battery and that gets recycled. On the surface this looks like a good argument but I recently attended a conference on Energy Harvesting and Wireless Sensor Networks. Now wireless sensors are something you do put in vehicles, the most common being tyre pressure sensors.

So perhaps energy harvesting is worth looking at a bit further. Everyone is aware that the wiring harness in a vehicle is expensive and heavy, which is why most manufacturers are trying to find a way to reduce its size. Perhaps wireless sensors that harvest energy from the environment are a way to make that happen. Imagine an engine bay with no wiring to any of the sensors!

Let’s start with the easy ones. A sensor that generates energy from rotational movement is relatively easy. There are magnets, coils, piezo sensors and Wiegend sensors that can all generate enough power to conduct useful electronic functions. Energy harvesters that get their energy from vibration are also possible, which can be very effective if the frequency and amplitude of the vibration is well defined. Similarly, collecting energy from an RF field is not so hard if you can tune your antenna to the right frequency. The engine bay of any vehicle is an oasis of opportunity to harvest energy from movement, heat, vibration or RF. So it would seem that harvesting the energy is not technically that difficult.

Ramtron recently released an innovative memory called WM72016. Its a wireless memory that can be read from and written to via an RF port and serial port. You can think of it as a dual-ported memory. The nice thing is that it can be powered either from a local power source or from an RF field. In a wireless sensor application the data could be retrieved even when the sensor is not generating any energy and data could be written by the harvested energy from the sensor without the need for a constant RF field.

Energy harvesting may offer solutions to automotive sensors that could reduce wiring. Ramtron’s Wireless Memory with a serial port gives a simple solution to the problem of transporting data without wires based on the energy harvested from the RF field. Now come on. Be honest. Did you start out thinking that a nonvolatile memory with low-power, high endurance and fast write could actually be a tool for the removal of sensor wiring?

Putting that complex question aside for another time I soon realized that F-RAM could be part of a rather neat solution for the weak-fingered applications. The problem with the electronic control is that connecting the vehicle battery after servicing could power up the light in the on state and that could drain the vehicle battery. In the schematic below a Ramtron F-RAM State Saver is used to remember the status. This is a simple D-type latch where the output uses F-RAM to recall the previous state. This means that if the lamp was off before the vehicle battery was removed it will be off after the vehicle battery is reconnected. It doesn’t get much simpler than that. Now, I wonder how to measure finger strength?

I once heard that if you consider all the systems a typical car needs, you will need about four hours for programming and configuration. Although I’ve never been able to verify this, I was reminded of this figure when I recently came across another programming problem.

The problem was programming a data table during production. The look-up table (LUT) contains data to control an engine and maps all of the engine parameters under various operating conditions. The table was about 256Kbits. That’s no not unusually large so why the problem? The problem was that the LUT had to be measured and calculated during production.  To compound the problem further, the table is arrived at iteratively and needs to be re-written many times.

Let’s just run through the calculations if you stored the LUT in an EEPROM:

 Now I don’t know about you but I was surprised to calculate that the cost of programming a 32Kbyte 256Kbit) EEPROM is 8.5c. This of course assumes that you can only program one device at time.

Compare this to using an F-RAM. The programming time is basically the bus speed.

 Now if you go the production department and tell them that you could save them 1min 41s of production time and will save €0.82 (at €30 per hour) you will make lots of people happy!

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.

Wear-leveling is commonly used for Flash or EEPROM parts to ensure that the application does not exceed their typical 1-million write cycle specification. With F-RAM’s 100 trillion cycles, in real world applications, you will never reach the endurance limit.

There is another way that F-RAM’s high endurance can save development effort. A lot of systems need to save data upon power failure. This requires extra hardware to perform an early detection of the power fail as well as some software to control the reaction. The power fail software needs to be written and tested, both of which require engineering time. At this point I should throw in lots of warnings about slow power ramps with noise and power glitches that cause problematic detection of a power fail. The faster you save the data the better!

Saving the time and effort to write and test software for wear leveling and data saves on power fail is valuable. A customer recently said that removing both of these tasks from his schedule would save him three months.  How much money could you save if you eliminated several months of development time? Generally speaking, I don’t have a precise answer, so here is my challenge. If you are an engineer, go and ask your boss how much extra your company can add to the BOM cost for ABC if it saves you X person-months of development effort!

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 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.