The Fatal Blow

I have to confess being a fan of crime TV shows, particularly ones involving forensics that put together clues and finish by tying it all together and explaining what happened. I like the medical series ‘House’ for the same reason. I wish that I had been blessed with half his diagnostic skills when I was a design engineer. Instead I spent hours looking at logic analyser output, scope traces, and software dumps searching for clues as to what had happened to cause the failure that I was debugging.

The same problem occurs with modern electronic systems. As automotive manufacturers look for greater reliability, we are looking at how we can prevent failures in the parts-per-billion range. What chance do we realistically have of ever witnessing a failure that occurs at a 1 ppb rate?

The answer is simple. The designer has to build systems into the car that perform self test and error logging functions that leave the vital trail of clues to help identify any problem. A good example of such as system are the batteries in a hybrid or electric vehicle. To get the most out of the battery, a great deal of data needs to be collected and processed including charge cycles, charge rates, temperature while charging and discharging, etc. If the manufacturer provides a 5-year warranty on the batteries, they will need to know that during the entire life of the battery, that it has not been mistreated. If it has, they will need to implement fixes to prevent the failure from happening again. Cue the need for a data log.

One key element of the data log is that it must be nonvolatile (so the data is preserved when the battery fails) and it helps if it can be written quickly and as often as possible. It’s no surprise then that F-RAM has been chosen in some of these applications. It has virtually unlimited endurance and its fast write allows designers the option of writing data continuously until a failure occurs. The F-RAM will then contain a short record of the operation that contains the trail of clues that would even impress Sherlock Homes!

Do I have Weak Fingers?

Maybe its me but occasionally questions pop into my head that no one else seems to ask. This happened recently when we were talking about map lights found in cars. My car is five years old now and the map light switch is the classical push-to-make/push-to-break switch. The latching action is accomplished with a combination of plastic and springs inside the switch. In more recent or maybe just more expensive cars (note to myself: check the bosses car) the push-button is a very delicate push-button. The latching action has probably been accomplished by a small micro. The implication is that we all want delicate push-buttons with electronic control to replace the firmer push-buttons. This is when that question filtered into my conscious – do I have weak fingers?

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?

Making Friends with the Production Team

Most engineers are aware of design for test and design for manufacture. The goal is to design products that are easy to test and manufacture. If you achieve this, you’ll have lots of friends in the production department.

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!

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’s the value of development time?

One of the features of F-RAM is very high endurance. It’s so high in fact that you can write as many times you like if you are using an F-RAM device over a serial bus. Simply put, serial busses are not fast enough to ever reach the endurance limit of F-RAM. This allows engineers to re-think their non-volatile memory strategy and entirely eliminate routines that perform wear-leveling and data saves on power fail.

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!

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.

F-RAM State Savers added to auto product line

Auto grade F-RAM State Savers

Ramtron’s FM1105-GA and FM1106-GA State Savers have recently received AEC-Q100 Grade 1 qualification.  Our state saver devices save the state of signals on demand and restores them to the correct state automatically upon power up.  F-RAM technology uniquely enables this capability due to its fast write time, virtually unlimited write endurance, and low-power requirements.

The Grade 1 temperature qualification allows the FM1105-GA and FM1106-GA to operate over the entire automotive temperature range of -40 to +125 degrees C, enabling designers to benefit from F-RAM in systems throughout the car. The nonvolatile state saver is as simple to use as a D-type flip-flop.  It operates like conventional logic, but stores and retains the logic state in the absence of power, simplifying the design of system control functions.  The nonvolatile state saver with Grade 1 automotive qualification is well-suited for diverse applications including door lock and child safety position electronic latches, airbag deactivation switch, blower speed and vent position for cabin ventilation systems, dome light mode switch, and electrically heated steering wheel and seats, among other applications.

For more information, visit the State Saver page on the Ramtron corporate website. go>