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Old 31st October 2006, 05:55   #1
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Cool What diference does the no of bits in an ECM make ?

For example, Santro is having a 8 bit ECM while Wagon R is having a 32 bit ECM although they are competitors in the same price segment.

So how will the difference in the number of bits in the ECM affect the overall performance of a vehicle ?

Should we always select the vehicle with more number of bits in the ECU since that vehicle may have better control of Fuel Injection, Ignition Timing, Variable Cam Timing etc ?
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Old 31st October 2006, 11:32   #2
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Here is some half baked knowledge I picked up at Bosch.

The ECUs are closed loop control systems. So a small difference in ECU performance can show up in the end result. Here is how.

The sensors used can be providing/loading the data busses with 16 or 32 bit data. If the car has simple yes/no sensors that are proving single bit values like door ajar warning, then it does not matter much. A complex sensor such as the lambda sensor, and the oxygen sensor picks up tiny analog voltages and converts it to 10 bit or a 16 bit digital values. A 8 bit ECU will have to play around with this large value before it completes the processing, while a 32 bit processor will be much more efficient, and can take the necessary steps sooner. Basic logic and arithmetic will be easier, easy to add multiply divide etc. These actions are often binary, such as turn the valves off, if the rpm falls below a certain rpm. The signals carrying the action needs to be conditioned as well. That needs horse power as well.

Feel free to disagree or prove me wrong.

-Prasadee

Last edited by prasadee : 31st October 2006 at 11:39.
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Old 31st October 2006, 11:36   #3
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More bits means more amount of data can be transmitted and processed in similar time.
A 32bit ECU has a 32bit data bus, which enables it to have higher data processing capabilities.
In simple engines where ECU is handling just basic stuff, 8bit 16bit or 32bit will not make much of a diffence. Its like editing a text file. Wether you do it on a 500$ top of the end nvidia card or you do it with your motherboards inbuilt puny SiS card, it wont make a difference.
But if you are into heavier stuff, like gaming, it will matter.
So if the ECU is looking at stuff like Valve timing, Brakes control, throttle control, climate control etc., etc., a powerful processor is needed, which is true for modern cars.
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Old 31st October 2006, 12:09   #4
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The santro has a 32-bit ecu.

Commonly used automotive spec controllers come in 8, 16 and 32 bit variants. To the end user, it really dosent matter which one is used.

Higher spec processors (32-bit) are needed to meet emission norms ( Bharat -II , III and IV ), as a lot of computing power is utilized. But this does not mean better economy or performance.

From a performance perspective of basic engine control, an 8-bit can do the job, although it is not the case now.

Last edited by rdkarthik : 31st October 2006 at 12:11.
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Old 31st October 2006, 12:22   #5
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Whenever you convert an analog signal into digital, there is a quantization error. The greater the number of bits, the less the error.

For example, consider that some sensor in your fuel system produces an output ranging from 0 to 16 volts. With an 8-bit system, you can have 2^8, or 256 values. So, your ECU can recognise a difference of 0.0625 volts, or 62.5 mV.

Now, if the system had 32 bits, you would have 2^32 (over 4 billion) "steps" in the same range (0-16v), so now, your ECU can recognise a difference of a few microvolts - so it's that much more accurate.

In other words, with a greater number of bits, the digital part of the system is able to approximate the analog signal to a much closer extent, hence greater precision and more accurate response.
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Old 31st October 2006, 15:08   #6
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It is quite complex to explain.

1: Resolution of the aquired data. all that Ajitkommini mentioned. Thanks ajit. But there is a top limit to this. generally 32 bit here is a over kill. If you have a sensor which can be sampled by 32 bit ADC, and the reference voltage is 12 V (car battery) then you have a resolution (not accuracy) of 2.7 nano volts. In electronic systems you find the noise in the order of 10s of millivolts. anything beyond this is an over kill.

2: Speed of the processor. There are faster processors in the 32 bit family available than in 8 bit family. Technically there can be faster processors in 8 bit too, but the general availability is the point. So need for faster processors push the manufacturers to higher bit width some times.

3: ECUs do take decisions based on some pre defined charts called look up tables. (software guys understand this better) these are again approximated values. with wider data bus, these look up table seeks perform better. There are atleast 4 such tables and each table can have few 10s to few thousands of values in each of them. assuming the data is 16 bit and the bus width being 32 bit you can fetch 2 such values and process in the time of 1. In a typical car which has 4 pot inline engine, running at 7200rpm, more than 30 decision are being made per second. This is not a big number for a microprocessor but is significant if you consider, it runs its own OS, has to sample and quantise the analog data and look into the tables, then run an algorithm to decide engine parameters , look into the table, execute the the actuators etc....

4: Gives a marketing edge..


Funniest answer I have heard..from the dealer.

I: Why do you need 32 bit processor in a car?

Dealer: 8 bit processor you can interface only 8 sensors, 32 bit processor 32 sensors, more FE
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Old 31st October 2006, 17:33   #7
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In layman computer terms, what would the processing power of a typical cars ECM be? 1.4 Ghz (like in my laptop)? Or more?
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Old 31st October 2006, 17:50   #8
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More means good. more bits, better computing. no of bits also determine no of addresses it can generate. Bigger the address space, better the programs can be written to be loaded. etc etc...
But as rightly told, its marketing edge which these people get!
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Old 31st October 2006, 18:00   #9
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Quote:
Originally Posted by GTO
In layman computer terms, what would the processing power of a typical cars ECM be? 1.4 Ghz (like in my laptop)? Or more?
Way lesser. Even the space shuttle wouldnt have a processor as powerful as what you have on your laptop!
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Old 31st October 2006, 18:04   #10
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To a lay man driving to work in stop go traffic, I guess it makes little sense how efficiently his car is processing data, but I guess if you are Michael Schumacher, it would mean the World Championship.
When I am playing freecell on my laptop which has a 1.79 GHz processor, it works just as well as my 2.4GHz desktop, but when I am working on a heavily linked 11mb excel file, the difference in computing speeds shows.
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Old 31st October 2006, 21:00   #11
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Quote:
Originally Posted by GTO
In layman computer terms, what would the processing power of a typical cars ECM be? 1.4 Ghz (like in my laptop)? Or more?
With newer generation 32-bit ecus, typically around 100Mhz to 500Mhz (0.1Ghz - 0.5Ghz ). Older ones are slower still. And higher speeds does not mean better ecus.......

Last edited by rdkarthik : 31st October 2006 at 21:04.
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Old 31st October 2006, 21:48   #12
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Actually if you compare the speed with which ECU can do its calculations with a general purpose processor, it will perform as well as a 2GHz processor for the same task, though the frequency is as low as 200MHz(.2GHz). The reason is that your laptop processor is general purpose, jack of all trades, while ECU is geared towards specific tasks.
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Old 31st October 2006, 22:01   #13
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My petra runs on a Frescale 32 bit 266MHz processor... way way less than the typical laptop.

But these are RISC processors (sorry for diving into details), which work more efficiently (power wise).

One of the reasons they work so well is they do not have a key board, mouse and display whose management is much more complex than the present ECU tasks.

Even the simple task of engine management is handled pretty complex in an ECU (which again needs more powerful processors) because a lot of fail proof mechanisms built into it.
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Old 31st October 2006, 22:17   #14
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Even the simple task of engine management is handled pretty complex in an ECU (which again needs more powerful processors) because a lot of fail proof mechanisms built into it.[/quote]

@DRC so are we talking about Micro-Controllers which have programs hardwired into them with no alterations allowable at the users end??? Correct me if I am wrong, when it comes to processors they are programmable to some extent right?
Eg :Over clocking them .
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Old 31st October 2006, 22:28   #15
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Here again, I am diggin deep into the details.

No, They are not Microcontrollers with hardwired programs. They do have a processor, transducer interface and memory system, both volatile and not volatile. So they can store events and update tables with learned parameters (some advanced cars). Notice, when you take your car to the dealer, they do get a log of errors occured in the car, failures etc. Some cars remember the service schedule too and switch ont he check engine ligght when the service is due.

Over clocking is a x86 specific feature. It has more to do with thermal management. Let us not digress here..
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