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EEVblog => EEVblog Specific => Topic started by: cksa on September 11, 2010, 02:14:09 pm

Title: suggestion for blog topic -- low power design
Post by: cksa on September 11, 2010, 02:14:09 pm
I remember dave breifly talking about low power electronics design on the amp hour. It might be a good blog topic to expand upon this. What are some of the common techniques? How can I avoid pissing away current in using a 10k pull-up resistor?

I'm sure many of us will find such an episode interesting :)
Title: Re: suggestion for blog topic -- low power design
Post by: Simon on September 11, 2010, 03:08:31 pm
it is probably a very broad subject that starts with choosing low power parts and knowing the details specs of your parts and how far you can push them
Title: Re: suggestion for blog topic -- low power design
Post by: Zero999 on September 12, 2010, 08:47:29 am
It depends on the input impedance and the logic levels.

Here's a PIC tutorial I'm following which discusses the issue of pull-up resistors, read the first couple of pages.
http://www.gooligum.com.au/tutorials/baseline/PIC_Base_A_4.pdf (http://www.gooligum.com.au/tutorials/baseline/PIC_Base_A_4.pdf)
Title: Re: suggestion for blog topic -- low power design
Post by: Simon on September 12, 2010, 09:01:29 am
you using assembly ?
Title: Re: suggestion for blog topic -- low power design
Post by: mikeselectricstuff on September 12, 2010, 09:48:22 am
Quote
it is probably a very broad subject that starts with choosing low power parts
That's not always the case - the knack is understanding how to get the result you need with the lowest power, and in some cases, the lowest power part is not the best choice.
In many cases the critical parameter turns out to be the length of time tnings are turned on, not the current draw whilst on.
A good example is sensing analogue values, e.g. temperature or pressure, on a low duty cycle.
In this sort of application you need to  turn on the sensor and often an amplifier, wait for it to settle, read the value and then turn off.
In practice, the settling & measurement time is often the  most critical value, as everything is powered, and often the CPU running as well. 
Where there is an amplifier, a fast opamp is generally more desirable than an ultra low power one, as the latter will be trading speed for power draw.

For a microcontroller, things like the number of different selectable clock rates and watchdog rates, provision of different levels of sleep, available wakeup methods and oscillator start-up time can make a bigger difference to overall power draw than the actual current draw in each mode.
Title: Re: suggestion for blog topic -- low power design
Post by: djsb on September 12, 2010, 12:41:35 pm
I wonder how NASA manage it on their Mars rovers. That's got to be the ultimate design challenge. Pity they don't have any design info available. They probably use FPGA's anyway. It'd be great to learn some of their tricks.

David.
Title: Re: suggestion for blog topic -- low power design
Post by: Simon on September 12, 2010, 01:06:27 pm
yes well the mars rover panels did not need to generate enough power to lift their weight into the sky, moving something along the ground is feasible, but lifting it under it's own generated power is a bit more tricky
Title: Re: suggestion for blog topic -- low power design
Post by: NiHaoMike on September 12, 2010, 01:14:11 pm
I wonder how NASA manage it on their Mars rovers. That's got to be the ultimate design challenge. Pity they don't have any design info available. They probably use FPGA's anyway. It'd be great to learn some of their tricks.

David.
I'm sure that given their budget, they probably have their own ASIC design team and perhaps some chip fabs.
Title: Re: suggestion for blog topic -- low power design
Post by: migsantiago on September 12, 2010, 01:35:29 pm
http://voyager.jpl.nasa.gov/faq.html (http://voyager.jpl.nasa.gov/faq.html)

Quote
What kind of computers are used on the Voyager spacecraft?

There are three different computer types on the Voyager spacecraft and there are two of each kind. Total number of words among the six computers is about 32K.

Computer Command System (CCS) - 18-bit word, interrupt type processors (2) with 4096 words each of plated wire, non-volatile memory.

Flight Data System (FDS) - 16-bit word machine (2) with modular memories and 8198 words each

Attitude and Articulation Control System (AACS) - 18-bit word machines (2) with 4096 words each.

According to my calulations, that's a total of about 541KB, or small potatoes compared to today's microprocessors. We probably could perform all functions with one of today's boards and still have room for solid state data storage and much more fault detection software. We would still need a second unit for redundancy. Today's microprocessors are also much faster than the chips used on Voyager and a comparative system would use less electrical power. On the other hand, software might be more complicated as opposed to that used in an interrupt type system, but it would be much more capable and more flexible.

http://wapedia.mobi/en/Intel_8086?t=3. (http://wapedia.mobi/en/Intel_8086?t=3.)

Quote
As of 2002, NASA was still using original 8086 CPUs on equipment for ground-based maintenance of the Space Shuttle Discovery, to prevent software regression that might result from upgrading or from switching to imperfect clones. [15]

Nasa may not always use the state of the art microprocessors. I've read they're still using old Intel processors, since their reliability has grown during all these years. They are mature processors.
Title: Re: suggestion for blog topic -- low power design
Post by: Zero999 on September 12, 2010, 01:36:22 pm
you using assembly ?
What me?

Yes, I'm getting there.

Give it a go, it's more efficient, meaning you can use cheaper MCUs for the same tasks.
Title: Re: suggestion for blog topic -- low power design
Post by: Simon on September 12, 2010, 01:42:16 pm
you using assembly ?
What me?

Yes, I'm getting there.

Give it a go, it's more efficient, meaning you can use cheaper MCUs for the same tasks.

I've not had something yet that maxs out the resources of the pic, as it is my dynamo controller needed a delay putting in the program to keep the software in sync with the rest of the world