Scientific, embedded, biomedical, electronics contents.

Voltage references in embedded design

scienceprog — Thu, 08 May 2008 21:21:17 +0000

Embedded systems that are dealing with DAC require a good voltage references. Despite all voltage references have their nominal values there are specific tolerances that indicates how much values can vary from nominal value. References as a rule are semiconductors that characteristics are effected by temperature.

For instance if we decide to use a 2.5V voltage reference then (zener) diode LM336A-2.5 may be a good choice for this.

These diodes have low temperature coefficient. It can guarantee that voltage will be in range between 2.44 and 2.54 at 25ºC. This precise zener diode has a third lead that can be used to adjust break down voltage. In the following figure you may see diode with potentiometer where breakdown voltage can be adjusted in range of ±120mV:

Adjusting break down voltage can be important feature where precision ADC are required. As we mentioned real reference voltage value can diverge from nominal so the ADC readings. And the bigger voltage needs to be read - the bigger error occurs.

We know that compensation can al so be done in software by adding some known offset value. But in many cases compensation leads to floating point operations where regular microcontrollers would waste their resources - especially in time critical measurements. As alternative there can be a reference table stored in memory, where calculations need to be made only once. But again for 10bit ADC readings we would need 1024 table entries - so this is a 1kB of non-volatile memory. Such implementations would also need a calibration in order to fill table with calibration values.

So probably it is better to avoid errors in hardware level than correct them in software. In precise embedded systems I would really recommend using precise voltage references. If you need more precision than zener diode can provide you can chose Maxim MAX6225 2.5V reference where value is in range between 2499V and 2.501V at 25ºC. MAX6225 also has ability to connect potentiometer that can adjust voltage range by about 40mV.

And probably I don’t need to mention that some microcontrollers have internal voltage reference sources that can be used for ADC capture. But if you need accuracy you should avoid using them.

>>>: Embedded projects from around the web Microcontroller projects everyday

Peggy 2.0 led display board

scienceprog — Thu, 08 May 2008 06:16:03 +0000

Guys from Evil Mad Scientist Laboratories have released 2nd version of their Peggy LED display kit. It is a 25×25 LED display board which is capable to display various LED effects, output text and even animation with individual LED addressing.

WPvideo 1.10

Download!

This is quite smart construction allowing to drive any number of LEDs (from one to 625). Peggy board has several buttons that can be used to control various actions like games or other effects. The board itself is nicely designed so it wouldn’t be a shame to use it as a dynamic LED sign, FFT sound grapher, RSS reader or simply as general purpose giant display. You can order peggy board as a kit or try to build it by yourself as all project files are available for building, hacking or simply analyzing. [Peggy v2.0]

Yet Another Simple AVR ISP cable

scienceprog — Mon, 05 May 2008 21:49:25 +0000

Probably this programmer cable can be found on every AVR fan website. It very simple and easy to build. You just need few passive components that can be packed inside LPT connector. From my perspective I wouldn’t recommend using this one programmer if you are more serious in microcontroller projects, because it has poor computer port protection. It connects directly to LPT port without precautions. I definitely recommend using similar ISP circuit with buffer IC between LPT and target board.

But let’s don’t pretend like we are very smart. The whole beauty is in its simplicity. This thing does its job perfectly as long as target board power supply is OK.

Note for those who are thinking what is this thing for… I answer - this is an in Circuit programmer adapter for AVR microcontrollers that can be used to program MCU’s without removing them form the board. This is especially important feature if you are using SMD parts where you cannot use any sockets to place your IC. [build one]

Cornell ECE Final Microcontroller Projects - Spring 08

scienceprog — Mon, 05 May 2008 21:28:39 +0000

It’s been ten years when Cornell University Computer Engineering students were publishing their final embedded projects. I always enjoy browsing over that big list of complete projects. You can get some crazy ideas here but also some good lessons that could be helpful while building your own projects.

As every year this one is not an exception. A new list of 35 projects id in front of you for your judgment. Here you can find some interesting solutions like Midi trumpet, Neural Net helicopter, and a big bunch of various game solutions. Happy browsing and learning. [Read]

DIY unique PCB Drill

scienceprog — Sun, 04 May 2008 21:33:41 +0000

This PCB drill is made of wood and looks really cool. Wouldn’t mind to have one on my table. Design is very unique - it uses various custom made parts, motors from old printers and VCR’s. Author even simulated all design code on a Labcenter’s Proteus VSM.

“Made from Dremel drill and drill press. I removed the bottom plate & mounted the column to the back with U-bolts. Acrylic is so easy to work with. It can be cut, drilled milled & glued and it’s very strong, doesn’t warp or break easily. The design is the same as the big PCB drills, the table moves, not the drill head. With the exception the real machine tables I’ve worked around ride on air. Like the old air hockey tables Important note: These NEMA teen motors don’t have enough torque to drive the table (no bearings).”

If you want to clone or own this masterpiece - you probably should contact author to get more details. [link]