2014-05-24

Current increase for MAX6957

This will be a mostly Analog Electronics post...

In a project I made with friends we had a Raspberry Pi driving a MAX6957 for driving some LEDs. The MAX6957 was connected to the SPI bus of the RPI and the driving functions were developed in python (if you're interested in this you can post a question in the comments but if google it it's faster).

We were driving a few piranha RGB LED but then we bought a bigger LED (something like this one). In order to keep the software simple and unchanged I wanted to use the MAX to drive the bigger LED. If the MAX drives the LEDs with a current source that can be stepped (in 16 equal current steps), so I would have measure the current of the MAX and amplify it for the new LED.
This is what I've done:



First I have a current mirror, as the MAX sinks current I used PNP transistors, on the other side I have a small resistor to convert current into voltage.
This voltage drives a voltage to current converter with a protection for over current (in case the MAX is removed, goes crazy or it's output is miss configured). Then repeat for each of the 3 channels.

2014-03-19

We're back..

I've been busy (very), but I'm still here...
Starting the first post of the year on the end of March is not the best indication of a success blog (which I don't have)... but I'll try to post some old stuff that I did meanwhile.
I've lost a bit of touch with the Arduino and last time I check things have gone mainstream and big (check this one out, an ARM Arduino). I have an old project done two years ago to post.
Meanwhile I've started with the RaspberryPi and how to connect it to some hardware. Although I don't really like Python (the strict indenting drives me mad), you can also use C to program it.
Well all the people of posted questions I've published them, for those with questions just shoot.

2013-08-25

AVRStudio 6.1 and Code Composer Studio 5.4

A new second hand laptop and a new windows XP re-install and all the tools for micro-controller development... If you are looking for a comparison between the above tools stop reading. **** This is a public bashing of both tools, they are s##t ****
I recently bought some MSP430 development tools from Texas Instruments (ez430 chronos) and went on to install the "Offical and supported Development tool" Code Composer Studio 5.4...


It has the following problems:
- very large program, based on Eclipse (that also suffers from some of the Emacs problems of growing ad nausea in features that only a few people use).
- incredible tools for managing projects from the IDE, fantastic.. except this is a micro-controller development platform. If you program has more that 32 code files (.c or .asm) you probably should rethink you program... and if RCS or CVS cannot manage your multiple projects... you should not be developing for micro-controllers.
- After many tries (I am not very patient, nor skillful with point and click interfaces) I managed to compile my program and debug...
- the main window has so many subwindows that it is impossible to use on a laptop, you need a 32 inch TV to use the interface... it is nuts!

I thought that was horrible, but then I installed AVR Studio 6.1, if the former program (CCS 5.4) is large the latter is huge...

Has the following problems:
- during install, further "support modules" are downloaded, including one of 1.2Gb!!! Microsoft Visual Studio something - HORRIBLE!
- combining AVR and ARM tools is a bad idea, why do I have two toolchains when I only want one of them!
- same problem with the windows as CCS but in some way we have less subwindows.. maybe its a bit better..
- to update you need to register! why ? did't I already had to register to download... leave me alone! And then it fails with a crash! 
- I tried to install AVR Studio 5.1 first... but doesn't work, fails during install....
- I'm going to try to install 4.19... I need to update my AVR Dragon and AVR ISP2... that's why!

Ahhhhhhh! were did all go so wrong! Give us an editor, a simple project manager, an assembler, a c-compiler,  a simulator and a debugger, that's it how difficult can it be! give us some icons in the interface and nothing else!

TODO:
Uninstall both horrible tools, install AVRSTudio 4.19 (because I need the simulator a bit), avrsimul in linux is getting better so maybe in the future there will be no need even for this.
Developing with MSP430 appears simple in linux (although the install is a bit more complicated), the tools for AVR install quite easily with a special one-click install for OpenSUSE (installing arduino tools).


2013-03-24

HF on the ATtiny15

I wanted my class E amplifier to be driven by an Attiny15. The idea is to build a HF (6.78MHz) low voltage and low power beacon.
The maximum PWM frequency attainable using the factory OSCCAL value, and using OCR1B=2 and OCR1A=1 is 8.525MHz with the only possible duty cycle is 33%. With OCR1B=3 we get 6.393MHz, OCR1B=4 we get 5.115MHz.

I want to use my beacons in the ISM band of 6.78MHz so the second value (OCR1B=3) is the closest. I managed to output a duty cycle of 25% (OCR1A=1) or 50% (OCR1A=2), this is perfect for a class E drive as 50% is available. The final tuning was done with trial and error on the OSCCAL value until I got the correct frequency.
In my tests, the factory value for OSCCAL (of all the Atmel Tiny15L I tested) would get you within 1% of the nominal frequency 1.6MHz, but with frequency measurement and tuning I got to within 0.5%.

Did a couple of other idiot measures and observations.
- The program memory of the ATtiny loops, the PC is only 9bits and when the last instruction of Program memory is reached execution continues for 0000.
- A NOP consumes less current that an rjmp +0, not significantly but noticeable. I fill the memory with each instruction and measure the current with or without BOD. The second instruction also take double the time of execution.

RJMP+0,BOD - 3537uA (AVG)
RJMP+0,NO BOD - 3494uA
NOP,BOD - 3503uA
NOP, NO BOD - 3458uA

- OSCCAL after reset is cleared and the measured clock frequency is 1.1MHz, when taken to 0xFF the frequency is 2.083MHz.
- Clock jitter of the oscillator. I filled the program memory with the following:
lp000:
.rep64
sbi portb,portb1
cbi portb,portb1
.endr
rjmp lp0000

I would then trigger the scope at the low level at the end of the loop. This gap would last longer than normal, then it is easy to trigger on it. You can just see it under the trigger arrow in the picture above.


I would the trigger to the flank increase persistence to max and let measure. the jitter.


The waveform goes up quite fast (in less than 10ns) with the oscilloscope probe as load. With a gate of MOSFET... it's not so good...