jgmdesign 
Greetings!!
Somebody out there spare me some research! I am using a MAX522 8pin SPI D/A converter. It is a dual unit, and it works great. Trouble is, that it is damn expensive in small quantities here in the states($5.00each).
Does anyone know of an 8bit, SPI, 8pin device that is lower in cost? It does not need to be a dual as I am only using one output. Oh yeah, it needs to operate on 2.7-5vdc
DIP package preferred
Jim
I guess we need to know more of your constraints. If I go to the
Analog Devices website, products -> DA converters, parametric
search, enter SPI and 8 bit resolution as constraints, I get 18
matches. The cheapest match is the AD5601 which comes in a tiny
SC-70-6 case. Digikey sells it for USD 2.38 for a quantity of 1.
I'm sure other vendors have similar offers.
jgmdesign Thanks for the info, I admit I am beig lazy here. I was just asking since you can usually get more from a broadcasted thread to the world as opposed to going on a silicon search
Thanks again
Jim
geckosenator 
What I do for a DAC, just use a RC lowpass filter on a pwm output from the AVR. Then you can just set in a register from 0 to 255, or probably better resolution using the 16bit counter. You probably want the highest frequency, and trade noise for update rate for power usage when picking the values for C and R.
jgmdesign Good Idea, but i need the resolution, and accuracy as i am gauging a thermistor. This is a thermometer that is running my attic fans as part of my home automation system.
Keep the ideas coming!!
Jim :mrgreen:
bobgardner Thermometer sounds like a job for an a/d to me. How does the hi res d/a come into play here again?
jgmdesign Thermometer sounds like a job for an a/d to me. How does the hi res d/a come into play here again?
Bob:
Maybe Hi-res is not the proper term. I am building a total home automation system, skilight control, lights, heat, a/c etc. My attic hits over 135' in the summer with the sun beating on the roof(you being a Floridian can relate!). The house becomes very hot as a result. When the attic fan is turned on, it cools the attic extremely well, and the house too. my thought is twofold, one, monitor the temperature, and if it gets above a certain threshold, turn the fan on automatically. Two, since this node is part of the home network, it can relay the temperature back to the host cpu(Mega162 with 64k of external RAM and EEprom) for display on the remote terminals in the house. The host cpu can also turn the fan on through the node network so the house is cool when my wife gets home.
Why i need the d/a is because the at90s2313 does not have an a/d internally, but it does have a comparator. I have a thermistor sensing circuit that provides an analog voltage to one input of the comparator. THe other input is tied to the d/a. by incrementing the d/a until I get a comparator match, I can then do a simple binary to bcd conversion and display the temperature.
The micro also has a sensor for the attic stair door that shuts the fan off should the door open so no one puts their hand in the blades accidentally. and turns on the attic lights as well.
I know this sounds complicated, but it really is not.
Maybe I should open a project page in that area of the Freak site. Can I post the nodes, cpu, code and documentation as I complete them?
Incidentally, in case anyoone is wondering why I just don't use an AVR with a built in a/d, the reason is i have a boatload of the older 2313's and I want to use them up.(My wife is also not letting me buy more stuff at the moment - DAMN!! :oops: )
Regards,
Send Comments - PLEASE!!
Jim
Lennart Since you have so many 2313 couldn't you use a port on one of them and make a R/2R resistor ladder?
jgmdesign Yes, I could do that, but that is a pain in the ass. The d/a is an 8pin spi device. I can create a simple bit-bang spi port on the 2313. the only thing that sucks is that d/a's in 8 pin dips cost around $5.00 each. Oh well.
Jim
bobgardner You can read a string of dallas 18b20 thermometers... one in the attic, one outside, one in the house using 2 pins... (alternate plan, less analog, more digital)
Lennart Maybe I misunderstood...
My thought was to feed the result of R/2R ladder to analog comparator of same 2313, the other signal to comparator being the temperature voltage and then do a bit-bang SPI on same chip (if that's needed, I suppose you wan't to turn on fan with a relay?).
jgmdesign You can read a string of dallas 18b20 thermometers... one in the attic, one outside, one in the house using 2 pins... (alternate plan, less analog, more digital)
I am going to have to look at that part. Great idea as it eliminates the thermistor, and the dual op-amp circuit I am using now. I never thought of that. How much are the devices?
Lennart:
Yes, the fan and the lights are relay controlled.
Jim
Lennart Don't know if you have seen this app note...
http://www.atmel.com/dyn/resourc...
BTW, I have built something "same but different" that you are aiming at. I don't control any lights but an heating system with mega16 as the host and AT90S2313 as remote devices (I also had a bunch of them).
I measure 7 NTC's (they are very cheap and can use long tiny cables), send results to mega16 ADC. Then use I2C to send computed results to slaves that display temperatures on LCD, control a circulation pump and movements of a shunt motor.
tpappano For reading a thermistor, it seems like using the 16 bit timer1 pwm with the comparator would be an accurate, elegant, low parts count solution. (thermistor, two resistors, and a capacitor)
Tom Pappano
Tulsa, Oklahoma
polus What about a resistor ladder on PIO pins? Obviously not as good as a IC-DAC but its certainly cheap!
jgmdesign Hey Bob,
Those converters look cool. A little software intense, but I could deal with that. The TO-92 package is great. Could you give me a pointer on converting the 9bit temp from C to F? Me and math are not the best of bud's.
Also, I code in assembler. I suck at 'c'. Until I can get Smileys book and butterfly combo I am gonna be a little challenged.
Thanks
Jim
jgmdesign Bob:
I am looking at the dallas part and I like what i see. The only issue is it does look a tad software intense as I suspected. I downloaded the app notes. but do you have any code I could look at?
Jim
bobgardner They read out directly in 9 bits of 'quarter degrees' C. I will leave conversion to F as an exercise for the reader. Several projects show reading 'onewire' protocol... check out microsyl.com
jgmdesign great
thanks
