Picaxe 14M 6 bit Digital to Analog Converter

[Svejk]

An inexpensive way to achieve a Digital to Analog Converter (DAC) with a Picaxe 14M. This uses R-2R resistor ladder. The tolerance of resistors is important, the more accurate, the better the results are.

I've used 100/200 ohms resitors 1% tolerance (quite common, out of the junk box).

Test code:

#picaxe 14M

for b0 = 0 to 63
  pins = b0
  wait 1
next b0

do:loop

This is no invention, just a reiteration of a well known circuit.

 

[premelec]

A reminder too that you can get a pretty decent sine wave using weighted R values - Don Lancaster has some discussion of this in the CMOS Cookbook I think it was... this could lead to the TT output 2 tones occasionally needed...

 

[hippy]

TT output 2 tones

You've lost me there and perhaps others; TT = Teletype ?

The main problem with bit-banging tones, and more so for analogue waves, is the speed needed to get reasonable frequencies.

 

[MFB]

You might find my post on just this subject of interest. Search under FSK + DAC

 

[westaust55]

Yes have done a similar circuit in 8-bit using a DS2408 1-wire chip but that needs a chip with 1-wire comms (X1 or X2 parts).

Easy when the outptus can drive high and low. A little more complex to adjust/trim with outputs that are open collector but it can be done.

How is the linearlity and monotonic nature of the R-2R DAC output with 14M driven circuit?

 

[fernando_g]

Indeed, ladders are useful and simple D/A converters.
With a 20M you could do the full 8 bits.

I would only add to your circuit an op-amp buffer at the analog output. It has to be of the rail to rail variety, unless it is being powered from a voltage higher than 5 volts.

 

[BeanieBots]

Or, if 5-bit is good enough, simply use a PICAXE which has an internal DAC

 

[westaust55]

Or, if 5-bit is good enough, simply use a PICAXE which has an internal DAC

Aka 18M2

 

[premelec]

@Hippy TT= Touch Tone [Teletype = RTTY or TTY].... SMASLT [So Many Acronyms So Little Time]

 

[aduy]

If you use the advanced features of the 14m you can make it into 9bit I have made a circuit and even a PCB that would work for 9bit

 

[Svejk]

I think the serial programming isn't exactly the right one. Maybe add a decoupling capacitor as close to the power pins. Another thing to consider when using the resistor ladder is the resistor tolerance: more bits resolution leads to more accurate resistors.

 

[westaust55]

If you use the advanced features of the 14m you can make it into 9bit I have made a circuit and even a PCB that would work for 9bit

Can I suggest that you also post your program for completeness which may at some future date enable others to follow in your foot steps without hassles.
We are afterall here to help each other.

 

[hippy]

Also see the comments made in the following thread where the same circuit was posted -

http://www.picaxeforum.co.uk/showthread.php?t=17724&page=4

 

[aduy]

Yes I over looked connecting the ground into the R2R part circuit.