From the mbed microcontroller Handbook.

AnalogOut

/media/uploads/mbedofficial/analogout_interfaces.png

The AnalogOut Interface is used to set the voltage of an analog output pin.

Hello World!

Create a sawtooth wave

#include "mbed.h"

AnalogOut signal(p18);

int main() {
    while(1) {
        for(float i=0.0; i<1.0; i+=0.1) {
            signal = i;
            wait(0.0001);
        }
    }
}

API

AnalogOut	An analog output, used for setting the voltage on a pin
Functions
AnalogOut	Create an AnalogOut connected to the specified pin
write	Set the output voltage, specified as a percentage (float)
write_u16	Set the output voltage, represented as an unsigned short in the range [0x0, 0xFFFF]
read	Return the current output voltage setting, measured as a percentage (float)
operator=	An operator shorthand for write()
operator float()	An operator shorthand for read()

class AnalogOut : public Base

An analog output, used for setting the voltage on a pin

AnalogOut( PinName pin,
const char * name = NULL )

Create an AnalogOut connected to the specified pin

void write( float value )

Set the output voltage, specified as a percentage (float)

void write_u16( unsigned short value )

Set the output voltage, represented as an unsigned short in the range [0x0, 0xFFFF]

float read()

Return the current output voltage setting, measured as a percentage (float)

AnalogOut& operator= ( float percent )

An operator shorthand for write()

operator float()

An operator shorthand for read()

Details

The AnalogOut Interface can be used on mbed pin p18.

The AnalogOut Interface can be used to set the voltage on the analog output pin somwhere in the range of 0.0v to 3.3v. The 0.0v to 3.3v range of the !AnalogOut can be represented in software as a normalised floating point number from 0.0 to 1.0, or directly as volts or millivolts.

Page history

Last modified 22 Sep 2010, by Chris Styles

No tags | 12 comments

12 comments on AnalogOut:

Steve Ravet

# 22 Sep 2010

This page is missing the link to the library API details.

Chris Styles

# 22 Sep 2010

Hi Steve,

Thanks for pointing that out. Its now fixed!

Cheers, Chris

hide maru

# 09 Dec 2010

How to change the settling time?

chiky manzanares

# 31 Dec 2010

Hi! My name is chiky. I have a question. I have a fan that works 12V. I get move it with mbed Vout (3.3 V). But I wanna control it giving differents voltage. For this, I though I could use the analogic out, however I set 3.3V in analogic out [out.write(1.0f)] but the ventilator doesn't move and I check with multimeter that the measure is 3.3V. I think that is because there is no enough amp. I wonder if I could solve this with an Electronic assembly to give more AMP. Or another solution. Thanks.

Sorry for the tirade!

Titi Tix

# 31 Dec 2010

For increasing the current capabilities you must use a buffer. Remember that the mbed itself output is about 40mA .. for a FAN you need about 100mA for 1w type 12v fan. At 3,3V I think you might get only 2 speeds ( "spinning from time to time" and "stop" ). You must consider using a voltage conerter like AO for extending the range from 0-3,3 to 0-12 ( something like 4x ) and a repater (transistor) to be able to drive the fan properly. And also it might be easier to control the speed of the fan on a PWM output.

chiky manzanares

# 31 Dec 2010

thanks "not important" your help is important for me. Could you recomend me an AO to buy? As you say mbed would have only 2 speed and I need more precision to get differents speed.

chiky manzanares

# 20 Jan 2011

I solve my problem with a control for motors, I say because sb could have the same problem. Bye

zhengyuan shen

# 16 Jun 2011

chiky manzanares wrote:

I solve my problem with a control for motors, I say because sb could have the same problem. Bye

Hi, I have a similar problem as you had before. I got a 5V 0.16A DC motor, and I don't know how to drive it. Can you give me some advice? THX

Philips Philips

# 16 Jun 2011

zhengyuan shen wrote:

chiky manzanares wrote:

I solve my problem with a control for motors, I say because sb could have the same problem. Bye

Hi, I have a similar problem as you had before. I got a 5V 0.16A DC motor, and I don't know how to drive it. Can you give me some advice? THX

I think maybe you can use transistor

Here is my "prototype" for driving a small electric motor using one of the Mbed's digital out pins (actually a PWM pin, but that's not important).

The Mbed can only source about 40mA from a single pin, so in order to drive an electric motor, one needs to use a transistor... The only transistor I had lying around were some BC337's, which are pretty normal NPN transistors, but the maximum current they can drive is 800mA... so I used this tool to find out what value resistor I needed on the Base... as the 5v electric motor I use had a resistance of 2.2ohms, I decided to use 400ohm resistor... which, if I've understood everything, will mean the maximum current through the transistor will be 654.5mA... Hooray! That gives me a 145.5mA tolerance for variance in the components :)

See below for a screenshot :)

/media/uploads/bloodline/mycircuit.jpg

-Edit- I've modeled the electric motor as a 2.2ohm resistor with a flyback diode, which I included in my design to protect the transistor from any reverse current off the motor (which I know happens with solenoids... not sure about motors).

http://mbed.org/forum/electronics/topic/2392/

Paul Shipley

# 10 Nov 2011

I have noticed that the AnalogOut isn't behaving as expected. In the above example program, each step increase should increase the voltage by 0.33V. Connecting it to a oscilloscope, we should be able to see this in action. The 1st step from 0V to 0.33V is detected but the consecutive steps are in error. It seems that the steps are decreasing in value exponentially. The last step causes the output value to be at 550mV instead of the expected 3.3V.

Is there an explanation for this?

Wim Huiskamp

# 10 Nov 2011

I havent used the analog out yet, so assuming the software library is OK.

How do you measure that value? Perhaps you are loading the output. Is there a protective diode in your circuit that starts conducting and limits the voltage to 550mV? Curve could be exponentially in that case.

Paul Shipley

# 14 Nov 2011

I am measuring the value using the oscilloscope.

The only thing the AnalogOut is connected to is a resistor. I will be testing the AnalogOut without connecting it to anything today.

Edited: It has worked as expected.

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