microcontroller
Please help me at how to generate a variable analog signal(0-5V) from a in-built AVR microcontroller's PWM(0-5V).
I am doing a project on I-V data logger.I am using a MOSFET as variable load & want to drive the gate voltage(Vgs) of MOSFET by a variable voltage which comes from pwm of microcontroller.
I would be very happy if somebody come up with answer.
Thanks in advance.
Nagu Bhanoth
You are not waiting for the ADC to finish the conversions. You would have to do something like this:
for (;;)
{
ADC10CTL0 &= ~ENC;
while (ADC10CTL1 & BUSY); // Wait if ADC10 core is active
ADC10SA = 0x200; // Data buffer start
P1OUT |= 0x01; // Set P1.0 LED on
ADC10CTL0 |= ENC + ADC10SC; // Sampling and conversion start
__bis_SR_register(CPUOFF + GIE); // LPM0, ADC10_ISR will force exit
P1OUT &= ~0x01; // Clear P1.0 LED off
}
}
You should read the user guide - chapter 20 (I think) ADC10 and look at the code examples on ADC10 with DTC
You can do this in software and keep track of all the required timing yourself, but most microcontrollers have hardware that can generate PWM autonomous after you initialize it. The basic things are a timer/counter and an Output Compare Register (OCR):
The counter continuously increments, represented by the slope, and is compared with the OCR. The PWM output goes low when the counter exceeds the OCR, and goes high again when it resets. The 5 % resolution you need (20 steps) is not a problem; a 16 bit timer can generate PWM with a better than 0.002 % resolution, and even an 8 bit timer can do 0.4 %.
All you have to do is program a new value in the OCR. The lower the value, the smaller the pulse width. The following animation is from this site:
Further reading
Using the AVR’s High-speed PWM, an Atmel application note.
ATMega32 datasheet describes on page 69 ff. how to use Timer0 for PWM operation.
Best Answer
you need a resistor, a capacitor and an opamp.
opamp is not really necessary when you are driving a mosfet, but will make life a bit easier.
just be aware, that this dac will be quite noisy or/and slow. The bigger cap/resistor values, quiter and slower the output.
another thing to be aware is that you better use 16bit pwm, as 8 bit pwm will give you only 256 discrete values - that could be enough or too coarse, depends on your application, of course.
just google "PWM DAC" for more info and calculations.
EDIT
one more thing - you could put several resistor/capacitor stages in series to improve on the noise side: