Electronic – Arduino PORT manipulation analogRead()

analogarduinoport

I am trying to figure out on my Arduino UNO how to read analog input using port manipulation. I have found a lot of tutorials on how to do that for digitalRead / digitalWrite, but nothing with analogRead / Write.

I want to read the analog data coming from the FSR sensor but I cannot figure out how to do that.

I found that for analog pins I have to use the C port of ATMega, and I tried by testing the code below.

but the only thing I get as result is either 0 or 1 when I get over half of the read values.

How can I get the values as an integer by using port manipulation? Or a double/float, the format of the result is not that important as long it is not boolean.

Best Answer

The analog readings for pins are obtained using the ADC peripheral, not a GPIO port. Additionally, you might not get the performance benefits you are looking for, since the relative overhead of analogRead is not as large as the overhead of digitalRead, and most of the time is spent actually waiting for the ADC to do a conversion.

The exact registers and details are given in the atmega328p datasheet, but I'll give an overview of a simple approach. Of course, you'll want to consult the datasheet for full details and explanations.

First, you'll need to enable the ADC and set the desired prescaler for an ADC clock of between 50 and 200 kHz by setting the ADEN, ADPS2, ADPS1, and ADPS0 bits in ADCSRA. For an Arduino clocked at 16 MHz, this gives an ADC clock of 125 kHz, well within the required range:

^{Datasheet page 258}

You'll then need to set the high bits of ADMUX, which specify the analog reference you want to use (external, 5 V AVCC, or 1.1 V internal reference). The default behavior of Arduino's analogRead is the 5 V AVCC reference, meaning that you'd need to set the REFS0 and clear the REFS1 bit.

^{Datasheet page 257}

You can then select the input you want to read by setting the lower four bits of ADMUX (following table 24-4 on page 258), and then set the ADSC (A/D Start Conversion) of ADCSRA to start a conversion. You can then loop waiting for ADSC to return to 0 indicating the conversion finished (or set an interrupt instead); once the conversion is done the results will be present in ADCH and ADCL.

However, if you're looking to optimize the time for reading a single analog port, you'll want to use a free-running mode. By setting ADATE in ADCSRA to 1 and leaving ADCSRB at all-zeros (default indicating free-running), the ADC will automatically perform analog reads, putting the result into ADCH and ADCL when it's ready -- you can read it at any time, for only the cost of two port reads. Note that in this case, you'll want to read ADCH and ADCL atomically by reading ADC (a syntactic sugar that leads to the compiler reading the two registers atomically):

 uint16_t result = ADC;

If you wish to save power, you may want to set the appropriate bits of DIDR0 as well; digital input circuitry can sometimes use excess power if an analog input not close to 0 or 5 V is provided, and disabling the digital input using DIDR0 will prevent this excess power consumption.

Related Solutions

Electronic – arduino – digitalRead sampling rate for Arduino Uno

You have two problems right now that are separate from each other.

1) Button bounce

2) Improper button wiring

First, you need to introduce a delay into your interrupt function so that it won't keep registering each rise and fall of the bounce as a separate button press. Use "delayMicroseconds()" because apparently you can't use regular "delay()" inside interrupts. Id suggest a 20,000 microsecond delay.

Second, you need to put a pull down resistor onto the digital pin that you are sampling.

The problem you are having is that the pin is in input mode and thus at high impedance (very high resistance) and thus has a tenancy to "float". If you put a voltage to it, it charges up like a capacitor and since the pin itself has very high resistance to ground, it takes a while for the charge to dissipate to the point where the Arduino will register a LOW again. A 10k resistor from the pin to ground will force the "capacitor" to discharge quickly after current from the button has stopped flowing.

Finally, the Arduino is pretty quick at it's read/write operations (I'm a mechanical engineer, ALL electronics happen "quickly" in comparison to mechanical systems, so it's relative I guess). Here's a link to some test results.

TL;DR verson:

What these values tell me, is that we can do about:

10 analog 10-bit readings per millisecond with analogRead()
128 pwm settings per millisecond with analogWrite()
220 pin reads per millisecond with digitalRead()
224 pin writes per millisecond with digitalWrite()
1056 pin reads per millisecond with direct port reads
1059 pin writes per millisecond with direct port writes

How to read these (Arduino) schematics

This is a horrific schematic and I can't blame you for being confused.

"Are there connections omitted?"

Yes, one would wonder if they forgot to connect them. They do have a net name next to them, however, and you'll find that same net name back elsewhere on the schematic. Everything with the same net name is connected. Personally I prefer the use of ports to indicate this, so at least it's clear it's not a forgotten connection.

enter image description here

I commented on the following part of the schematic in another answer:

enter image description here

Here there's too much unnecessary information shown, and required information is missing. It's probably not clear on first sight but that "TS42031-160R-TR-7260" refers to the switch at the top, just like the word "RESET". That's the reset switch, but it would be much clearer if it only showed the refdes (that's short for reference designator). Each component has a unique refdes which identifies it. For a switch it could be SW1. The long type number is completely irrelevant to understand the schematic and it shouldn't be here. Your schematic should only show basic information, like "R22" as the refdes for a resistor together with its value, like "12k". That should do. Whether it's a 1/4W or 1W is not important to read the schematic, so leave it out. This kind of information, just like the manufacturer's ordering code belong on the BOM (Bill of Materials) which you want to have next to your schematic in case you need the extra information. The question about the power of the resistor may come up in a design review: check the BOM.

enter image description here

This may be the worst part of the schematic, and you'll only understand it with a lot of experience and some guessing. It doesn't say anywhere, but these are the power connections for the dual opamp U5. It seems to be the only IC for which no power connections are shown, and a dual opamp in an 8-pin package often has its power on pins 4 and 8. (That's where experience comes in).

All in all, this is not a good example to learn reading schematics.

Best Answer

Related Solutions

Electronic – arduino – digitalRead sampling rate for Arduino Uno

How to read these (Arduino) schematics

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