Electronic – Using the ATMega328 with the internal oscillator

atmegaavrclockoscillator

I have a project that I think would be best suited for an ATMega328P. However, in every simple project I've seen, people always hook up a 16MHz external oscillator. From what I can see, it should have an 8MHz internal oscillator. My project doesn't require a lot of processing power, nor does timing need to be very precise(other than for a UART and I2C). I have a programmer also, so I don't need to worry about bootloaders.

Is there any reason for me to use an external oscillator?

Best Answer

What you don't say is what the accuracy of this internal oscillator is. It took me some time to find it in the datasheet, on page 369.

10%. Ten percent! And that for a calibrated oscillator? This is awful. It isn't unreasonable to expect error as low as 1% for this. Microchip/Atmel provides a document for calibrating the oscillator yourself to 1% accuracy.

I2C is a synchronous protocol, and timing accuracy isn't relevant as long as minimum and maximum pulse times are respected.
UART on the other hand is asynchronous, and then timing accuracy is important indeed. Most UARTs allow a half bit error in the last bit (the stop bit), so that's 5% for a 10 bit transmission.

The factory calibrated oscillator won't do here. You'll have to go through the calibration procedure to get to 1%. In that case you can use the internal oscillator. Otherwize you'll have to use a crystal.

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Electronic – arduino – Can ATmega88 be run at 3.3V with internal clock

Yes, you can use internal clock with Mega88 + Arduino.

To achieve this, you need to burn the bootloader differently:

Download the 8MHz Bootloader

if using AVRDUDE to flash the bootloader, change the fuse settings like below:

# fuses for 8MHz internal RC Oscillator
avrdude -e -U lock:w:0x3f:m -U lfuse:w:0xd4:m -U hfuse:w:0xca:m

Change settings in your arduino environment (preferences.txt)

change      build.f_cpu=16000000L   
to          build.f_cpu=8000000L

Edit the makefile:

In <arduino>/lib/targets/arduino edit makefile
change    F_CPU = 16000000
to        F_CPU = 8000000

Electronic – Do I need an external oscillator for PIC16F887A

It doesn't appear to have an internal oscillator capable of generating its own clock, but if you check page 146 of the PIC16F87XA datasheet it shows how you can use an RC oscillator if cost rather than the number of I/O pins is an issue. The PIC Mid-range Family Guide oscillator section includes the following recommendations:

For REXT values below 2.2kΩ , oscillator operation may become unstable, or stop completely. For very high REXT values (e.g. 1 MΩ), the oscillator becomes sensitive to noise, humidity and leakage. Thus, we recommend keeping REXT between 3 kΩ and 100 kΩ.

Although the oscillator will operate with no external capacitor (CEXT = 0 pF), we recommend using values above 20 pF for noise and stability reasons. With no or small external capacitance, the oscillation frequency can vary dramatically due to changes in external capacitances, such as PCB trace capacitance and package lead frame capacitance.

While the design is based on a relaxation oscillator I found several references to Microchip not committing to an exact time determination formula. However on page 200 of the datasheet you'll find a graph of typical frequencies for several resistor / capacitor combinations. If you're running at 5V it looks like 20pF and a 5.1 kΩ resistor should be close to the mark.

I guess you're using a chip you already have on hand, but the more modern devices include internal clock options that are much more accurate and leave the I/O pins available for other uses. If you won't really need that much speed you could also consider running it as a lower clock rate, as 4MHz is the highest recommended rate and component tolerances may push it over that limit.

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Related Solutions

Electronic – arduino – Can ATmega88 be run at 3.3V with internal clock

Electronic – Do I need an external oscillator for PIC16F887A

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