atmegacrystaloscillator
I've received the PCB that I have designed.
This PCB has an atmega2560 on it.
I have designed it to use a Crystal Oscillator such as this figure shows :
Is this designed supposed to work on an atmega2560 ?
Because, when I look at the schematic of the arduino mega, I see that they don't use a crystal oscillator, but they use a resonator with a 1M resistor :
Since the question asks for working AVR code for servo control, this article, though written for the ATmega32 microcontroller, would be an excellent resource.
On an Arduino board, the primary clock typically used for the Timer/Counters 0..n is the crystal on the board, typically 16 MHz, though some Arduinos / clone boards use 8 MHz or 20 MHz crystals as well. Section 18 of the datasheet addresses selection of clock source / prescaler from either external or internal clocks.
You do have the option of using the ATmega2560 "calibrated internal oscillator", essentially an inbuilt RC timer, but that lacks precision / stability, as might be needed for the servo signal, depending on the application. To use the internal oscillator, set CKSEL3:0 to 0010. From Section 10.3.1 of the datasheet:
The device is shipped with internal RC oscillator at 8.0MHz and with the fuse CKDIV8 programmed, resulting in 1.0MHz system clock. The startup time is set to maximum and time-out period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0").
An additional suggestion:
If precision of servo control is desirable, then instead of using Timer0 which is an 8-bit Timer/Counter, one of the 16-bit timer counters (1, 3, 4, and 5) would be a preferred option.
With the details provided, it does seem like the crystal may be damaged - it's the most sensitive component on the board, and there's a good chance the pulser has been dropped a few times during it's long life.
The circuit seems reasonable and if it was originally designed with the resistor (rather than the capacitor), then I'd leave it like that. There are similar circuits out there that do use a capacitor as you say:
I'd go for replacing the crystal - choose a crystal designed for series operation such as the FOXLF160 (as opposed to the FOXLF160-20) While you're grabbing the crystal, getting a couple of 74LS38s wouldn't be a bad idea either, given they are only around 50 pence/cents each.
Also, given the price of decent CMOS oscillators nowadays, the other option is just to buy a 16MHz CMOS oscillator (5V supply tolerant) and replace the first stage with that.
Best Answer
Yes.
Using a crystal oscillator to provide the clock input, is shown in section 10 of the ATmega2560 datasheet (as you linked). It is also discussed in the Atmel Application Note: AVR042: AVR Hardware Design Considerations which I linked in an answer to one of your previous questions.
That application note also:
highlights that PCB layout is important;
discusses some of the differences between crystals and ceramic resonators, but doesn't focus on one of the main reasons why ceramic resonators are used - low cost!
explains how to choose the values for the external capacitors used with a crystal; your schematic showing 22pF may or may not be optimal for your specific 16MHz crystal (check your crystal's datasheet for more information).
Note that the partial schematic which you included with the crystal oscillator, is not using the correct
XTALpin numbers for the ATmega2560 (as the text shows, the schematic seems to be for an ATmega16 etc.)Also note that you need to consider which fuse settings to use - again, this is discussed in section 10 of the ATmega2560 datasheet.