Atmega328P 32.768KHz Crystal Issue

atmegaavrcrystalmicrocontroller

I am having an issue with 32.768khz crystal connected to an Atmega328P (which I am using as an external source to timer2 in asynchronous mode to wake up the Atmega from sleep every second).

On a breadboard everything works fine and as expected.

However upon assembling the circuit on a PCB, the AVR never comes out of sleep. For some reason the crystal is not oscillating

Can someone explain what is the right way to design the PCB as far the crystal is concerned to ensure correct operation (including selecting the capacitors). I have read about it here n there but never given much thought to it.

To remidy the issue, I tried the following but none of them worked:

Tried multiple crystals
Soldered the crystal directly to Atmega328P TOSC1/2 pins
Added 22pf/10pf caps connecting the crystal pins to GND

Here's a close up picture of the crystal attached to the PCB. I had to add some jumper wires because I broke off the traces trying out multiple crystals.

Best Answer

The problem here is that you don't have the load capacitors on the crystal. These connect from each pin to ground, and their capacitance is specific to the crystal you are using.

A resistor in the oscillator input may also be needed (\$R_L\$). The ATMega's datasheet will tell you about that I would suspect.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

When you were on a breadboard the breadboard's inherent stray capacitance was providing the function, but now you are on the PCB there is considerably less stray capacitance.

The capacitors are usually in the range of 30pf or so, but read the datasheet for your crystal to get its load capacitance and use that to calculate the right capacitor values:

$$ C_{X1} = C_{X2} = 2(C_L - C_{stray}) $$

\$C_{stray}\$ is the stray capacitance of the PCB (it can probably be ignored as it will be tiny), \$C_L\$ is the load capacitance from the datasheet for the crystal.

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Most likely the 22pF values used by TI are a compromise (cost / availability). The crystal can generally tolerate a few pF plus or minus the calculated value. I would guess that some empirical testing went into the decision to use 22pF instead of a closer value, or perhaps 22pF was already on the BOM.

Ultimately, even a calculation like what's in the datasheet is based on stray capacitance 'guesstimation'. You have to test whatever capacitor value you come up with and make sure that it works in your end product.

Also, page 20 of the C1111 datasheet that you linked to says 12-18pF is the range to use for the 32.768kHz crystal. Your mileage may vary.

The most important thing to keep in mind is that the capacitor should be tight tolerance with an appropriate dielectric material (one that isn't highly temperature dependent, such as NP0/C0G).

Further reading: here's a link to a good explanation of the topic of how crystals and capacitors interact.

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Here is the simplest possible LED flasher program that works on the STM32VL Discovery board:

/* Test.c
** Simple program for STM32F100RB to flash LED on PC_9
**
*/

#include <stm32f10x.h>

void delay(void);

void main(void)
{
  // I/O port C clock enable
  RCC->APB2ENR = RCC_APB2ENR_IOPCEN;
  // Set PC_9 to output 
  GPIOC->CRH &= ~(GPIO_CRH_MODE9 | GPIO_CRH_CNF9);
  GPIOC->CRH |= GPIO_CRH_MODE9;

  while(1)
  {
    GPIOC->BSRR = (1<<9);
    delay();
    GPIOC->BRR = (1<<9);
    delay();
  }
}

void delay(void)
{
  volatile unsigned int i;

  for (i = 0; i < 20000; i++)
    ;
}

Best Answer

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