Hi every one,it's me again asking questions.
I have built an RTC on AT90USB1286 with a 32.768kHz Crystal attached to the TOSC pins using Asynchronous Timer feature and I am trying to figure out what should be the values of the capacitors connected to the pins.
Firstly,do I need external capacitors?
I believe I do.Datasheet states "The device can operate its Timer/Counter2 from an external 32.768 kHz watch crystal or a external
clock source. See Figure 6-2 on page 42 for crystal connection". Figure 6-2 shows capacitors connected to pins,not the TOSC pins but the XTALs. So,to calculate the required external capacitors, I use this formula:
Ce = 2 * CL - Cs
Ce - is the external capacitor
CL - is the load capacitance for a 32.768kHz crystal specified by the crystal vendor
Cs - is the total stray capacitance for one TOSC pin.
So,is this correct?I will assume Cs=3pF.
But if that is correct,should I be worried about "an internal load capacitance at each TOSC pin" as specified to the ATMega48PA family datasheet(p.32,doc8161.pdf)?
And it would be understandable if someone asked:"is he blind?that's an other mcu and has deferent specs".
But Atmel's tech support thinks deferent.
To Atmel tech support:
"I am currently developing a product that includes a Real time clock.I am using the AT90USB1286 device with a 32.768khz crystal attached to the TOSC1 and TOSC2 pins.I would like to know the total stray capacitance for each one of the TOSC pins because it's not clearly stated in the datasheet of this particular avr.I have found the corresponding values for the ATmega48PA/88PA/168PA family as described in the datasheet at page 32 Table8-8 (doc8161.pdf). Are these values similar to the mcu I use?"
"The value is similar to that of the mega device ATmega48PA. Sorry for not having that mentioned in datasheet."
That changes Cs to 18pF+3pF for TOSC1 and 8pF+3pF for TOSC2.
So which is correct?