Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Question about AVR and high frequency xtal

Status
Not open for further replies.
R

Rooftop

Full Member level 2
Full Member level 2
Joined
Oct 28, 2004
Messages
134
Helped
4
Reputation
8
Reaction score
2
Trophy points
1,298
Activity points
1,241
I want to use 14,7456 MHz xtal for my avr. I'm using ATMEGA128 in 5 Volt, for the capacitor i'm using 22pF but it looks like it's unstable. I choose the highest rising time for the fusebit and CKOPT programed. May i know some additional information about how to using high frequency xtal for the avr, about additional hardware or fusebit etc...


Thanks a lot for the attention


Best regards
 

For this type of circuit, you want a parallel crystal, and you need to know the load capacitance the crystal is designed to operate with. This is NOT the value capacitor to build into the oscillator, but the total equivalent capacitance.

The relationship is:


CL=Cs+(C1*C2)/(C1+C2)

where CL is the load capacitance given by the manufacturer, C1 and C2 are the capacitors to ground on either side of the crystal, and Cs is the stray capacitance from the circuit board such, say 5pF or so.

Usually, C1 and C2 are chosen to be equal.

Large values of C1 and/or C2 may cause start up problems.
 

    R

    Rooftop

    Points: 2
    Helpful Answer Positive Rating
Rooftop said:
I want to use 14,7456 MHz xtal for my avr. I'm using ATMEGA128 in 5 Volt, for the capacitor i'm using 22pF but it looks like it's unstable. I choose the highest rising time for the fusebit and CKOPT programed. May i know some additional information about how to using high frequency xtal for the avr, about additional hardware or fusebit etc...


Thanks a lot for the attention


Best regards
Click to expand...

Do you have a parallel resistor (>1MOhm) with xtal?
 

    R

    Rooftop

    Points: 2
    Helpful Answer Positive Rating
No, i don't have a parallel resistor with xtal. Is it necessary? In ATMEGA128 datasheet it just tell that i should use about 20-22pF for the C1 and C2 with equal value.

Anyway thanks a lot for the help.... Please help me because i can't get a stable baudrate with this xtal.

Thanks a lot
 

Do you grounded the metal casing of xtal?
 

    R

    Rooftop

    Points: 2
    Helpful Answer Positive Rating
i have grounded the metal casing, now i'm trying to parallel the resistor.

Thanks for the attention
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top