crystals for atmel chips

[aes92000]

a newb question----

i have some atmega644pa-au chips-
datasheet states:
 Speed Grades
̶ 0 - 4MHz @ 1.8 - 5.5V
̶ 0 - 10MHz @ 2.7 - 5.5V
̶ 0 - 20MHz @ 4.5 - 5.5V

does that mean that there are only the 3 speeds for external or is it possible to use a 16MHz???

[chucktodd]

a newb question----

i have some atmega644pa-au chips-
datasheet states:
 Speed Grades
̶ 0 - 4MHz @ 1.8 - 5.5V
̶ 0 - 10MHz @ 2.7 - 5.5V
̶ 0 - 20MHz @ 4.5 - 5.5V

does that mean that there are only the 3 speeds for external or is it possible to use a 16MHz???

This is telling you that the processor can be clocked at upto 4mhz with a 1.8V power supply. Up to 10mhz needs at least 2.7v and 20mhz needs Vcc to be at least 4.5v.  The 5.5v is the maximum the processor can safely handle.

So to use 16mhz crystal you need Vcc to be higher than 2.7v  I use 3.3v with 16mhz resonator on a 328p.

Chuck.

[jboyton]

I use 3.3v with 16mhz resonator on a 328p.

Have you ever had a problem over clocking it like that?

[DrAzzy]

Those are marked 0 - 20, not 20mhz - as in, it's a range, from 0mhz to 20mhz, and anything in that range is within spec.

So to use 16mhz crystal you need Vcc to be higher than 2.7v  I use 3.3v with 16mhz resonator on a 328p.

Chuck.

See page 310 of the datasheet:

"As shown in Figure 29-1, the Maximum Frequency vs. VCC curve is lin-
ear between 1.8V < VCC < 2.7V and between 2.7V < VCC < 4.5V."

10mhz @ 2.7, 20mhz @ 4.5

4.5-2.7=1.8v
20-10=10mhz

1mhz per 0.18v

That implies that 16mhz is only within spec at 2.7+6*(0.18)= 2.7+1.08 = 3.78v or higher.

That said, it's widely reported that the chips work at 16mhz @ 3.3v, despite this being out of spec. Might not work over the whole temperature range or something, though.

[chucktodd]

Have you ever had a problem over clocking it like that?

nope.

Here is the DigiKey partnumber 490-1198-1-ND 16mhz resonator. I connect a 1M resistor in parallel.  I have not had any problems.

chuck.

[DrAzzy]

Is there a reason you use the resonator instead of a crystal?

[chucktodd]

Is there a reason you use the resonator instead of a crystal?

Acceptable accuracy, cheap price($0.32 per 100), fewer parts(no Caps needed)

Chuck.

[CrossRoads]

I use these
http://www.dipmicro.com/store/XC7-16000
less than a quarter each,
and two of these, less than 7 cents each,
http://www.dipmicro.com/store/C1K22-50
No problems. Stock up, you'll use them in all your projects.

[chucktodd]

I use these
http://www.dipmicro.com/store/XC7-16000
less than a quarter each,
and two of these, less than 7 cents each,
http://www.dipmicro.com/store/C1K22-50
No problems. Stock up, you'll use them in all your projects.

Wow, those HC49's are big.  The resonators I use are 3.5 x 1.5 x0.6mm. Those HC49's are 11.5 x 4.7 x 3.5mm, bigger than the 328 TQFP!

 
Chuck.

[larryd]

Hard to manufacture a small 16Mhz xtal that are cheap.
Small solid state devices are easy.
I prefer the stability of a crystal over a resonator.

.

[DaveEvans]

nope.

Here is the DigiKey partnumber 490-1198-1-ND 16mhz resonator. I connect a 1M resistor in parallel.  I have not had any problems.

chuck.

why the resistor in parallel?

[john1993]

Hard to manufacture a small 16Mhz xtal that are cheap...
I prefer the stability of a crystal over a resonator.

imo silly to consider ceramic over quartz in this day and age. unlike decades ago quartz is much cheaper and far more accurate. on world market hc49s 2-3 cents ea and 16mhz watch crystal size or 5x3.2mm smd less than a dime. sometimes as low as a nickel.

personally i never uses any caps unless absolute accuracy required. in that case tcxo best bet anyway.

[chucktodd]

why the resistor in parallel?

From ECS Inc. Ceramic Resonator Principles

page 3:



The feedback resistor increases the probability that the resonator starts oscillating.

Chuck.

[jboyton]

The feedback resistor increases the probability that the resonator starts oscillating.

The Uno R3 has this resistor. But what is the probability that it will fail to start oscillating without the 1M resistor?

By the same token overclocking a little increases the probability of the processor failing. But apparently not by much.

[chucktodd]

The Uno R3 has this resistor. But what is the probability that it will fail to start oscillating without the 1M resistor?

By the same token overclocking a little increases the probability of the processor failing. But apparently not by much.

My projects have never had a clock failure, so I cannot give you a probability.

Chuck.