Clock Choices

Using Arduino Microcontrollers
1

Thank you for moving this, I guess it deserved its own thread.

This question concerns the Atmel programmer's fuse options and the fuse choices as listed on the fuse calculator at AVR® Fuse Calculator – The Engbedded Blog

One thing I have never understood well is the clock choices. I always use a 16mhz crystal with 2 22pf caps and select the last on the list and it works. What do the clock selections mean?

Ext. Clock; - true square wave signal? Applied to which pin?
Int. RC Osc. 8 MHz - OK, I know what this means.
Ext. Low-Freq. Crystal - no idea how this varies from the bottom one.
Ext. Full-swing Crystal - no idea how this varies from the bottom one.
Ext. Crystal Osc. - OK, I get this, but have never seen anyone suggest one other than the 16 mhz crystals. Some of the chips are rated for 20mhz, do you just use a 20mhz crystal to achieve that?

What do the other values mean, e.g. "Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms;" Total confusion there.

Would love a quick schooling on these matters if anyone wants to chime in.

Ext. Clock; Start-up time PWRDWN/RESET: 6 CK/14 CK + 0 ms;
Ext. Clock; Start-up time PWRDWN/RESET: 6 CK/14 CK + 4.1 ms;
Ext. Clock; Start-up time PWRDWN/RESET: 6 CK/14 CK + 65 ms;
Int. RC Osc. 8 MHz; Start-up time PWRDWN/RESET: 6 CK/14 CK + 0 ms;
Int. RC Osc. 8 MHz; Start-up time PWRDWN/RESET: 6 CK/14 CK + 4.1 ms;
Int. RC Osc. 8 MHz; Start-up time PWRDWN/RESET: 6 CK/14 CK + 65 ms;
Int. RC Osc. 128kHz; Start-up time PWRDWN/RESET: 6 CK/14 CK + 0 ms;
Int. RC Osc. 128kHz; Start-up time PWRDWN/RESET: 6 CK/14 CK + 4.1 ms;
Int. RC Osc. 128kHz; Start-up time PWRDWN/RESET: 6 CK/14 CK + 65 ms;
Ext. Low-Freq. Crystal; Start-up time PWRDWN/RESET: 1K CK/14 CK + 0 ms;
Ext. Low-Freq. Crystal; Start-up time PWRDWN/RESET: 1K CK/14 CK + 4.1 ms;
Ext. Low-Freq. Crystal; Start-up time PWRDWN/RESET: 1K CK/14 CK + 65 ms;
Ext. Low-Freq. Crystal; Start-up time PWRDWN/RESET: 32K CK/14 CK + 0 ms;
Ext. Low-Freq. Crystal; Start-up time PWRDWN/RESET: 32K CK/14 CK + 4.1 ms;
Ext. Low-Freq. Crystal; Start-up time PWRDWN/RESET: 32K CK/14 CK + 65 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 258 CK/14 CK + 4.1 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 258 CK/14 CK + 65 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 1K CK /14 CK + 0 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 1K CK /14 CK + 4.1 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 1K CK /14 CK + 65 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 16K CK/14 CK + 0 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 16K CK/14 CK + 4.1 ms;
Ext. Full-swing Crystal; Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 0.4-0.9 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 0.9-3.0 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 3.0-8.0 MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 258 CK/14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 1K CK /14 CK + 65 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 0 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 4.1 ms;
Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms;

2

Best schooling is to read the data sheet. Section 9 is clock and start up cycles section.
Then come back & ask again.

3

That was all I was asking for and it was extremely informative. I didn't know that if you used the internal clock you automatically got those GPIO ports freed and got one freed with an external clock. There were good definitions of why when when to use the different clocking options and what those delays are for. It answered all my questions. Thanks again.

Telecommando, thank you too. I didn't really grasp the difference between full swing and low power crystals and now I am going to do some more research on that topic.

4

All of those can be found in the datasheet.

Ext. Clock; - true square wave signal? Applied to which pin?

Doesn't matter. Applied to XTAL1.

Ext. Low-Freq. Crystal - no idea how this varies from the bottom one.

32Khz. Those other frequencies (typically less than 1Mhz) also work.

Full-swing Crystal - no idea how this varies from the bottom one.

High drive power. Typically for high frequency crystals or noisy environment. More current / power consumption.

Ext. Crystal Osc. - OK, I get this, but have never seen anyone suggest one other than the 16 mhz crystals. Some of the chips are rated for 20mhz, do you just use a 20mhz crystal to achieve that?

Yes, you can use other frequencies as well - from DC to whatever specified. Overclocking is also possible - I have ran them at 32Mhz no problem.