Clock source fuses startup time

Go To Last Post
2 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hey guys; I know I should know this from working with AVRs for so long, but I have never known?! I guess I just never had a reason to investigate it before; but the datasheet does not explain why.

When you select a clock source you also must select the startup time and such; for example on those fuse calculators; you can select like:

Quote:

High Freq - Startup-Time 6CK + 4ms
High Freq - Startup Time 256CK + 2ms
and so on...

I always assumed thats just for allowing the power to even out and/or giving yourself a bit of time to enable other peripherals before the MCU starts or whatever; but I don't know forsure...

What is the purpose of selecting start-up times? And what are some examples of when to select XCK & Xms over say XCK & Xms?

Thanks.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Some timing devices take longer to start than others.

Ceramic resonators start much faster than crystals, for example. Both use the same same clock source setting. Internal oscillator starts very quickly.

You can always use the longest possible startup time. The only thing that will be hurt is how long it takes to get going. Its rarely very long, anyway.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net