Oscillator selection

[Christopher4187]

I have to buy an oscillator for an 18F4550. The demo board uses a 20MHz osc and I use the prescaler at /5. That means I'm really running at 48MHz? If so, are there any benefits to using a 48MHz? In other words, can I use the prescaler at /5 with a 48MHz oscillator and make it go even faster?

[mackrackit]

48Mhz is required for USB operations and that is as fast as you can go with the 18F4550. Yes, there are some that have played with over-clocking but if you need faster get a chip that supports it.

[Christopher4187]

Thanks for the reply. I've been using the 18F4550 with a 20MHz resonator for a USB 2.0 application since 2005 and never had any issues. Are you saying the oscillator has to be 40MHz or the 18F4550 has to "think" it's running at 48MHz? My question wasn't due to any issues, I asked because I'm making a new design and the 48MHz and 20MHz oscillators are the same price and was curious if there are any benefits. In other words, would I see a difference using a 20MHz oscillator with /5 or using a 48MHz oscillator with /1?

[mackrackit]

How you get 48 does not matter, I normally use a 4Mhz resonator PLL'ed up to 48. I only use the 4 because that is what I had at the time I started playing with the 4550, then I found out that you can run a "standard" 18F4550/2550 at 3.3 volts and still achieve the 48Mhz with PLL. So that would imply that the slower the external the less power consumed, as with most things...

[Christopher4187]

You learn something new everyday......

Thanks for the info.