XT and HS

[gramo]

What exactly is the difference between these too fuses on PIC micro's, i.e. how do you know what option too pick for the Crystal speed in use?

 

[Nigel Goodwin]

What exactly is the difference between these too fuses on PIC micro's, i.e. how do you know what option too pick for the Crystal speed in use?

It's a VERY vague distinction - but HS (High Speed) gives more drive to the crystal.

There's no specific changeover point - but 4MHz would be XT, and 20MHz would be HS. Often either will work, but perhaps a reasonable changeover would be 10MHz?, or even 4MHz and below is XT, and anything higher is HS?.

 

[gramo]

Hehe. that’s the vaguest explanation I’ve seen you give to date Nigel!

 

[gramo]

So running HS for 4Mhz+ is safe and accurate?

 

[ericgibbs]

hi gramo,

From datasheet.

EDIT: part2

Eric

 

[gramo]

Great find, thanks for clearing that up!

 

[Nigel Goodwin]

Hehe. that’s the vaguest explanation I’ve seen you give to date Nigel!

It's a very vague subject! - notice that the chart posted says 'guidance only'.

 

[eng1]

The application note AN588 says something about the power consumption:

The LP oscillator
The LP, or low power oscillator, is designed to trade
speed for low power operation. Although this circuit
shares the same topology (schematic) as the XT
oscillator, the transistors used in the LP oscillator have
a higher Rdss value and draw considerably less
current. This configuration is optimum for low frequency
operation, because it trades the away unnecessary
high frequency responses for dramatically reduced
operating currents.

The XT oscillator
The XT oscillator is designed to give a compromise
between high frequency performance and modest
power consumption. The gain of this oscillator is as
much as 15 times higher than the LP oscillator. This
middle range will be used for frequencies up to 4 MHz.

The HS oscillator
The HS oscillator is designed to give the maximum gain
and frequency response. The current consumption is
accordingly higher. The gain is roughly five times higher
than that of the XT oscillator. This gives the PICmicro
the ability to operate at frequencies up to 20 MHz.

 

[gramo]

Soo, by running HS, I will have the best frequency response, would that make timing more accurate?

 

[blueroomelectronics]

If you need accurate timing use a 32768Hz crystal on TMR1 osc. These are more accurate than most high speed crystals.

 

[Nigel Goodwin]

Soo, by running HS, I will have the best frequency response, would that make timing more accurate?

No, it will make no difference.