question about using a crystal with a propeller
dhoyos
Posts: 2
Hi all,
This is my first post, I'm just starting to learn how to use the propeller, and analyzing some circuit diagrams on the internet and datasheets, I saw something I've never seen before in other microcontrollers.
Normally, both crystal pins are connected to each of the two leads of the crystal and also to a 22pF (or a similar value) which are connected to GND. However, I've seen a lot of circuits using the P8x32A that have the XI and XO pins just connected to an external crystal and nothing else. What's going on there?
Also, to be able to use the Propeller at full speed, which is the correct crystal value to use? Is it 5Mhz? Thanks!
This is my first post, I'm just starting to learn how to use the propeller, and analyzing some circuit diagrams on the internet and datasheets, I saw something I've never seen before in other microcontrollers.
Normally, both crystal pins are connected to each of the two leads of the crystal and also to a 22pF (or a similar value) which are connected to GND. However, I've seen a lot of circuits using the P8x32A that have the XI and XO pins just connected to an external crystal and nothing else. What's going on there?
Also, to be able to use the Propeller at full speed, which is the correct crystal value to use? Is it 5Mhz? Thanks!
Comments
Welcome to the forums!
The 5Mhz crystal:
http://www.parallax.com/product/251-05000
is already rate for 20pf. Drop in. This is rated for 80 Mhz operation.
You could also use a 6.25 Mhz Xtal:
http://www.parallax.com/product/251-06250
to acheive 100 Mhz operation.
Please ask more questions if need.
In many cases, people that were using external capacitors were getting the values wrong and their crystal would not clock. The microcontroller would never start.
But it would be wisest to buy your crystals from Parallax as other source might have crystals that run with different capacitor requirements. This was the source of all the problems. A machine that tests the exact capacitance required is expensive (thousands of dollars) and people were buying crystals without knowing the right capacitance required.
As I recall, the faster the crystal, the more you have to be careful. So a 5mhz crystal might worked even though it is not quite the right capacitance, but an 50mhz crystal might not work without precisely the right values.