MacGyverS2000
Electrical
- Dec 22, 2003
- 8,504
All of these years I've designed uP circuits with a blind eye towards the crystal portion. I've always felt guilty about it, so I decided to remedy the situation starting with my current project, which uses a PIC and a 20 MHz crystal.
Now that I've read the datasheets for the crystal (an Epson MA-505) as well as datasheets and design guides for crystals, I think I was happier when I was designing blind
My first question is this...what value of caps should I be designing in for proper oscillation?
More capacitance means a more reliable oscillation, but also means slower startup times. The suggested range from Microchip is 15-33 pF for a 20 MHz chip, so I chose 18 pF based upon the load capacitance listed by Epson for the chosen crystal...although I'm not sure if that's the correct way to do it (the datasheet for the PIC suggest testing was done with a similarly spec'd crystal, also from Epson, but doesn't specify anything other than the range given above).
I'm unsure as to where I can find drive power on the datasheet, or if I'm supposed to calculate it based upon the external cap values and crystal frequency.
My second question...do I need a series resistor?
The PIC datasheet says it's needed if the crystal is AT cut (which I don't believe it is), but Epson's design notes goes into some pretty heavy testing to determine the value.
Please, someone help me out with some useful rules of thumb on this one to get me back in the game without a minimum of headache.
Thanks!
Now that I've read the datasheets for the crystal (an Epson MA-505) as well as datasheets and design guides for crystals, I think I was happier when I was designing blind
My first question is this...what value of caps should I be designing in for proper oscillation?
More capacitance means a more reliable oscillation, but also means slower startup times. The suggested range from Microchip is 15-33 pF for a 20 MHz chip, so I chose 18 pF based upon the load capacitance listed by Epson for the chosen crystal...although I'm not sure if that's the correct way to do it (the datasheet for the PIC suggest testing was done with a similarly spec'd crystal, also from Epson, but doesn't specify anything other than the range given above).
I'm unsure as to where I can find drive power on the datasheet, or if I'm supposed to calculate it based upon the external cap values and crystal frequency.
My second question...do I need a series resistor?
The PIC datasheet says it's needed if the crystal is AT cut (which I don't believe it is), but Epson's design notes goes into some pretty heavy testing to determine the value.
Please, someone help me out with some useful rules of thumb on this one to get me back in the game without a minimum of headache.
Thanks!