Hi,
I'm wanting to interface a Wolfson sigma delta DAC through I2S. Unfortunately, my STM32F0 board can produce the sufficient I2S signals but it doesn't produce the clock signal permanently which is required for the Wolfson IC. I could use the MCO output etc but for portability, I want to use a seperate dedicated master clock for the DAC. With regards to the DAC, I'm wanting to use the WM8762GED. I'm using a 48kHz sample rate and with an oversampling ratio of 512x, this results in the DAC requiring a 24.576MHz clock. Fortunately, this is a pretty standard crystal which makes things a lot easier! It also means I can use a 96kHz sample rate in the future oversampled to 256x or 32kHz with 768x oversampling. So what I'm here to ask: I know that you can create a clock using a crystal, two capacitors, a series resistor (sometimes required?) and an inverter. I'd like to use a standard HC49 crystal. Whenever I'm doing this with a standard microprocessor, a pair of 18pF/22pF capacitors always ensures that the crystal starts correctly though I don't have the equipment to accurately measure the frequency at which it is oscillating.
With regards to this application however, is the capacitance required dependent on frequency or crystal package? One of the crystals I'm looking at on RS states an 18pF capacitance (
http://uk.rs-online.com/web/p/crystal-units/6938838/). With respect to the inverter, I'd rather not use a full 74HC69 package or 4069 so I'm looking at the fairchild range of single package logic gates - notably the MC74VHC1G04. What I'm wanting to know however is whether this inverter will be fast enough to drive the crystal and produce the required clock frequency? A propagation delay of 7.1ns into a 15pF load at 3.3v is stated though under my conditions, the capacitance would be higher so I'd expect to take longer. Regardless, surely that propagation delay should allow for >100MHz operation? What are the main parameters to look for with respect to using inverters to drive crystals and produce oscillations? The inverter also states an input capacitance of maximum 10pF but typically 4pF. This coupled with PCB capacitance should surely be considered when choosing the load capacitors for the crystal, right?
Any help is appreciated!
Cheers,
Harris