OK, first real project, first real post…
I built an MP3 player with the VS1002 and the LPC-H2148 boards from Sparkfun & Olimex. Now I’m bulding one with the newer VS1033 chip from VLSI.
Since the MP3 chip needs a 12.288MhZ and the ARM will run on the same (or close) , how can I share a single crystal for both devices?
Trying to run two will change the frequency a bit, right?
You may use an external oscillator that drives both chips. See the section 3.4 of the 2148 data sheet, and look for the slave mode.
mred:
OK, first real project, first real post…I built an MP3 player with the VS1002 and the LPC-H2148 boards from Sparkfun & Olimex. Now I’m bulding one with the newer VS1033 chip from VLSI.
Since the MP3 chip needs a 12.288MhZ and the ARM will run on the same (or close) , how can I share a single crystal for both devices?
Trying to run two will change the frequency a bit, right?
It is possible, but you will need to takes some steps to minimize the loading on the oscillating circuit and be sure that it is never turned off by a sleep mode. The normal on chip oscillator consists of the crystal and two capacitors connected to two pins like “OSC_OUT” and “OSC_IN”. The safest place to tap in to the oscillator is at the “OSC_OUT” pin, and it should be done through a series resistor to isolate the input capacitance of the external circuit. Depending on the external circuit receiving the signal you may also need a series capacitor for DC voltage isolation and and a set of bias resistors to set the “OSC_IN” of the external circuit to the center of it’s voltage range. Many people find it easier to just use an oscillator module to drive both chips simultaneously, but if cost and board space is an issue it can be done as described above with a little work.
–Dave
You could also use a clock oscillator to drive both chips. They are essentially an oscillator with a TTL buffer in one chip, such that you can generally connect them to several inputs (so long as the devices being clocked support this).
Thanks for all the feedback and suggestions.
I suppose this is the same as any design in that I want to simplify the design, save real estate, and lower the costs.
I looked a moving to a clock oscillator and at using slave mode per the User Manual and posted suggestions - thanks. One thing I was trying to accomplish is fewer sources of noise on the board by having multiple crystals.
So, the two options are;
Separate crystals
The combined parts would be two crystals and four capacitors. The cheapest CSM-7 or HC49 form factor is about $.60 each. If I shop around and move to a smaller footprint I can do a 5mmx3mm for $.57 each. (NDK NX5032GA Series SMT)
Oscillator
If I move to a clock oscillator, I would need the oscillator and two caps. Cheapest oscillator is about $2.13 at +/-50ppm in a 7.3mm x 4.8mm package. (Conner Winfield CWX Series HCMOS SMD Oscillator)
After pouring through the Digikey catalog, it looks like my best option balancing cost and real estate might be to just stick with a couple of smaller SMT crystals