Replacement of Crystal Oscillator
LutzK
Posts: 7
Dear All,
We have used the Propeller 1 for a couple of applications over the last years. We always used a 8 MHz crystal (ABM7-8.000 MHZ-D2Y-T) on our boards. Due to the general chip shortage this crystal is not available already for some time. Therefore I would like to replace it by another crystal with the same footprint. The 32 MHz crystal, ABM7-32.000 MHZ-D2Y-T, for instance is (still) on stock at the major suppliers.
Now I am wondering if the replacement would work and how I have to change my SPIN programs correctly.
The programs working with the 8 MHz crystal use the following settings:
_clkmode = xtal2 + pll4x
_clkfreq = 32_000_000
Am I right if I assume that the correct settings for the 32 MHz crystal would be?
_clkmode = xtal3
_clkfreq = 32_000_000
Actually, I just thought to test it myself, but it is difficult with SMD parts and we need to order at least 25 assembled boards to get a reasonable price. It would be a pity if all the boards would not work.
Thanking you for your help in advance.
Comments
That should work, though I haven't seen any design with a fast crystal like that. Always 5 or 6 MHz or something similar. Presumably because the PLL only works between 4 and 8 MHz input clock and most designs don't want to run below half the rated speed.
There is an upper limit mentioned in the datasheet. Have you checked that 32MHz is Ok? I can't remember the specs, but that feels a little high.
Is this the right section of the datasheet?
If so, it looks like 32MHz should be fine as long as you don't use the PLL.
I'd recommend changing to an oscillator module providing direct drive on XIN. You can still use the PLL if desired and 8 MHz oscillators is common value.
PS: The following clock mode should suit:
_clkmode = xinput + pll4x _clkfreq = 32_000_000I use a small SMD 10MHz crystal. Always worked perfect for me
and
Probably not what the OP had in mind. The 32 MHz crystal should work just fine.
-Phil
That's the one!! Thanks Duane.
I'm travelling at the moment so couldn't pull that up. Seems the PLL restriction was on my mind, and yeah, all good for XTALs up to 80MHz that run without PLL.
As someone else mentioned, I've also used 10MHz parts on a few projects, seemingly without issue, although that's not guaranteed by Parallax across all temperatures/operating conditions.
You need to proceed with caution, as before the SW issues, you may bump into a HW one.
Getting a 32MHz crystal to oscillate is not trivial, and they prefer lower load capacitors and the P1 may not deliver enough amplifier gain at 32MHz to start oscillate.
It is supposed to:
Thank you to all.
I conclude from the discussion that the attempt using a 32 MHz crystal should work. I am aware that the PLL circuit must not be used in this case which ties the Propeller to run on 32 MHz with no possibility to change the speed by software. But 32 MHz is sufficient for our application. We did not use the full speed in order to reduce the energy consumption slightly because the Propeller runs on batteries.
You could improve that a little by using a 4 MHz crystal with PLL8X instead of PLL4X. This is because the multiplier is really a divider. The 8 MHz crystal gets multiplied by 16 first, producing 128 MHz from the PLL's VCO, then gets divided by 4 (PLL4X) to get 32 MHz as sysclock.
With a 4 MHz crystal it would be multiplied by 16 to 64 MHz then divided by 2 (PLL8X) to get the same 32 MHz sysclock. Less power needed for both the crystal oscillator and VCO.