K, nice theoretical analysis of the ADXL001 on the LPC 10 bit ADC (Ai
Ling, take note :).

I don't think we'll need an external ADC. The 10 bits (0.2g/bit) range
is JUST fine. Worst case, we'll throw an amplifier at the problem in
order to cut down the enormous 70g range of the sensor. But I bet we'll
be fine with just hooking it up directly. Or perhaps "worst case" is
that we oversample by 20x and squeeze out an extra 2 bits of resolution
by oversampling then LPFing.

Again, all the pyro node needs to do is to detect launch reliably
(although down the road it might do some crude integration, too) so
accuracy is not a huge concern here.

But, K is right Jeremy and Mike: you should be up and running with the
ADC as soon as you can. Oh, and also the SPI port so we can use the
SCP1000 pressure sensor :)


K wrote:
> Hi Jeremy & Mike,
> I don't have Mikes email...I don't know where I put
> it. Can you please forward? Thanks.
> --------------------------------------------------------
> http://www.analog.com/static/imported-files/data_sheets/ADXL001.pdf
> g != grams in this discussion.
> g = 9.8m/s^2, use a different number if you like.
> I was looking at the specs for ADXL0001. So as the
> firmware team, you guys need to stay ahead of the
> game as much as possible. I did some quick reading and
> here is what I came up with:
> The lpc2148 has a 10 bit ADC. The ADXL is biased
> to 1.65V (Vsupply/2) Typical. So we get 1.65V to 3.3V
> for +g and 1.65V to 0V for -g. Unlikely that the part
> would hit either rail in practice (+/- 70g) or
> that the circuitry would allow it. But we will assume
> that it can go full range.
> Given:
> a 10 bit ADC on lpc2148
> 512 steps from 0g to 70g (half the range)
> 16mV/g Typical sensitivity on the 70g part.
> 0g  = 1.65v
> 70g = 3.3v
> Then:
> 512 steps in 1.65 v = 1.65/512 V/step = 3.2 mV/step
> 1g/16mV * 3.2mV/step  = 0.20g/step = (1.97m/s^2) / step
> Using this ADC we can discriminate down to two meters
> per second acceleration in +/- direction, under ideal,
> no noise conditions, with ADXL->LPC. I didn't mention
> conversion times, but on the LPC it is about 2.5uS, it
> takes 11 conversion times to get a valid read, so you
> get down to about 400khz. Ewww. Ick.
> So, my advice is to be familiar with the lpc2148 ADC
> (you can use the on-board potentiometer to test, it
> is hooked up to the ADC GPIO on the Olimex board) before
> Friday's meeting, since you'll probably be asked
> what you can/'t achieve with the lpc2148 and whether
> to use an external ADC instead (with perhaps 24 bits resolution,
> so you'll need to calculate your minimum resolution &c based
> on the specs for that part...My crystal ball sees a spreadsheet
> in your future. :)
> An external ADC would probably communicate with SPI.
> ------------------------------
> Tuesday:
> We should start looking at freertos. We'll need Dave's help
> to get this going.
> Freertos:
>  git clone git:///git.psas.pdx.edu/git/freertos.git
>  (Do 'make clean' first, then 'make')
>  http://www.freertos.org/
> See you Tuesday!
>  -K

Andrew Greenberg

Portland State Aerospace Society (http://psas.pdx.edu/)
and...@psas.pdx.edu  P: 503.788.1343  C: 503.708.7711

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