I'm working on an Energy Meter and I need to add 8 fully differential channels to a MSP430F5437A. I will use the ADS130E08.
My question is about the clock source for the ADS130E08. In my application Clock accuracy (in particular over the temperature) and stability are critical. At the same time I need a really cheap BOM and a small foot print. I'm thinking about using a smd cystal (32768 KHz) for the MSP430 and then sourcing the clock (from an IO PIN) to the external ADC.
Would this solution be the cheapest and with the most accurate clock source?
Thank you.
Salvatore
What should be the input clock frequency of the ADS?
I assume that you want to use the internal DCO on the F5437A device and use the 32kHz crystal clock as reference for the FLL+ module.
Is that correct?
I need 8196 SPS and considering that DRDY (Data Available) toggles at fclk/256 the input frequency should be 2.098.276 Hz which is within the limits of the ADS130E08 for an external Clock (0.7 to 2.25 MHz) . I'll need to sample 5 channels in continuous mode: (24 + (16*5)) = 104 bits.
Yes, I want to use the internal DCO on the F5437A device and use a crystal clock as reference for the FLL+ module. Again, unless there is a cheaper and smarter option. Would you recommend a high frequency crystal instead ? What are the pros and cons?
Also the F5437A should run at full speed since I need to perform a lot of calculations.
Salvo
The 5437 has an universal clock module, not an FLL+. It is a superset of the FLL+ with more options.
if you can live with a small tolerance (see datasheet), you can use the internal REFO instead of than external crystal as FLL reference.
However, the FLL only produces an average frequency. The momentary DCO frequency can be higher or lower, so you'll always get a jitter.
You can run the MSP with 16MHz, put a /8 divider on SMCLK and output 2MHz SMCLK to the SMCLK ouput pin to drive your external ADC. Or, by (ab)using a timer, you can even run the MSP on 24MHz and implement a /24 divider for a 2MHz output on the timer CCR1 output.
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