There was a question from Class 3 slide 19 on Serial Ports in Low Power Designs. One of the bullets is -
- Transmission/Reception
- Data length
- Phase, MSB, Polarity
These are just common characteristcis of serial ports and don't have any appreciable effect on power dissipation.
The key low power point of this slide is really the table that shows the Total Charge needed to transfer data by UART, I2C and SPI. Typically the faster you can send data the lower the overall power (total charge) will be. Backwards from what you might expect!
Actually, as BruceMcLaren points out, we are not talking about Ohm's Law here, where the voltage is applied to a fixed impedance (R or Z). We are talking about I=CdV/dt, the charging of capacitances. So P = VI and P = V * C * dV / dt where the dV would equal V, since the internal logic swing for the MCU will be rail to rail. and 1/dt is the frequency. So now you have P = V * C * V * f or P = V^2 * f * C
rruther2: Keep in mind the formula on slide 8 (P=V^2*f*C) is particular to dynamic power, the power CMOS gates consume by charging and discharging internal capacitances. It brings out the effects of transistor scaling very effectively.
Sure -- hopefully your cap leakage is small (if not, get a better cap). A 100nA leakage on a 1 uF will drop 0.1 volt in 1 second. Of course if the load you are going to be measuring is much larger you will need a larger cap or the voltage will drop too much to keep the MCU functioning.
Anatolly1086- It is important to check the internal value. I have seen some recent MCUs with much better pull-up characteristics. I think I saw this on a recent PIC MCU, but I'm not positive.
@WarrenM I believe that a typical internal pullup value is from 1k to 4.7k that is very weak pullup considering temperature variations and some other factors in design to avoid leaks.
@nschneck- I have seen these but have not used one. It seems like a great approach as long as it can be accuate enough for your MCU. If you need to measure very low currents (nA range) I would make sure the current measuring device is accurate enough.
@WarrenM I think I asked this earlier, but have you used external current monitoring ICs that communicate via SPI or I2C to the MCU and measure current on a common voltage bus? We are considering using one in a design.
@Anatolly1086- Most Low Power MCUs have very low power pull-ups available internally. I prefer these unless I need a specific value then I guess you need an external resistor.
@DonH- Good point on the capacitor leakage rate. One tehcnique is to let the cap just sit, unconnected to anything except your meter. This will measure all the leakage and you can adjust you MCU current measurement to account for the leakage. measure the volatge
@GarySxt- I don't have a specific recommendation, but this technique was given in the MicroChip app note I include in the reference section. It might call out specifics on what Cap to use.
I've used amnmeter, of course. Also used shunt resistor with a scope measuring across it to get an idea of dynamic current. Better results when the shunt resitor can be in the ground path, but that isn't always reasonable.
I have used resistor/scope. I like the capacitor idea but question accuracy. You would need a precise high value capacitor with very low leakage. Warren: Can you recommend a specific cap?
Measuring current with a shunt does not work well since the voltages are so low. The capacitor is good, but you need to measure the internal leakage discharge rate of the capacitor to ensure that is not significant.
We've just used an ammeter to measure current on our project so far - in the range of several hundred microamps. But your capacitor idea is pretty clever and we might try it...
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