wake up through external interrupt

[info@ckmintech.com]

We need to wake up a PIC when it is very close to a RFID 125KHz reader. We meaured the voltage generated by the coil is as littler as 0.15V.

Is it sufficient to serve as L-to-H external interrupt to wake up a PIC?

[jecottrell]

Why not use an op amp to bring the voltage up to the required logic level high of the chip/Vdd that you are using?

[info@ckmintech.com]

The device cannot use more then 3 uA current. The deisign is now using 1.8 uA (< 3uA) when sleeping. If I use a opAmp in standby mode, the current must be far much higher than the requirement.

[jecottrell]

Ahhhhhhh......

OK. Are youusing a PIC with a comparator? If so, there may be a way to use a 0V ref and your input. One of the pros might be better able to say whether it's possible. I use a scheme in one project where I set a ref value and bring it out on one pin and then feed it back into the comparator on another. (I didn't come up with the idea, a consultant that I use did.... and it works pretty slick.) The problem is that you're going to start using current by turning on peripherals (comparator), so it may not meet your needs.

So we get back to your original question:

[Ttelmah]

I'm afraid you are stuffed.
0.15v, is not even out of the 'low' state specification for the PIC, left alone enough to be seen as high. You _need_ to either amplify it, or use a comparator. Unfortunately the comparator module itself, typically draws more than you have available, as does the internal voltage reference.
The voltage is so low, that you cannot easily use a FET as a simple amplifier, without significant extra parts, and probably power consumption...
Even a low power voltage detector (such as the EM6353), draws 3uA, and needs a minimum of 1.3v.
However some questions do come to mind. What about a larger coil?. Is this the output just of the coil, or have you got tuning components present?. If not, adding a capacitor, and tuning the circuit to 125KHz, can give vastly more output.

Best Wishes

[treitmey]

Just as a side note: How did you come to 3uA spec?
That is soooo tiny.

[shalt]

Well, if you need to use something consuming more than 3uA for a short time then you can charge a capacitor in long time until you need to use your opamp, then you can use the opamp with supplying from the capacitor.

just a view . of course if possible for your application.

Shalt

[libor]

In a tipical RFID application you don't really need RFID-field sensing at a 100% duty cycle.
I suggest to wake up the PIC every e.g 100-500 ms (depends on the application) using a low power real-time clock IC sending a normal CMOS level alarm edge every 100-500 ms into the PIC. (there are RTC IC's running at tipical 0.5uA, but the PIC's low power external oscillator can do the same running at 32768Hz, this also takes very low current, 2-3uA tipical)

Waking up only for some ms time you can power on the comparator (on-board or an external one) using the PIC as a power supply and check for the RFID field with the PIC running at full power, then you put the PIC to sleep again if no RFID signal is present, so no activity is needed.
With an arbitrary duty cycle (RTC wake-up frequency) you can bring down the average current consumption anywhere between the sleeping current and the full-power running state.

I would also think about another solution: not waking up the PIC itself with the RTC, but rather power on only the comparator IC somehow using the RTC IC's periodical alarm pin. (maybe the RTC's alarm pin can serve as a power supply for the comparator IC, - depends on the alarm pulse-length and the comparator power-on delay, but there may be some false signals powering on the comparator...)