PN7462 LPCD Wakeup

[s_ali]

Hi

I am using PN7462C development board.

Started with DoorAccess Example project, have enabled STANDBY and are using LPCD. I can see that PN7462 goes into standby and then wakes up every timer interval (500ms) to check for card detection. Why does it need to wake up using timer? I was of the impression that while LPCD is being used, it will continue to run in the background and there will be an option to wake up PN7462 only when LPCD detects a card presence like it does in case of CLRC663 PLUS. Is that not the case?

While LPCD average current is quite low, having to wake up the PN7462 regularly on timer definitely takes the bulk of power consumption. With this setup, the average current draw for TVDD_IN is about 14uA and about 120uA for VBUS, VBUS_P. If we disable the wake up timer, VBUS and VBUS_P average current draw is about 20uA but then card detection doesn't work.

[IvanRuiz]

Hello,

The timer is used so that the field is not always turned on. Anyway, the LPCD feature unlike the discovery loop can detect a Tag on the field based on demodulation of the field and not necessarily starting a communication, which saves even more power.

Hope it helps!

BR,

Ivan.

[s_ali]

Hi Ivan,

Thanks but I understand what LPCD is and why the timer is used - it polls by using LPCD mode and then goes back to standby again.

What would save even more power is that the LPCD keeps on running in the background while the main processor is in standby and on detection of a Tag, the LPCD component interrupts the processor to wake it up like it does with devices like CLRC663:

We have now spend plenty of time to reduce current consumption and this is the lowest figures that we have got:

With 1 second polling interval (that timer you mentioned):

Overall average current is 90uA.

75uA of this current is taken by the processor (cycling between standby and waking up every second)

13uA is taken by RF/LPCD (TVDD)

2uA is drawn by P_VDD

With 0.35 second polling interval which is what we need (that timer you mentioned):

Overall average current is 200uA.

175uA of this current is taken by the processor (cycling between standby and waking up every second)

13uA is taken by RF/LPCD (TVDD)

2uA is drawn by P_VDD

As you can see that the bulk of the consumption is the processor waking up, manually initiating the LPCD and manually checking LPCD status. Is it not possible with PN7462 or are we doing something wrong in the firmware?

For the project that we are working on, we expect to produce about 10k devices by end of Sep. If we can't use the LPCD the way it is supposed to be used then we will have to move away from PN7462.

Thanks

[IvanRuiz]

Hello,

Yes, Increasing the timer for polling definitely will reduce power consumption. Nevertheless, since PN7462 has embedded the core, as mentioned in the previous pdf image you shared, the interrupt is internal, on the other hand, with CLRC663 there is the flexibility to keep a more optimum control of the host MCU of your preference because it does not embed a core and notifies through external IRQ.

BR,

Ivan.

[s_ali]

Hi Ivan,

How can this internal interrupt for LPCD on PN7462 be used to wake up PN7462 then?

[IvanRuiz]

Hi,

Basically what can be configured for LPCD is the timer and AGC reference value in PN7462. A proper AGC value for your system shall only wake up the reader when an actual card is detected and not a false caused by any detuning on the antenna caused by external noise. It would be helpful to refer to the following document: https://www.nxp.com/docs/en/application-note/AN11785.pdf 

Hope it helps!

BR,

Ivan.

[s_ali]

Hi Ivan,

Which interrupt will wake up based on AGC value? That is the missing piece of information.

The timer you mentioned - that timer will wake up PN7462 MCU irrespective of whether or not there is a card since this is an interval timer and it wakes up the MCU core.

In case of CLRC663, the LPCD feature keeps on running by CLRC663 alone and the host MCU can go to sleep indefinitely as long as there is no card detected. The host MCU gets awaken by CLRC663 interrupt line only when CLRC663 LPCD detects a card. This doesn't require any interval timer to be run on the host MCU at all and therefore saves a lot of power. Can this be achieved with PN7462?

[IvanRuiz · Accepted solution]

Hello,

This is an internal connection which notifies the core and is not configurable as in other readers. As I mentioned before the only setup possible in PN7462 is the timer and the threshold as mentioned in the Application note I have referred you to, which shall be properly set to accurately notify the reader of a card detection while it is in LPCD.

Hope it helps.

BR,

Ivan.

[s_ali]

Thanks for clarification