ST25R3916 Adjust Regulators not completing without a tag in the field

[LltWc]

As we've been integrating the ST25R3916 into our final hardware, there's been an issue with the Adjust Regulators command. On external supply power, the command works without any problems. On some (not all) of our battery-powered units, as tested in the field, the Adjust Regulators command never returns, so NFC is unable to initialize. The weird part is that when a tag is held inside the field during initialization, the command completes without any issues and NFC initializes. I assume this is because the tag increases impedance while it pulls power, but this should not be necessary and our system is able to handle the current. The two traces below show initialization with a tag in the field (first), and failure without a tag in the field (second). At no point does the 2.8V rail dip, so I don't see this as an issue with the power supply.

VDD_2.8 (green)
VDD_D (red)
VDD_AM (blue)
VDD_RF (yellow)

I tried with every value of RFO driver resistance, and none worked. Setting the regulated voltage manually did, but that's not a solution that we can use going forward. Is there something I'm missing here to ensure this command completes successfully? The application is for NFC-A only, so the VDD_AM jumping around makes me wonder if there's amplitude modulation happening that shouldn't be, and messing up the initialization somehow.

[LltWc]

I wasn't able to test distances for Adjust Regulator, but for anyone who might come across this problem in the future, there were two things that fixed it together.

First was a race condition; there were some cases where the IRQ semaphore was already given (and shouldn't have been), so Adjust Regulators completing couldn't notify that it was complete.

Second was setting Tx/Rx enable before the Adjust Regulators command. I don't know why this helped, my assumption is that it blunts the power draw somehow, but we didn't see any voltage supply issues. I think detuning having knock-on effects is likely, but I didn't have time for any more experimenting after I had it in a state that worked consistently. Plus we're not going to typically be booting our devices sitting on metal.

The state of the firmware now is that we do three Adjust Regulators retries, and check the actual register value after each one in case the command completed but couldn't notify via IRQ properly, and then if we fail all three (hasn't happened yet!), the regulated voltage is just set to the observed 2700 mV that the command would have.