Can't understand AGC values

[dakhnod]

According to AN11742.pdf Fig. 3, the measured AGC value should rise with a card moving closer to the reader.

I have a piece of code, that constantly reads 0x26, without writing to it.

To my understanding, this should perform a quick AGC reading.

What strikes me is that the value gets lower with a card moving closer, not higher.

The values hover around AGC Gear 0, AGC Value 300.

Card absent: ~310

Card fully in contact: ~290

This seems like a fairly small variability...

Here are the questions that arose:

1. Why is the AGC value getting lower, not higher with the card moving in?

2. Why is the AGC value variability so low? Is 10 +- enough to trigger LPCD?

3. Should the mechanism theoretically switch to a gear with a higher variability?

For completeness, here is my EEPROM readout, with the DPC regs. It seems to be turned on, but my AGC is not getting high enough to switch gears, I think:

73 DPC_CONTROL DPC enabled, step size 1, start gear 7

20 4e DPC_TIME 20000 / 1ms

00 DPC_XI

40 25 AGC_CONTROL duration 320, duration enable, step size 0, step size enabled

72 01 73 01 72 01 6e 01 66 01 63 01 5d 01 4e 01 90 01 ad 00 a7 00 9e 00 96 00 87 00 4a 00 DPC_THRSH_HIGH
370, 371, 370, 366, 358, 355, 349, 334, 400, 173, 167, 158, 150, 135, 74

00 00 DPC_THRSH_LOW

00 DPC_DEBUG

05 DPC_AGC_SHIFT_VALUE

09 DPC_AGC_GEAR_LUT_SIZE

f9 f1 f3 f5 f7 f0 f2 f4 f6 96 66 46 36 26 16 DPC_AGC_GEAR_LUT

20 4e DPC_GUARD_FAST_ MODE

a8 61 DPC_GUARD_SOF_ DETECTED

90 01 DPC_GUARD_FIELD_ON 

5d 91

Thanks for any hints!

[KaiLi]

Hello @dakhnod 

Please see the comments as the below:

1. Why is the AGC value getting lower, not higher with the card moving in?

Which is related the threshold.

2. Why is the AGC value variability so low? Is 10 +- enough to trigger LPCD?

AGC value variability will not trigger LPCD, LPCD needs to be abled via command.

3. Should the mechanism theoretically switch to a gear with a higher variability?

A card is entered into the antenna field, which increases the load, and therefore the output power. As soon as the measured AGC value exceeds the AGC high threshold value of the corresponding gear (DPC_THRSH_HIGH), the PN5180 (resp. PN7462) automatically switches to the next gear with lower output power. This automatically reduces the measured AGC value, until the card is moved closer to the antenna.

[dakhnod]

Hello,

thanks for taking the time.

I understand that the chip should be switching gears by itself anf exit LPCD, if configured so.

Yet, my question still stands: Why is the AGC value getting lower upon inserting a card, without the gear switching?

Thanks!

[KaiLi]

Hello @dakhnod 

Which is related the threshold. Do you know what is the threshold in your device?