Changing the MFRC522 Receiver Gain about rfid

[omersiar]

Once I tried Setting Register but obviously failed because of bit shifting. I was really confused as a Newbie, Now it is clear to me, thank you.

What do you suggest where to use Gain Control ? at Library level or Sketch level? Maybe a simple define help others to know there is a Gain Control with Dump Example or Calling PCD_Init(VariableGain); ? other MFRC522 Library for another platform comes with Maximum Gain by default.

byte gain = 0x07 ; // defines Receiver's gain dafault 48db max

/*RxGain [2:0]
  defines the receiver's signal voltage gain factor:
  000 18 dB HEX = 0x00
  001 23 dB HEX = 0x01
  010 18 dB HEX = 0x02  
  011 23 dB HEX = 0x03
  100 33 dB HEX = 0x04
  101 38 dB HEX = 0x05
  110 43 dB HEX = 0x06
  111 48 dB HEX = 0x07
  3 to 0 reserved - reserved for future use
*/



void MFRC522::PCD_Init(byte gain[]) {
  if (digitalRead(_resetPowerDownPin) == LOW) { //The MFRC522 chip is in power down mode.
    digitalWrite(_resetPowerDownPin, HIGH); // Exit power down mode. This triggers a hard reset.

    // Section 8.8.2 in the datasheet says the oscillator start-up time is the start up time of the crystal + 37,74ms. Let us be generous: 50ms.
    delay(50);
  }
  else { // Perform a soft reset
    PCD_Reset();
  }

  // When communicating with a PICC we need a timeout if something goes wrong.
  // f_timer = 13.56 MHz / (2*TPreScaler+1) where TPreScaler = [TPrescaler_Hi:TPrescaler_Lo].
  // TPrescaler_Hi are the four low bits in TModeReg. TPrescaler_Lo is TPrescalerReg.

  PCD_WriteRegister(TModeReg, 0x80);        // TAuto=1; timer starts automatically at the end of the transmission in all communication modes at all speeds
  PCD_WriteRegister(TPrescalerReg, 0xA9);   // TPreScaler = TModeReg[3..0]:TPrescalerReg, ie 0x0A9 = 169 => f_timer=40kHz, ie a timer period of 25�s.
  PCD_WriteRegister(TReloadRegH, 0x03);     // Reload timer with 0x3E8 = 1000, ie 25ms before timeout.
  PCD_WriteRegister(TReloadRegL, 0xE8);

  PCD_WriteRegister(TxASKReg, 0x40);        // Default 0x00. Force a 100 % ASK modulation independent of the ModGsPReg register setting
  PCD_WriteRegister(ModeReg, 0x3D);     // Default 0x3F. Set the preset value for the CRC coprocessor for the CalcCRC command to 0x6363 (ISO 14443-3 part 6.2.4)
  PCD_WriteRegister(RFCfgReg, (gain<<4));       // Set Rx Gain to max
  PCD_AntennaOn();              // Enable the antenna driver pins TX1 and TX2 (they were disabled by the reset)
} // End PCD_Init()

[mdxs]

Well, I think the PCD_WriteRegister approach for RFCfgReg shown in above modified PCD_Init might be incorrect; as that is setting more bits than what the specification says are to be used... this is why that bit shifting is needed.

I would recommend to change:

  PCD_WriteRegister(RFCfgReg, (gain<<4));       // Set Rx Gain to max

Into:

  PCD_ClearRegisterBitMask(RFCfgReg, (0x07<<4));
  PCD_SetRegisterBitMask(RFCfgReg, (gain<<4));       // Set Rx Gain

In that customization of PCD_Init; and also consider why byte gain[] is used as parameter... shouldn't that be simply byte gain (and check the prototype declaration in the .h file)?

Now that may help you customization of the library, but I wonder if it should be done this way in the miguelbalboa/rfid version of the library. As the gain can be changed in the Sketch after calling PCD_Init, it doesn't make sense to me to force everybody to consider the extra parameter.

And it might not be correct to assume a proper default in the library... I guess the hardware/chip provides a default if it is not specifically set via the library. So perhaps manufacturers have a specific reason to use their setting as default.

So, what to do? I guess the best would be to provide some #define entries to describe the RxGain table you provided in a comment block. Together with a helper function to set it to one of those values, using the Clear- and SetRegisterBitMask combo approach. Perhaps adding an if-then guard to help preventing accidental out-of-bounds.

@miguelbalboa would you like a PR for that; and if so, can you suggest some function name to use?

[mdxs]

Perhaps something like this:

void MFRC522::PCD_SetAntennaGain(byte mask) {
    if ((mask >= (0x00<<4)) && (mask <= (0x07<<4))) {
        PCD_ClearRegisterBitMask(RFCfgReg, (0x07<<4));
        PCD_SetRegisterBitMask(RFCfgReg, mask);
    }
}

where the if should actually check for some defined values (not using #define as I said before, but rather some enum in the .h part).

[mdxs]

Closed with #47 being merged into the library.

[albapa]

Hi, should there be a difference whether PCD_SetAntennaGain() is called before or after PCD_Init()? Thanks in advance.

[omersiar]

Should be, because when you call gain control it modify pcd's specific
register you should init before any other register change.
21 Tem 2015 16:25 tarihinde "albapa" [email protected] yazdı:

Hi, should there be a difference whether PCD_SetAntennaGain() is called
before or after PCD_Init()? Thanks in advance.

—
Reply to this email directly or view it on GitHub
#43 (comment).

[albapa]

Thanks for your reply.

I was asking because in the original version the antenna gain was set in PCD_Init() before calling PCD_AntennaOn(). In the update, if you call PCD_SetAntennaGain() after PCD_Init(), this will naturally happen also after switching on the antenna.

I suppose my question really is whether setting the appropriate register corresponding to maximum gain should happen before or after switching on the antenna or it doesn't matter. Thanks!

[YIGITER-CLOAK-AND-DAGGER]

When I use the function in my code mfrc522.PCD_SetRegisterBitMask(mfrc522.RFCfgReg, (0x07<<4)) after initializing the reader The RFID reader will misread the codes for the RFID. It doesn't matter the level of gain. 20% of the time it reads it wrong. When I remove it there is no error. I have tested it with multiple types and the error persists.