When I read a single block which is 4 bytes long, why do i get 5 bytes back?

[MBenn]

I have got a program running that goes through the following steps

• Nfcv Poller Initialisation
• Nfcv Poller Check Presence
• Nfcv Poller Collision Resolution
• Nfv Poller Select
• Nfv Poller Read Single Block

This is reading iCode tags using the ISO15693 protocol, but the response i get back from the Read Single Block is 5 bytes long, with what appears to be an additional 0x00 at the beginning. I know this because the Read Single Block routine takes several arguments, one being the rxbuffer, its size and the rcvLen - it is this last parameter that is getting set to 5.

To confirm my code was working correctly, I used the ST25R3911B Disco Demo board and the windows software to set values into block zero that were non zero values. For example

Value Set on PC: 0/0: FF 0/1:00 0/2:00 0/3:00

Value read on Raspberry Pi: 00FF000000

Value set on PC: : 0/0: 00 0/1:01 0/2:02 0/3:03

Value read on Raspberry Pi: 0000010203

So I am confident that I have an extra value at the beginning of the returned dataset. Is this a code bug?

[Grégoire Poulain · Accepted solution]

Dear Matthew,

The Read Single Block method will compute the Request and place the Response on the given rxBuf.

The response is composed by:

On the rxBuf you will receive the card's reply, expect: SOF, CRC and EOF.

Therefore the first byte will always be the Response_Flag.

Additionally, on the method's argument list it is mentioned that the Response_Flag will part of the response.

* \param[out] rxBuf    : buffer to store response (also with RES_FLAGS)

That is the reason you receive overall 5 bytes, where only 4 are Data.

Kind regards

GP