Problems with using load modulation in transparent mode

[Jack_Chen]

在目前的项目中，已经实现ST25R3911B作为主机，ST25R3916B作为从机，使用透明模式收发数据。
我希望可以在透明模式中使用负载调制，以实现自定义的协议，我尝试使用ST25R3916B在透明模式中使用负载调制，但是没有成功，现象是将位targ=1之后，ST25R3916B的MISO没有收到主机的任何数据，且没有办法使用MOSI发送任何数据。
请问ST25R3916B如何在透明模式中使用负载调制？位targ需要如何配置？

Here is the machine translation:

In the current project, ST25R3911B has been implemented as the host and ST25R3916B as the slave, using transparent mode to send and receive data.

I hope to use load modulation in transparent mode to achieve custom protocol, I tried to use load modulation in transparent mode with ST25R3916B, but failed, the phenomenon is that the MISO of ST25R3916B did not receive any data from the host after targ=1. And there is no way to send any data using MOSI.

How does the ST25R3916B use load modulation in transparent mode? How do I configure a bit targ?

Best regards

[Ulysses HERNIOSUS]

Hi Jac_Chen,

did you set SCLK=1 during that operation to enable the receiver?

Even so I have my doubts if that can work as you imagine: Load modulation as performed by the ST25R3916 will have only small effect on the external reader field. The receiver in card mode on the other hand is built to receive strong AM/OOK reader modulation. It can be that the own load modulation will not trigger the receiver which is active at this point of time.

Regards, Ulysses

[Jack_Chen]

很高兴收到回复！
我尝试过设置targ=1和SCLK=1，ST25R3916B作为从机仍然无法收到主机的信号，如果targ=0，那么接收是正常的；
另外，当我控制MOSI不停翻转时候，使用示波器观察主机天线上面的载波信号，载波的幅度没有任何变化，说明ST25R3916B并没有进行负载调制。
我需要确认ST25R3916B使用负载调制，是否需要设置位targ=1。

Glad to receive a reply!
I have tried to set targ=1 and SCLK=1, ST25R3916B as the slave still cannot receive the signal from the host, if targ=0, then the reception is normal;
In addition, when I controlled the MOSI to keep flipping, I used an oscilloscope to observe the carrier signal on the host antenna, and the amplitude of the carrier did not change, indicating that the ST25R3916B did not carry out load modulation.
I need to confirm that ST25R3916B uses load modulation and whether it is necessary to set bit targ=1.

Best regards

[Ulysses HERNIOSUS]

Hi Jack_Chen,

targ=1 is required to perform load modulation. Depending on setting of lm_ext and lm_ext_pol I expect also that you will be able to observe its operation on EXT_LM pin.

I assume you have both ST25R3916B in transparent mode? The host/PCD with targ=0 and the target/PICC with targ=1. Correct? Who is modulating now? At which subcarrier frequency? 

Can you share logic analyzer traces (combined of host and target) and scope shots?

BR, Ulysses

[Jack_Chen]

从机使用的开发板是X-NUCLEO-NFC08A1（ST25R3916B），计划使用板上的线圈在透明模式中进行负载调制，我认为和lm_ext和lm_ext_pol的设置没有关系，与EXT_LM引脚也是不相关的。
主机和从机都工作在透明模式，双方都使能tx和rx，主机/PCD配置targ=0，从机/PICC配置targ=1。目前从机并没有发送实际的数据，只是在MOSI引脚上输入一个方波（如图，脉宽10us），简单验证主机是否可以正常接收。预期是主机可以收到与从机相同的方波，目前使用逻辑分析仪在主机的MISO上没有观察到任何输出。

The development board used by the slave is X-NUCLEO-NFC08A1 (ST25R3916B), and it is planned to use the coils on the board for load modulation in transparent mode, which I think has nothing to do with the setup of lm_ext and lm_ext_pol, and is also unrelated to the EXT_LM pins.

Both host and slave work in transparent mode, with tx and rx enabled on both sides, targ=0 for host /PCD and targ=1 for slave /PICC. At present, the slave does not send actual data, just input a square wave (as shown in the figure, pulse width 10us) on the MOSI pin to simply verify whether the host can receive normally. The expectation is that the host can receive the same square wave as the slave, and no output is currently observed on the MISO of the host using the logic analyzer.

Best Regards

[Ulysses HERNIOSUS]

Hi Jack_Chen,

On your slave/PICC verify that actual modulation goes out:

1. Look at EXT_LM, it should basically be identical to what you put on MOSI. This should verify that the ST25R3916B is actually trying to load-modulate the external field
2. Sniff the field: Use an oscilloscope, connect the ground to the tip (building a loop) and put that into the field. You should see the 13.56 MHz carrier and the load modulation on top (especially when close to the PICC antenna) and also (weaker) when close to the PCD antenna.

Even so it can easily be that some of these low frequency changes are filtered by the PCD receiver: It is built for 848kHz subcarrier and not for 100kHz. Also on the pin you should see correlator (correlation to 848kHz) output.

BR, Ulysses

[Jack_Chen]

很高兴收到你的回复！
我现在不确认EXT_LM是否与透明模式的负载调制是相关的，我在规格书的2.2.1发送器章节看到关于使用RFO1和RFO2的驱动电阻进行负载调制的方式。
我将尝试按照你的描述进行测试，目前的情况是当targ=1时，PICC的MISO无法收到PCD的调制波形输出。

Glad to receive your reply!
I am not sure if EXT_LM is related to transparent mode load modulation, as I see in the 2.2.1 Transmitter section of the specification about the way load modulation is done using RFO1 and RFO2 driver resistors.
I will try to test according to your description, the current situation is that when targ=1, the MISO of PICC cannot receive the modulation waveform output of PCD.

Best Regards

[Ulysses HERNIOSUS]

Hi Jack_Chen,

in the same section 2.2.1 you will also find the sentence "The transmitter can also drive an external MOS transistor via the EXT_LM pin to generate the load modulation signal."

Using lm_ext=1 you should be able to verify that the chip is trying to do load modulation.

BR, Ulysses

[Jack_Chen]

我想我已经可以实现PICC透明模式的负载调制了，实现的方式与你的说法有所不同，配置targ=0，使用的是RFO1和RFO2的驱动电阻进行调制，这个与EXT_LM并不相关。
如图所看到的，基带信号的脉宽是10us，看到调制后的波形类似于ASK调制后的波形。放大信号波形，也没有看到关于848KHz负载波的信号。
最后，还是非常感谢你的回复对我的启发。

I think I have been able to achieve PICC transparent mode load modulation, the way to achieve it is different from your statement, the configuration targ=0, using RFO1 and RFO2 driver resistance modulation, this is not related to EXT_LM.
As can be seen in the figure, the pulse width of the baseband signal is 10us, and the waveform after seeing modulation is similar to that after ASK modulation. Amplification of the signal waveform, also did not see the 848KHz negative carrier signal.
Finally, thank you very much for inspiring me with your reply.

Best Regards

[Ulysses HERNIOSUS · Accepted solution]

Hi,

was never claiming that with targ=0 that you would have something on EXT_LM. Only with targ=1 while doing load modulation on the PICC. 

Good that it works for you but just again the warning: targ=0 transparent receiver should not give a demodulated signal on MISO but a correlated signal.

BR, Ulysses