TMS37157: Cannot get full response, using TMS3705

Hello,

Have you been able to read/write any other pages successfully?

When you say you've written 5 bytes via MCU, for full clarity, you mean writing 5 bytes from MCU into TMS37157 without using RF communication, correct? If so, did you verify you could read the bytes out correctly? Just sanity checking for starters...

hi~ thank you for replying.

I’ve checked that 5 bytes are correctly written into TMS37157 using SPI Command.

And I'm mostly confused that according to TMS37157 datasheet I should  receive a 10 bytes response, consist of  1 start bytes(7E), 1 selective address(00), 5 user bytes(whatever), 1 bytes Read Address, 2 bytes BCC.

But I always get an 8 bytes response. And  the response seems to be discontinuous which is quite different from expected:

I‘ve changed my code to read page 3 from tms37157 so we can make a comparision between them.

I got 7 bytes this time which is as blow:

It's a zoom of signals in scope above.

And the watch window of IAR tell me that the 7 bytes are 0x7E, 0xFF, 0x00, 0x0E, 0xD5, 0x00, 0x00. It's quite the same as the scope implied.

It seems to me that whatever page I read from TMS37157, the response data become weird after 5 bytes transmitted.  

I have no idea what may be the problem.

As you can see，diagnosis phase is not recorded in the scope. But I am sure diagnosis is performed well because I wrote my code in such a logic that read phase is carried out if only 0xAF is received in diagnosis phase.

I'll upload screeshot of diagnosis part as soon as possible after I finish my business travel.

Besides，I trimmed tms37157 again after my last post. Trimmed byte change to 0x6c from 0x58. And the response of tms37157 changed too. Sometimes no response at all，sometimes 10 or 9 or 7 bytes received if I change the position of tms37157 slightly.
I am wondering whether the parameters of RF passive component on tms3705 side are not best matched，like Q factor or resonate frequency .

Thank you for your advice.

I'll try and post it here if any progress is made.

Hi~

Being busy with another project for some time, the research into communication between TMS37157 and TMS3705 has been delayed .

According your last advice，I replaced series resistors with antenna from 4.7Ω to 22Ω to lower Q factor of antenna with TMS3705. Then communication seemed to be in better case.

Reading Page 9, I got response as below:
start bytes (1 byte)| Page 2(1 byte)| UserData(5 byte) |ReadAddr(1Bytes)| FrameBCC(2Byte)
0x7E | 0x00 | 0x4F,0x50,0x51,0x52,0x53| xxx(random) | xxx(random)

And what exactly I expected to get is this:
start bytes (1 byte)| Page 2(1 byte)| UserData(5 byte) |ReadAddr(1Bytes)| FrameBCC(2Byte)
0x7E | 0x00 | 0x4F,0x50,0x51,0x52,0x53| 0x24 | 0x75,0x5C

ReadAddr in response I got can sometimes be correct(i.e. =0x24). But FrameBCC I got always seems to be random and meaningless.

Beside, I've found something interesting that response became strange whether I make antenna distance larger or smaller. In my opinion, smaller distance will always make communication easier. Correct me if I am wrong.

I'll adjust series resistors to some other values to find the difference. And any advice from you is appreciated.

OK, thank you for your advice.

I'm quite new in RFID communication. Could you please tell me that why in my experiment smaller distance makes communication more difficult not easier? Or it's also associated with the antenna  problem? 

In my project, the distance is required to be among 5mm and 20mm, however my design now only worked(means that only user data ,not  including CRC, correctly received) when communication distance is among 15mm and 30mm.

Here is something about antenna I used:

for TMS3705: antenna size : 32mm*43mm;  inductance :  425uH; wire diameter: 0.2mm; cable length: 150mm; Resistance: 6.7Ω; Circles: not sure;

And here goes the picture:

Capacitor is not included in the picture. It's mounted on a demo PCB.

And the schematic:

The waveform  of L2. I measured is like this:

Maybe you can help me figure out whether the antenna is working well or not.

And I wonder that since response from responder is uploaded during discharge phase, can it be the cause that the duration of discharge phase is too short so that the last three or four bytes can't be successfully uploaded ?

Thank you again for your patience.

I’ve  done what exactly you suggested, twisting the lead, removing R3 and C2 then shorting both of them, replace C10 with a 22uF capacitor. Beside I adds a  paralleling capactitor to CR, and the frequency changed from 135.1 to 134.4, a bit closer to 134.2. However things didn't go any better.

Then I started put my focus on reviewing design of responder. After looking into waveform of RF pin on TMS37157, I was confirmed that response was not sent completely. The waveform is as below:

A response from TMS37157 will always be 96 bit which make the duration about 12.2ms. Compared to the waveform of a properly working read action from online document as below,  I believe that something goes wrong with responder.

I wonder why amplitude of my response waveform keeps decreasing and then suddenly disappers, ending with a 2V constant level. It seems that responder stops response by itself. Can it be a antenna problem?

Here goes design of responder :