SPI communication issue on TMC429+TMC389

[japhy]

Hi, Dr.LarsLarsson,
This is an urgent case and need your help.

After 2 months waiting, finally I got my ES samples back and soldered 2 prototype boards. The TMC429+TMC246 looks pretty good and got to run. BUT the TMC429+TMC389*3 still got a communication problem with SPI. The circuit is followed the figure 18, page 44 of TMC389 datasheet with separated CS pins. The issue is, when I just give 3.3V to the system, my MCU can talk to TMC429 without any problem, but TMC389s will return all 0xFF at the SDO pin. If I apply both 3.3V and Vm_24V to the TMC389, the both TMC429 & TMC389 will return 0x00 to MCU. While SDO pin (MISO line) remains high, the output level is only 0.72V. Till now I didn't find any short solder on my boards.

Any advice on this? Appreciated if you can put this case in a high priority. Thanks!

[TRINAMIC_BD]

Dear Japhy, this looks like a simple problem caused by TMC389 control in my view: Overlapping NSCS? Test pin not firmly tied to GND? These are the only two reasons I can imagine for SDO shorting out the data line.

[japhy]

Yes, It's SDO short problem. All connections had been double verified. After cut down the copper of the SDO pin of each TMC389, I made one of them to work. After check, 2 TMC389 had been damaged. If Chip selected on those 2 IC, then SDO pin will drop to 0.7 V if I give a pull up resistor.

We tried to soldering 6pcs TMC389 with hot air gun on my board, but only 3 pc can work properly. All 3 failures got same SDO short problem. But the resistance check is passed by multimeter (resistance is infinite). That means no solder short.

The performance of the only one is really good. Any way, we still decide to use this chip. I'm affriad that it's reliability issue of TMC389, please do have a investigate on this problem. Or to see if can do any thing to strengthen the SDO output?

[TRINAMIC_BD]

Dear Japhy,

i tested the SDO output with resistive load, a 390 Ohm resistor to GND causes no problem for communication. High level drops by only about 0.8V.
We have done some larger designs with up to 45 motors with these ICs and find them to be completely reliable.

Most probably, several ICs become selected at a time, e.g. TMC389 and TMC429, or two drivers, e.g. two TMC389. This will short out the SDO signal, and the driver driving a "low" signal wins, as low drivers are stronger for a TTL compatible output. This is not a matter of relative driver strength, only short circuit current would increase. The output pad we selected will be able to drive even SDO on a large designs with up to 50 SPI slaves, as load seen is only capacitive.

Please try measuring with an oscilloscope multiple CSN resp. NSCS lines at a time. Especially for TMC429 please be sure, to use the SDOZ_C for SPI, as only this pin can go to hi-Z.
Probably you will find a SW issue setting multiple CSN at a time.
At any time, only one CSN time is allowed to be active (low).

Further, hand soldering of the QFN migh cause a problem. You could have a single IC, where the TST_MODE pin or CSN is floating, then it will disturb the complete SPI bus. This could even be the working IC, as sometimes pins couple capacitively, if soldering joint did not get hot enough.
You could find the driver the follwing way: Pull the SDO line to a medium level like 2.5V, when no IC should drive it, using a resistor, e.g. 1kOhm. As it should be floating, it should exactly follow in the time between SPI transmissions. If some driver is still active, you will see the line being pulled high or low.

[TRINAMIC_BD]

Dear Japhy,

we re-tested some engineering samples of the device, which were not tested with the final test program, and found also one IC with short circuit on SDO - it might in fact be, that this error was not detected in the early engineering sample tests. If you did not get to a conclusion with checking NCS signals, please try exchanging the IC, which you suspect to block the SDO line - it could be, that you have an IC showing the same behaviour. This will not occur anymore with final silicon test.