Hi, I have a circuit where I run JTAG through the 74CBT to a TXB0108 part to translate the 3.3v to 2.5v to interface to a FPGA. Once the signal transistions to 2.5 volt it goes to a xilinx fpga.
I am running the JTAG as low as 750 Khz and still have the severe ringing - so it seems to be a voltage conflict between the devices, not a signal integrity issue per se.
Distance between the 2 parts is ~ 0.4 inch. We tried a series resistor of 50 ohm between the JTAG pod and the 74CBT device with no impact to signal ringing.
Basically the circuit is unuseable right now, I am in the midst of respinning the PCB and need to solve this issue. I don;t have board access between any of the parts as both are BGA devices, also there are no pullups or pulldowns on the signal lines, nor are there series resistors.
Thanks for the assistance.
For better understand your design and issue, Could you please list your detail design for this? Diagram or schematic is prefered.
Also for the ringing issue you talk about, could you please provide some screensnap shot for the signal?
Hi Junjie, Thanks - here is the partial schematic with the signal. The ringing is severe, once the part is removed, the rest of the circuit exhibits good signal integrity.
I cannot provide you an entire schematic given the project guidelines.
Junjie, Any insight? I have to redesign this over the weekend and have not heard a response.
I am not clear where is the ring happened? Whether at the input side of TXB0108 or the output side of TXB0108?
You mentioned that if we remove the part, there is no ring happened. What is the “part”? 74CBT or TXB0108?
Could you please provide the waveform for the ring?
Your application is 3.3V->2.5V? Is 3.3V in B side or A side?
Hi Brian, Answers below.. Thanks Luke
Ringing is evident at output of TXB part // If I remove the TXB part and run the JTAG chain straight from the Xilinx programming pod at 2.5v, there is no ringing.
Application is with 2.5v on A side, 3.3v on B side.
Checking to see if we saved the waveform we took the part off the board for the prototypes, so I don't have a quick way to recreate.. Will send it if I do.
However it was a severe ring / feedback even as low as 750 KHZ rate. We put series resistors in the 3.3v signal lines of 50 and 100 ohms with no effect on the ringing.
We are having a similar problem with the TXS0108. After trying various methods of eliminating the ringing,
we have decided to eliminate the TXS0108 from the design. The part is edge sensitive and it seems that it is fairly
particular about the capacitive loading. I get 20-25 Mhz ringing during some transitions on one line. However, the one
line is physically and electrically similar to the rest of the lines which are routed and terminated in a similar way etc.
This also only happens with some parts. When we switch out the TXS0108, sometimes the problem will stop.
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