Part Number: TS3A27518E
We are trying to utilise a TS3A27518E to switch a uSD card (com channel) between either our own controller (NC channel) or a camera's SD slot (NO channel) so that we can read photos back from the SD card when required.
With slower uSD Class 4 cards, everything works fine for a variety of cameras (and a clock speed of up to 50MHz) proving that the hardware connections are fine. However, for HC-I bus cards, the circuit is failing when it tries to switch clock/data lines from 3.6v to 1.8V and ramp up the clock speed to 100MHz. Although the tracks are short and a ground plane is used, my guess is there is too much parasitic capacitance as viewing the clock signal shows that the clock sometimes has a reduced amplitude of 1.0v (down from 1.8v) and also has a DC bias. I was considering using a low forward voltage schottky diode to remove the DC bias but this will also introduce a phase delay that I suspect would be too much at 100MHz. Has anyone had any success in using this type of switch for low voltage (1.8V) and high speed (100MHz) SD signals? I was wondering if I should have used two TS3A44159 devices instead as that has a lower R(on) of 0.3 ohms.
Although the circuit board allows for pull-ups, no pull-up or pull-down resistors are used on the data lines.
RHS screenshot: Ch1 is input to switch, Ch3 is the output.
Appreciate any ideas anyone might have to resolve this.
Hi Paul, Your schematic is very helpful for me to understand what you want to achieve at the system level. I have not found any show stoppers gating you not to use TS3A27518E. However, I will appreciate if you can walk me through the messages you want us to know from your two oscilloscope's screenshots. For example, what are the input and output signals you would expect to see and what is the timing sequence you are controlling? Here are 3 thoughts. 1. Ron should not matter between TS3A27518E and TS3A44159 because your pull up resistor is 47K and the Ron of both devices is insignificant 2. I am more curious the root cause of the DC bias added on your clock lines. To me, it should not exist.
3. If you are really concerned about the signal bandwidth, I would like to know how far you put two SD card slots and your microcontroller.
In reply to Fan.Wang:
In reply to Paul Deroon:
FYI, here are also the PCB layers from top to bottom:
Thanks, Paul. Your stack arrangement looks good to me. I think what we can focus on is why the clock signal corrupted. It looks very strange to me. I don't think it relates to the analog performance on the signal path. It looks to me as digital control or handshake issue from the transmitter/micro-controller side.
DC bias is more analog issue, but I bet we can fix that later and really quickly.
I had also tried using my boards in the Nikon cameras, with the TS3A27518E bypassed. In the first instance I used a bare bones board with only the uSD socket, a VCC link and then 10 ohm resistors across where the switch IC would be. In the second instance I removed the switch IC from an assembled board and bypassed it with ribbon cable. Please refer to the pic below demonstrating this.
In both instances, the clock signal was unclipped with no DC offset at 100 MHz (trace 3 in the lower photo).
Any other thoughts on what I can try?
Any more thoughts Fan (or anyone else)?
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