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2 simcard clk switch

My setup is as on png above, I am using sim800L gsm module, by trial and error i discovered that i can connect sim pins: (rst,gnd,io and clk) and the module detects and connect to GSM without connecting power to sim vcc pin, i also read that sim vcc is optional.

what i want to do is somehow switch clk signal so that one sim card is detected by module at a time, i can do it with manual switch but how can i do it with electrical switch?. pls help.

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  • \$\begingroup\$ You could use Mosfets or, to make your like even simpler, an I²C multiplexer IC. \$\endgroup\$ – Janka Feb 19 '17 at 23:47
  • \$\begingroup\$ I have used Mosfets in previous projects and surely i can get some in my scrap box but never used an I²C multiplexer IC, pls explain how to do about Mosfets, ill try and give feed back soon. \$\endgroup\$ – edb Feb 19 '17 at 23:51
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    \$\begingroup\$ If you want a completely self-contained solution (no discrete mosfets) take a look at analog switch ICs. For example, the MAX4599 is a very small SPDT switch that should work well in your application. \$\endgroup\$ – uint128_t Feb 19 '17 at 23:52
  • \$\begingroup\$ @uint128_t: That's also a good solution. \$\endgroup\$ – Janka Feb 19 '17 at 23:54
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    \$\begingroup\$ This is I²C, so it's all about pulling the IO and Clk lines to GND. You have to avoid the master pulling both device clock lines to GND through the body diode of the MOSFETs. So when you use e.g. use two n-channel MOSFETs, you have to wire each S to host, and the individual D to each card. In that configuration, the non-selected card could pull down the host's CLK line through the body diode but it wouldn't because this is only done due to clock streching, which needs a clock pulled low by the master first. \$\endgroup\$ – Janka Feb 20 '17 at 23:18
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Using analog multiplexer is simple and easy solution.
In the example diagram attached, the two audio source can be two sim cards. SIM-CLK and SIM-DATA can be now shared between two ICs, with the help of one spare GPIO line used to select between two cards.

enter image description here

Remember that it's a bidirectional switch, hence the applications are simply vivid.

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I have no idea what you want to achieve exactly by switching the clock signal but I'm sure that way you will violate the ISO/IEC7816 spec. You may not turn of the CLK signal while the card is processing data. Since data line is still connected and RST is obviously not activated all the time there will be data sent to the card while there is no CLK signal. I.e. so there is no guarantee that the SIMs will work the way you expect.

If you want to deactivate on of the two SIMs while the other one is working you should activate one of the RST lines but not deactivate only the CLK line.

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You can also use the TXS02326A which is described as:

DUAL-SUPPLY 2:1 SIM CARD MULTIPLEXER/TRANSLATOR WITH AUTOMATIC DETECTION AND SLOT DEDICATED DUAL LDO

enter image description here

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