Good afternoon!
There is a problem. We connected the HL8548 modem to the SIM card according to the recommended scheme. Collected 3 copies. Two works, and one is not very good. That SIM card is determined, then no. They started to understand. It turned out that if you pull the UIM1_DATA data line with a 20K resistor to the power line, then everything works! We started to understand further and it turned out that there were recommendations (ETSI TS 102 221 V11.0.0 (2012-06)), which indicated the need for this brace (paragraph 5.1.5).
I have two questions:
Why do two instances work in this case?
Why is there no pull-up resistor in the recommended circuit that is recommended in the specification for SIM cards?
It is recommended to use a oscilloscope to compare the good device and bad device SIM interface signal diagram. If you don’t see the any different, then it needs to take debug log for analysis.
Pull-up resistor at UIM_DATA is not a must. HL series dev-kit does not mount the pull-up resistor. Could you please check your design (schematic and layout) can meet the following suggestions.
(1) If a ESD diode is used on the application, check the capacitance loading <10pF. ESD diode should be placed close to the SIM socket.
(2) A 100nF de-coupling capacitor on UIM_VCC. The capacitor value to be tuned on application. Capacitor should be placed close to the SIM socket.
(3) The length of the track between the module and the SIM socket should be less than 10cm.
Note: If the SIM card vendor recommends to have a pull-up at the UIM_DATA, then you can add it.