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?