I've got a sim connected to my microcontroller. The rst, i/o, and clck pins are wired correctly. There is a hardware UART on my board, but since it is full-duplex and not half, I've jumperd RX/TX together.
So far, I toggle RST according to ISO-7816, and my UART buffer fills up with the ATR the sim card responds with. Once I've received the ATR, I change the UART to TX mode and send it a PPS. After sending, I change the UART back to RX only mode. It follows the correct format as stated in ISO-7816, but I do not receive the confirmation bytes from the sim. The confirmation is supposed to be a repeat of the settings I sent.
While I'm not a protocol expert, the most likely cause seems to me, that you send PPS too early- "after sending" can be easily too early on modern microcontrollers. ISO 7816-3 states, that the guard time applies as usual and the waiting time is 9600 etu's. Sending PPS too early means, that the card does not yet listen, which perfectly explains receiving no response at all. Wrong format would cause an error block, which should also be visible on the scope, which supports my assumption.