How come I do not recieve a response from the SIM (ISO-7816)?

Question

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.

Answer 1

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.

Answer 2

I suppose your problem is of the same origin as I had with gsm modems. Sending a command you get an acknowledgement from the device, then send the next command, get ack, etc, etc. Soon or later the device hangs up.

The key is the interpretation of the acknowledgement. You may think the acknowledgement means the command is accepted AND executed. However - at least at ALL gsm modems I know - it means no more but the command was accepted and INTERPRETED - but not executed. In case of time consuming commands you send your next command during previous command is being executed. You do it because you may think acknowledgement means the command is done - but it is not true. The device may or may not buffering cumulative commands, but soon or later the device runs out of resources and hangs up.

I have no experience with device you use but the phenomena seems to be the same.