FreeCalypso > hg > fc-sim-tools
view doc/Serial-SIM-readers @ 58:871281cb0555
doc/Low-level-commands: apdu-checksw documented
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Tue, 23 Mar 2021 00:21:02 +0000 |
parents | b0cf75d0bb2d |
children |
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A serial SIM reader is a very simple hardware gadget consisting of just the following: * An always-on power supply (voltage determined by hw design) connected to SIM VCC contact; * An always-on fixed-frequency clock fed to SIM CLK contact; * SIM I/O line connected to both TxD and RxD of a general-purpose UART, with UART TxD driving SIM I/O by way of an open collector or open drain buffer; * SIM RST line controlled by one of the modem control lines, either DTR or RTS. In order to successfully communicate with the SIM, the UART baud rate needs to be set to clock_freq/372, where clock_freq is the clock frequency fed to SIM CLK contact, determined by the crystal oscillator used in the design of the serial SIM reader gadget. This baud rate must be used initially for receiving ATR and performing PTS, and it can also be used for the entire card session if the user is content with the default slow speed - however, if the SIM card supports speed enhancement (almost all cards do), this speed enhancement feature can be supported with serial SIM readers if the UART baud rate can be switched under software control from clock_freq/372 to clock_freq/64. I (Mother Mychaela) am a very late comer to this game, thus I am not able to speak with authority regarding the history of how serial SIM readers came about. However, my understanding of this history is that early serial SIM readers were RS-232 serial rather than USB-serial, thus they were constrained to use only standard RS-232 baud rates like 9600 bps. And even when "serial" SIM readers are really USB-serial, with a USB UART chip built directly into the custom gadget, most people are still used to standard RS-232 baud rates (and the most important OSes still make it unreasonably difficult to use non-standard baud rates on USB-serial and other ports that are capable of such), thus the classic baud rate of 9600 bps has stuck. When a serial SIM reader gadget is designed to operate at 9600 baud, the crystal oscillator frequency should ideally be 3.571200 MHz: 9600 * 372 = 3571200. However, most real-life serial SIM readers out there have been built in a "race to the bottom" fashion in terms of cost, and they typically feature crystals of some frequency that is close to the ideal one, but not exactly - apparently crystals of other nearby frequencies are cheaper than getting one for the ideal frequency of 3.571200 MHz. (As a practical example, the USB-serial SIM reader I bought on ebay in 2021-03 features a 3.579545 MHz crystal.) These serial SIM readers with slightly-off crystals exploit the baud rate error tolerance that is inherent in all UART interfaces. The use of 3.571200 MHz (or close to this frequency) clocks in USB-serial SIM reader designs is a case of conservative thinking, as opposed to maximizing speed. The highest clock frequency allowed by SIM card specs is 5 MHz for class A (5.0 V) or 4 MHz for classes B and C (3.0 V and 1.8 V), thus if someone sought maximum SIM communication speed, they could use a higher frequency clock (for example, the same 4.8 MHz clock that is used in the USB CCID reader on which I measured it with an oscilloscope, HID Omnikey 3121), use a high-quality USB-serial chip with good support for non-standard baud rates, and design the software around the resulting non-standard baud rates, including baud rate switching for SIM speed enhancement. If anyone has ever made such a USB-serial SIM reader, our tools are ready to support it - but I don't know if anyone has ever built such a gadget. In case of serial SIM readers with 3.571200 MHz (or nearby) clocks, it is my (Mychaela's) guess that their designers probably never considered the possibility of using SIM speed enhancement, and that most programmers who write software tools for such readers don't consider it either, resigning themselves to just the default slow speed mode of F=372 D=1. However, if the SIM card supports F=512 D=8 speed enhancement (almost all cards do) and this mode is activated via PTS procedure, the baud rate becomes 55800 bps. This baud rate is non-standard - however, the USB-serial chip in the SIM reader gadget I got from ebay supports it just fine. (The USB-serial chip in this particular reader is a PL2303, so I initially thought that it wouldn't support non-standard baud rates - however, a closer investigation revealed that the popular notion of these chips not supporting non-standard baud rates is actually a myth: PL2303 chips support non-standard baud rates just fine, instead this support was lacking in earlier versions of the Linux kernel pl2303 driver.) Our FC SIM tools back end for serial SIM readers supports 55800 baud speed enhancement mode whenever the UART and its Linux kernel driver support it, and we also support even higher speed enhancement modes: if the UART can handle 111600 baud, we can use F=512 D=16 mode, and if the UART can handle 223200 baud, we can use F=512 D=32 mode which most newer SIM cards support. And yes, the PL2303 USB-serial chip in the SIM reader I got from ebay supports this fastest 223200 baud mode just fine. Using fc-simtool with serial SIM readers ======================================== The most basic usage is as follows: fc-simtool -d /dev/ttyXXX If the serial SIM reader uses a 3.571200 MHz (or nearby frequency) crystal and you are not going for speed enhancement (i.e., if you are sticking with 9600 baud for the entire card session), then no further options are needed. If the clock frequency stays at 3.571200 MHz (or nearby) but you do wish to use speed enhancement, add one of the following options: -e1 if your UART can handle 55800 bps; -e2 if your UART can handle 55800 and 111600 bps; -e4 if your UART can handle 55800, 111600 and 223200 bps. The actual selected mode will be based both on the UART capabilities declared with the -e option and on the SIM card capabilities indicated in the ATR TA1 byte. If your serial SIM reader uses a different clock frequency such that you need to use baud rates other than 9600 and 55800 bps for standard and speed-enhanced modes, respectively, then you need to use the more general -b option instead of -e, taking one of the following forms: -b base_baud -b base_baud,spenh_baud -b base_baud,spenh_baud,spenh_max base_baud is the initial baud rate to use for receiving ATR; if no further comma-separated parameters are specified, then this initial baud rate persists for the entire card session without any speed enhancement modes. spenh_baud is the baud rate to use for F=512 D=8 speed enhancement mode. spenh_max must be 1, 2, 4 or 8 (the default is 1); it is the maximum further multiplier that can be applied to spenh_baud, if any.