arbitrary lf em commands

Goals:

1. Improved logging of lf em commands and responses
2. Greater certainty in command sequences
3. Easier testing of new commands

Methodology

This is documenting the actual commands used by existing code. Phases include:

• Document the existing command sequences
• Document the existing logging APIs
• Define small set of timing-sensitive functions as abstractions
• Implement the abstractions
• Add logging

The goal is to improve logging and debugging, and allow easily testing new LF commands.

EM4x70 (aka ID48, aka Megamos)

Only six command sequences currently used:

#define EM4X70_COMMAND_ID 0x01 #define EM4X70_COMMAND_UM1 0x02 #define EM4X70_COMMAND_AUTH 0x03 #define EM4X70_COMMAND_PIN 0x04 #define EM4X70_COMMAND_WRITE 0x05 #define EM4X70_COMMAND_UM2 0x07

ID Command

Wait for LIW (listen window), and start transmission at next LIW:

UM1 Command

UM2 Command

Auth Command

Write Word

PIN Command

Abstraction required

Possible items to abstract:

• bits to send: quantity of bits to be sent + storage containing those bits
• bits to receive: expected bits to receive + storage to receive those bits
• LIW: special-case handling to synchronize next command
• ACK: special-case handling to wait for ACK
• HEADER: special-case handling to wait for HEADER
• DELAY: ticks to delay before processing next item

Special handling required for:

• HEADER --> 12-bits of zero, 4-bits of one. Consider a timeout: if tag disappears, no pulse found, while sometimes expect long time before HEADER appears (as in SEND_PIN). Read of header may miss the first few bits during transition, so need to special-case handling of this detection.
• LIW --> Timing-sensitive, syncs reader with tag ... reader must send during 32 period where chip's modulator is ON.
• ACK --> This is currently a time-to-delay. Should this be a maximum time to wait for ACK? Currently, could sit waiting for long time if no tag present, as check_ack() has no timeout.

WaitTicks(EM4X70_T_TAG_TWA);
if (check_ack())
    WaitTicks(EM4X70_T_TAG_WEE);
    if (check_ack())
        return PM3_SUCCESS;