Hang if no pull-up present

[xross]

The lib includes a wait on SCL high, this is either an event or a polling loop, dependent on implementation.

If this is called on a system where the pull up isn't present then the project can hang forever - not ideal.

If would be better if this error was detected and reported to the developer.

This could be done with timeouts or a simple check for pull-up present at init time.

Real world use case is running USB Audio project on a custom board - we ideally don't want it to hang.

[XMOS-JoeG]

Example:

lib_i2c/lib_i2c/src/i2c_master.xc

Line 58 in 648351b

p_scl when pinseq(1) :> void;

in release_clock_and_wait function. Yes function should wait until clock goes high, but not forever!

Should timeout and report error or similar.

[xross]

Polling loop example:

lib_i2c/lib_i2c/src/i2c_master_single_port.xc

Line 69 in 82e82f2

while (!(val & SCL_HIGH)) {

[xross]

Suggested implementation:

• Add check for pull ups on both SDA and SCL lines at startup in i2c_master() and set an bus error variable appropriately
  • This could be done multiple ways:
    • drive low then high-z and see how long it takes to read 1
    • check for input 1 for quite some period of time (i.e. avoid glitching on floating inputs)
• Extend i2c_res_t for error conditions (could have multiple errors cases for SCL or SDA pull ups missing for example)
• Modify read() and write() such if bus error var err set they perform no action but return error code
• Update read_reg() and write_reg() to inspect and respect return codes from read() and write()
• Do something appropriate with i2c_regop_res_t return value from read_reg() and write_reg()