The i2c driver for samd21 CPUs has timing problems with the standard compiler optimization -Os. The optimization level for this CPU was enabled in #2848. It seems that e.g. this code line is not executed which may lead to collisions on the bus. In my case I got stuck using tests/drivers_tmp006 with an Atmel samr21-xpro and an external tmp006 sensor. As a bugfix I see two alternatives (for now):

1. Disable size optimisation for samd21 again by reverting cpu/samd21: set optimization level to -Os #2848

• PRO: Who knows where else it introduced failures?
• CON: Huge code sizes

   text    data     bss     dec     hex filename
  15516     128    2792   18436    4804 ...samr21-xpro/driver_tmp006.elf

1. Stay with -Os and use puts() instead of DEBUG() at critical parts (as shown above)

• PRO: Smaller code sizes; Does not hurt in this case; Improves current situation
• CON: Seems a bit like a random solution; Probably compiler dependant?; Similar bugs in other drivers possible

   text    data     bss     dec     hex filename
  10236     128    2792   13156    3364 ...samr21-xpro/driver_tmp006.elf

BTW: I used gcc version 5.2.1 20151202 (release) [ARM/embedded-5-branch revision 231848]

What's your opinion?