Mono emitting a long tone, falling in pitch?
You might already have heard it. Mono emitting a long falling tone, lasting for about 3 secs. What is going on?
Protection against short circuits
The tone is an indication from the short circuit protection software, that is a part of Mono Framework. The power subsystem monitors the voltage supply on the auxiliary power rails, and if it detects a short circuit - it cuts the power immediately!
Then the power is cut, the display, Wireless module and the SD card loses power. This means that your software might not be able to continue to run. Therefore we have chosen to let the buzzer emit a long: pheeeeeeeeeeew-like tone, before putting Mono to sleep mode.
No harm is done.
Why you might hear it
Currently there is an issue where the power control circuit thinks there is a fault on the 3.3V power rail, and triggers the short circuit protection routine. This false positive occur only when mono has its USB port connected. We are currently investigating the issue.
Why this is needed
During assembly we became aware that the physical construction of the SD Card connector had a design flaw. The metal housing of the connector can be pushed and bend down to short circuit the SD card connector pins. This, in accordance with the design of the Micro SD Standard, makes it easy to short circuit the 3.3V power rail to ground.
We have counter measures that the situation. The 3.3V rail is current limited, such that nothing will be damaged. But it is still an undesired fast decharge of the battery. Also, the power circuit might get quite hot. Therefore we added a software based short circuit detection routine in Mono Framework. This means we are pleasantly safe guarded against short circuits on the 3.3V power rail.
So, if you hear the pheeeeeeeeeew-sound, do not be alarmed. Mono is OK and will just goto sleep.