What does red flashes at about 1hz right at startup mean ?
I have more or less isolated it to be a power problem, because if I apply 3.3v directly to the board the imp connects and starts flashing green, but if I connect a voltage trough my onboard switcing power supply, the imp flashes red.
I have measured the voltage provided, and its 3.25-3.30 so should be fine.
Probably, I will get a scope on it one of these days.
I usually use TPS54231D but I went with ADP2302 this time due to it needing alot less external components, but it seems that also makes it more picky about the components it does get and its layout.
Possibly starved of current at a critical time. Do you have a lab power supply? that you can set a current limit? You may be able to replicate a bad start-up by limiting the supply current (has happened to me). I think my blink pattern was not just red flashing. it was something else and it was not one of the documented patterns. I solved my ‘problem’ by turning up the current limit on the power supply.
Maybe a multimeter with a MIN function would catch it? The volts may be dropping but you don’t see it because it happens to fast.
hmm, good catch
If I limit current to 50mA I get the same red pattern on startup.
Switching PSUs are very layout sensitive. An incorrect layout could show what looks like a good voltage, but it will respond badly to transients and load… sounds like this is what is happening here.
Definitely sounds like a current issue. The imp module draws 250 mA for a few milliseconds on startup when it connects to WiFi, and if that isn’t available, it will blink red. (I’ve done the same thing during prototyping). The card may need a bit more power. Just remember, if something else in your circuit is also drawing power, you’ll need more than 250mA.
@mora did you follow the PCB layout guidelines for the ADP2302 precisely? Figure 53 in the datasheet shows what you should be doing, it’s always a very good idea to copy the suggested layouts as closely as possible if you want things to work well.
I could probaly have done a better job at the routing/placement, the in and output caps are further away than they need to be, and I used less vias.
I wonder if the slow start feature would be an issue?
When the chip starts up it outputs 2V for half a second or so before going to 3.3.
Sounds like your layout is problematic. The slow start on the ADP2302 is only 3ms according to the datasheet (fig 28 & 31) for the complete ramp to the preset Vout…