Maximum capacitance on 3.3V rail / maximum rise time for Teensy 3.2

[stefandz]

I have a project with a Teensy 3.2 where (according to the spec of an RFID reader module on the 3.3V rail) I have placed a 1000uF electrolytic capacitor across the 3.3V rail on the Teensy.

I have noticed that this often stops the serial port appearing when the Teensy is plugged in via USB and code will not run (I'm using blink as test code to keep as many variables out of this as possible). Removing the 1000uF capacitor fixes this.

Putting a scope on the 3.3V rail shows a 5ms rise time - my guess would be that this is too slow for the Teensy - so what is acceptable? Does anyone have specs on this?

Many thanks in advance

Stefan

 

[stefandz]

Having experimented with this a bit more, it seems that 600uF (2x 1000uF in series plus 100uF in parallel) fails to start, with a rise time (10% to 90%) of around 2.6ms works about 10% of the time. 500uf (above but with the 100uF removed) works 95% of the time (rise time roughly 2.1ms). I've looked in the datasheets for the MK20DX256 and can't find anything specific about required maximum VDD rise times, but unless anyone has any solid reason to think otherwise I would not recommend putting any more than 470uF across the 3.3V rail on the Teensy 3.2 and to be on the safe side probably better to use a maximum of 220uF.