Noise on the powerstep01 Busy line

Ron Kreymborg · ‎2019-12-21

We need to use the Busy line as an interrupt and have noticed that often there is considerable switching noise on that line, making use for that function unreliable. We currently pull it up to VREG with a 39K. We have tried reducing this resistor to 5.6K but that makes the noise worse. We can sort of fix the problem with a 2.2nF cap to grnd from the 39K resistor but that makes for a slow off switch.

Is there something we can do here? This was quite unexpected as the possibility is not mentioned anywhere. Please see the attached 'scope shot.

Ron Kreymborg

Cristiana SCARAMEL · ‎2020-01-09

The noise shown in the scope is not a normal condition. It seems a coupled noise, we suggest to investigate on the possible origin.

Some points to check:

Is the VREG pin correctly filtered? We suggest a 100/200 nF, good quality ceramic capacitor, connected as close as possible to VREG pin and GND of the IC.
Check the board layout:
- the path between VREG pin and the BUSY pull up resistor has to be as short as possible, just like the connection between pull up resistor and BUSY pin.
- verify others possible coupling noise due to tracks layout

Anyway to filter the BUSY pin a capacitor in the order of 100/200 pF will be enough.

oeliks · ‎2020-01-09

Crosspath between this pin and some switching action?

Ceramic cap seem to be best solution.

For further attenuation of remaining high frequency noise use ferrite beads (wont increase slope).