[Ovmsdev] SD CARD

Stephen Casner casner at acm.org
Mon Nov 6 15:13:21 HKT 2017

As I think you were implying, the varying resistor values in the
transistor circuit really shouldn't matter much because those are
there just to limit base current.  The transistor should be all the
way on or off, and in particular, we need them to turn all the way off
when trying to pass SDCARD data.

It is unclear to me why the cross-coupling between DTR and RTS is
required rather than just operating EN and IO0 independently (with the
transistor emitters going to ground).  I can see the difference in the
logic table where DTR and RTS both at 1 leaves EN and IO0 both at 1
rather than pulling them to 0, but is there a reason why we need to
let both DTR and RTS go to 1?  Or is that for the "open" condition
when the USB serial is not connected?

                                                        -- Steve

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