[Ovmsdev] SD CARD

Mark Webb-Johnson mark at webb-johnson.net
Mon Nov 6 15:19:32 HKT 2017

The logic and arrangement is designed by Espressif. Frustrating as they don’t explain the rational behind it anywhere, and are not answering my questions as to what has changed and why, or what is the current recommended approach?

As you say, the resistor values shouldn’t matter too much, so why are they 10K -> 12K -> 100K changing with each iteration of hardware?

I suspect that the open condition is, as you say, for when USB is not connected.

The thing that really confuses me is why, after flashing Geikcreit development board, the GPIO2 is at 0v. When controlling the GPIO2 pull-up / pull-down manually, the SD CARD seems to operate just fine.

Regards, Mark.

> On 6 Nov 2017, at 3:13 PM, Stephen Casner <casner at acm.org> wrote:
> 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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