Pogo pins power clarification - reading schematics
03-11-2021, 09:43 AM
(This post was last modified: 03-11-2021, 09:17 PM by dsimic.
Edit Reason: Added a couple of comments
)
I'll have a few comments later... In the meantime, @scholbert thank you very much for the "condensed" exceprt from the PinePhone schematic! It's nice and very usable when discussing the whole thing about the pogo pins.
Edit: Here's another weird "feature" of the keyboard case... With no additional "handshaking" between the phone and the keyboard case, the phone would actually never be able to turn itself off while the keyboard case is attached!
By design, the keyboard case would pretty much always provide 5 V to the DCIN pogo pin (unless the case battery is drained completely, of course), which in turn would cause the PMIC to automatically power the phone on after it was powered off.
The only way this behevior could be avoided is to have some additional logic in the keyboard case, besides the separate charger, and to have the phone instruct that logic over I2C (i.e. perform the aforementioned "hanshaking") to disconnect the case's power output from the DCIN pogo pin until further "hanshaking" notice, i.e. until the next phone power-on event initiated by the user. Anyway, it's probably going to be an error-prone mess.
Edit #2: Here's the comment I've mentioned at the very beginning of this response.
Having an isolation diode on the expansion board (connected to the phone via the pogo pins) that provides a power source to DCIN would be good, but another isolation diode would be needed as part of the phone circuitry, in order to provide complete isolation of two power sources. Relying on the USB charger "overpowering" the expansion board's power source (by providing higher voltage) would be very risky, even if possible at all.
Edit: Here's another weird "feature" of the keyboard case... With no additional "handshaking" between the phone and the keyboard case, the phone would actually never be able to turn itself off while the keyboard case is attached!
By design, the keyboard case would pretty much always provide 5 V to the DCIN pogo pin (unless the case battery is drained completely, of course), which in turn would cause the PMIC to automatically power the phone on after it was powered off.The only way this behevior could be avoided is to have some additional logic in the keyboard case, besides the separate charger, and to have the phone instruct that logic over I2C (i.e. perform the aforementioned "hanshaking") to disconnect the case's power output from the DCIN pogo pin until further "hanshaking" notice, i.e. until the next phone power-on event initiated by the user. Anyway, it's probably going to be an error-prone mess.
Edit #2: Here's the comment I've mentioned at the very beginning of this response.
(03-10-2021, 07:49 AM)scholbert Wrote: - Pogo-Pin Power Supply Input: AFAIK DCIN should be used, but you'll need to add a schottky diode to protect the power supplies from working against each other.
The trick would be, to not go higher as 5V with the input voltage at the anode.
In case a USB Type C charger gets connected the diode would prevent the a current flow form the phone into the breakout.
If you further take the assumption that USB type C charger delivers 5V, the following would match...
On the other side, a 5V supply on the breakout board would not be sufficient to overcome forward voltage of the diode to drive against USB Type C charger.
Having an isolation diode on the expansion board (connected to the phone via the pogo pins) that provides a power source to DCIN would be good, but another isolation diode would be needed as part of the phone circuitry, in order to provide complete isolation of two power sources. Relying on the USB charger "overpowering" the expansion board's power source (by providing higher voltage) would be very risky, even if possible at all.
