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Carambola 2 - an AR9331/SoC based PCB
GNU General Public License v2.0
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GPIO 14 bootstrap should be LOW but it is shown as HIGH in datasheet #48

Closed valentt closed 9 years ago

valentt commented 9 years ago

I have looked at your datasheet and it shows that GPIO 14 should be HIGH during bootstrap - selection_025

But in Carambola 2 dev board both GPIO 13 and 14 (ETH0 and ETH1 LEDs) are connected to GND (LOW) - selection_024

Could you explain this discrepancy or correct Carambola 2 datasheet so that GPIO 14 is labeled LOW instead HIGH.

valentt commented 9 years ago

Do you need any aditional info from me?

GiedriusM commented 9 years ago

Carambola2 modules have internal 10k pull-up/down resistors. GPIO14 bootstrap should be LOW, so no problems here. GPIO13 should be HIGH and due to voltage drop on the LED it is "high enough" at about 1.8V.

If this answers your question, please close the issue and next time for non-software problems please use the forums.

valentt commented 9 years ago

@GiedriusM thanks for quick reply.

Carambola2 modules have internal 10k pull-up/down resistors.

Are they programmable or fixed? Do GPIO 13 and 14 have internal pull-up or pull-down resistors and can that be changed?

GPIO14 bootstrap should be LOW, so no problems here.

Agreed.

GPIO13 should be HIGH and due to voltage drop on the LED it is "high enough" at about 1.8V.

Ok, this part confuses me. I see that both GPIO 13 and 14 are wired the same, and that they both are connected over LED to GND. So how is that one is Bootstap High and other Low? Do they have different internal pull-up and pull-down resistors? Any explanation would be really helpful. Thanks.

GiedriusM commented 9 years ago

1) They are physical (external) resistors. I don't remember whether GPIO13 and 14 have programmable internal resistors, but it is irrelevant in this case, because configuration could only be done after the bootstrapping stage.

3) In the reference GPIO13 and 14 are wired the same OUTSIDE of the module, but they have different wiring INSIDE the module (under the metal shielding). GPIO14 is connected to GND via 10k resistor (inside) and to GND via LED and 330Ohm resistor (outside), which results in overall pull to GND. GPIO13 is connected to Vcc via 10k resistor (inside) and to GND via LED and 330Ohm resistors (outside). That forms a non-linear resistor divider, which at low Vcc voltages outputs voltage approximately that of forward diode voltage drop. Yellow LEDs have a typical voltage drop of 1.8.-2.2V and that's what can be measured on GPIO13 when the chip is in reset state.

valentt commented 9 years ago

Thanks