1.8V Rail on Polaris GPUs

Here we'll talk about 1.8V rail on Polaris cards, how it's created, usage, and common problems.

The Controller Circuit
On Polaris cards, 1.8V rail is controlled by GS7103-A or similar LDO.

The output voltage is configured through the FB pin with a voltage divider (refer to the datasheet or figure 2).

Markings on the schematic and board could differ from GPU model to another but the circuit is almost always the same.

Usage
1.8V is used to power low level logic in the GPU core, straps, and clock.

No Voltage on 1.8V rail
Start by checking EN (figure 3) and VCC pins, if one of them is missing the controller will not operate. Enable pin should always be high from the moment the card receives power while Vcc comes from the 5V LDO either directly or through a low resistor.

After making sure both of the above are present on the controller, the next thing to check is the Vin pin, where 3.3V is supplied through some resistors.

If all of the above is okay but still no output, check the voltage divider on the FB pin according to the schematic (figure 2) or the datasheet.

Short on 1.8V
The 1.8V rail goes straight to the core, if you have low resistance or it's shorted then most likely you have a dead GPU on your hands. A short on 1.8V is accompanied by a dead core well over 95% of the time.

Though unlikely, a capacitor could be the source of the short. Using the steps from this guide "Base Voltage Rail Short on Polaris GPUs", try to find the source of the short.

No POK signal
POK signal from 1.8v controller enables the Display Rail. If there is no POK, the rest of the card will not work.

If 1.8V is being regulated but no POK is not rising, replace the controller. Otherwise, check FB resistors and make sure it's not being "actively held low" i.e. shorted to ground.