Display Rail on Polaris GPUs

Here you'll find information about the Display Rail on Polaris GPUs and common problems associated with it.

The Controller Circuit
Often called 0.8V rail, the display rail is most commonly controlled by either GS9238 or APW8713 step down voltage converters which takes 3.3V (in some cases 12V_BUS) and converts it to 0.8V. Sapphire cards tends to use an MPS NB671 for the Display Rail buck converter which has a slightly different pinout. Markings on the schematic and board could differ from GPU model to another but the circuit is almost always the same.

Note that the GS9238 makes its own 5V on the VCC pin and does not need external power to operate except Vin. On the contrary, the APW8713 needs external 5V on its VCC pin in order to function which it gets from the 5V Rail on Polaris GPUs.

Enable comes from 1.8V Rail on Polaris GPUs' POK pin which is the gate of the NPN transistor which shorts the signal shoring EN to GND as shown in figure 3.

Usage
The display rail as the name suggests, is used to power the display part of the GPU as shown in figure 4.

Inconsistencies on the output voltage could result in a flickering display or in cases where the controller is faulty to not output a voltage but does output POK signal, the card will "work" just without displaying anything.

No voltage output
As always, when missing a voltage rail you should start by measuring VCC and EN pins of the controller, make sure both are active high. The MPS NB671 generates it's own VCC, if it's missing it may be that the buck converter itself is faulty.

If they are, check whether Vin is provided to the controller, sometimes the resistors from the voltage source to the controller becomes too high resistance or open line and prevent the controller from receiving power.

If Vin is also okay, make sure the feedback resistors are the correct values.

Short or very low resistance
The display rail by design has a low resistance, though, if the resistance is very very low (sub 5 Ohms) this could signal a dead GPU. Still, you should look for a possible shorted capacitor using the method described here: Base Voltage Rail Short on Polaris GPUs.

No POK signal
POK from this controller is responsible for enabling Vmem and VDDCI. If the controller is not outputting POK, check the FB circuit, if everything is okay, replace the controller.