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| {{Explanatory Guide
| | 1070 q33 |
| |Device=RX 460, RX 470, RX 480, RX 560, RX 570, RX 580, RX 590
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| |Type=Circuit
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| |Difficulty=2. Medium
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| }}
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| '''''In this section, we will discuss the Display Rail on Polaris GPUs, including its controller circuit, usage, and common issues.'''''
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| ==The Controller Circuit==
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| Often referred to as the 0.8V rail, the Display Rail is typically controlled by either the [https://datasheet.lcsc.com/szlcsc/2005191033_GSTEK-Green-Solution-Technology-GS9238-ATQ-R_C532950.pdf GS9238] or the [https://www.icware.ru/pdf/0004112.pdf APW8713] step-down voltage converters. These controllers take 3.3V (or in some cases 12V_BUS) and convert it to 0.8V. Sapphire cards, on the other hand, may use an MPS NB671 for the Display Rail buck converter, which has a slightly different pinout.
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| [[File:Polaris 0.8v board.jpg|thumb|Location of the 0.8V controller on an RX 480 Reference card (Figure 1)]]
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| [[File:Polaris 0.8v schematic.jpg|thumb|Schematic view of the 0.8V controller (GS9238 version) (Figure 2)]]
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| [[File:Polaris 0.8v en schematic.jpg|thumb|Enable signal schematic for the 0.8V regulator (Figure 3)]]
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| While the markings on the schematic and board may vary between GPU models, the circuit design is typically consistent.
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| It's important to note that the GS9238 generates its own 5V on the V<sub>CC</sub> pin and does not require external power, except for V<sub>in</sub>. In contrast, the APW8713 needs external 5V on its V<sub>CC</sub> pin to operate, which it receives from the [[5V Rail on Polaris GPUs Explained|5V Rail on Polaris GPUs]].
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| The Enable signal comes from the POK pin of the [[1.8V Rail on Polaris GPUs Explained|1.8V Rail on Polaris GPUs]], which is the gate of an NPN transistor, shorting the EN signal to GND, as depicted in Figure 3.
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| ==Usage==
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| The Display Rail, as the name implies, powers the display components of the GPU, as shown in Figure 4.
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| Inconsistencies in the output voltage can lead to issues like flickering displays. In cases where the controller is faulty and does not output voltage but still produces a POK signal, the card may appear to "work" but without displaying anything.
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| [[File:Polaris 0.8v usage.jpg|thumb|Usage of the Display Rail in Polaris GPUs (Figure 4)]]
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| ==Common Problems==
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| ===No Voltage Output===
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| When encountering a missing voltage rail, begin by measuring the V<sub>CC</sub> and EN pins of the controller, ensuring that both are active high. The MPS NB671 generates its own VCC, so if it's missing, the buck converter itself may be faulty.
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| If V<sub>CC</sub> and EN are both confirmed to be present, check if V<sub>in</sub> is being provided to the controller. Sometimes, the resistors between the voltage source and the controller can develop high resistance or become open, preventing the controller from receiving power.
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| If V<sub>in</sub> is also confirmed to be present, verify that the feedback resistors have the correct values as shown in Figure 2.
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| ===Short or Very Low Resistance===
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| The Display Rail is designed to have low resistance, but if the resistance measures very low (sub 5 Ohms), this could indicate a dead GPU. Nonetheless, it's advisable to inspect for any potentially shorted capacitors using the method described here: [[Short Circuits - Repair Basics]]
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| ===No POK Signal===
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| The POK signal from this controller is responsible for enabling Vmem and VDDCI. If the controller is not producing a POK signal, check the FB circuit. If everything appears to be in order, replace the controller.
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