Molex Mini-Fit Jr – Know your Power, and Don’t Get Confused – “PCIe” and “Xilinx Not PCIe” Power Connector

Our Mission: If It Is Packets, We Make It Go Faster – today more on PCIe, this time, Powering PCIe Cards with FPGAs…

And with packets we mean: Networking using TCP/UDP/IP over 10G/25G/50G/100G Ethernet; PCI Express (PCIe), CXL, OpenCAPI; data storage using SATA, SAS, USB, NVMe; video image processing using HDMI, DisplayPort, SDI, FPD-III.

To move packets you need power, but how much can you draw from an interface?

PCI Express Cards Consumption Limit

PCIe has been around for a long time, since 2003, and many people know the maximum power draw of 75W per PCIe slot, but is it that simple? The short answer is no, but let’s have a look into the spec. It states that all cards can consume up to 3A on the 3.3 V power rail with the following restrictions applying:

  • x1 cards: 0,5A on 12V, but overall consumption limit is 10W
  • x4 – x16 cards: 2,1A on 12V, but overall consumption limit is 25W 

But where does the 75W come from? Well, exceptions apply:

High power devices can draw more power after the initialization and configuration:

  • x1 cards can consume 25W 
  • x16 cards can consume 75W

Using Additional Connectors for Higher PCIe Card Consumption

With additional connectors (6pin and 8 pin) cards can consume up to 300W. A 6 pin adds 75W while a 8 pin adds 150W.

But be careful, in the FPGA world there are two versions of the 6/8 pin Molex Mini-Fit Jr connectors, the Xilinx and the PCIe option. 

The 6 Pin option has 2 pins (3 and 4) swapped. If you mix up the connectors 12V will directly connect to Ground, this can destroy your device.

6 Pin Power Connector
Pin PCIe Spec AMD/Xilinx Dev Boards
1 +12 V +12 V
2 +12 V +12 V
3 +12 V Ground
4 Ground +12 V
5 Sense Sense
6 Ground Ground

The 8 Pin option the changes are less subtle, all pins have a different function and cause an electrical short.

8 Pin Power Connector
Pin PCIe Spec AMD/Xilinx Alveo Cards
1 +12 V Ground
2 +12 V Ground
3 +12 V Ground
4 Sense1 Ground
5 Ground +12 V
6 Sense0 +12 V
7 Ground +12 V
8 Ground +12 V


🌐 www.missinglinkelectronics.com

MLE (Missing Link Electronics) is offering technologies and solutions for Domain-Specific Architectures, which focus on heterogeneous computing using FPGAs. MLE is headquartered in Silicon Valley with offices in Neu-Ulm and Berlin, Germany.

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