Regarding PCIE clock of Jetson TX2

I wish to comfirm the following two things:

  1. Do REFCLK± pins conform to HCSL electrical standards ? (i can’t see any useful info regarding this, neither DS nor OEM Design Guide). Otherwise please point out to what electrical standard shall REFCLK± conform?

  2. Please show how REFCLK± pins are terminated on the TX2 module (if any). Or are these two pins
    simply not terminated at all ?

By the way, i have noticed that the PCIE reference clock is not always present, is there a way to turn on the clock
as soon as the module gets powered on even when no endpoint is attached

I’ll get back to you with the REFCLK questions soon.
Regarding, keeping REFCLK alive even when there is no PCIe endpoint device connected, please use the following patch.

diff --git a/drivers/pci/host/pci-tegra.c b/drivers/pci/host/pci-tegra.c
index 63c0e343d388..52c46b68981e 100644
--- a/drivers/pci/host/pci-tegra.c
+++ b/drivers/pci/host/pci-tegra.c
@@ -2194,7 +2194,7 @@ static bool tegra_pcie_port_check_link(struct tegra_pcie_port *port)
        } while (--retries);
-       return false;
+       return true;
 static void tegra_pcie_apply_sw_war(struct tegra_pcie_port *port,

The default behavior of TX2 is that, if there is no PCIe endpoint detected, it powers down the PCIe controller (thereby shutting off the REFCLK as well). The above patch changes that behavior.

Our engineering team says that irrespective of the source (i.e. LVPECL, LVDS, CML,HCSL) REFCLK should always conform to AC and DC specifications as mentioned by the spec.
FWIW, TX2 does conform to the spec and the source in case of TX2 is CML.

1 Like

thanks for your explanation!

But you still didn’t show whether REFCLK± are already terminated on Jetson TX2 module …
Please clarify this …
Thanks ~

I see one item in the DS that you posted
i.e., Zc-dc = 40Ω ~ 60Ω
can I interpret this as each pin of REFCLK± is terminated with a resistor of ~50Ω
as shown in the figure bellow

please help me with this, thank you

Yes. You can proceed with that understanding.

I have designed an electronic card that has Nvidia TX2 and a Intel FPGA Stratix V. Their PCIe design is referencing the schematics of Jetson TX2 development kit and Intel FPGA Stratix V development kit which do not have any termination resistors in their respective schematics for PCIe reference clock. In an earlier chat Jetson TX1/TX2 PCIE differential reference clock type , a question was posted about the I/O standard of TX2 PCIe reference clock and it was replied by Nvidia moderator Trumany that it is HCSL. Within Intel FPGA, I configured it to receiver PCIe reference clock as HCSL I/O standard.

When I encountered intermittent behaviour of the PCIe interface : there are times when TX2 cannot detect the Intel FPGA PCIe device with lspci command, there are also times when during TX2 boot up, there are PCIe error messages running non-stop. I did a measurement of the TX2 PCIe clock with an oscilloscope and discovered that the TX2 PCIe clock is not HCSL. A HCSL clock should be toggling between 0mV and 700mV. The measured has an positive offset voltage of about 600mV and toggling of about 150mV swing, riding on top of the 600mV. This signal appears to be more like a LVDS signal although LVDS should have a 1.2V positive offset voltage. As LVDS receiver can tolerate quite a wide variation of input offset voltage. I reconfigured Stratix V FPGA to receive the clock as LVDS receiver. With this change, the intermittent situations improve significantly with many previously failed electronic cards working. However, there are still failed cases.

I also measured the PCIe clock resistance to ground with a multimeter and received a very high resistance value. This means there is no 50 ohm termination to ground within TX2 for this PCIe differntial clock as shown in Phoenixlee’s last post.

According to Intel FPGA support, its PCIe Hard IP should be configured to receive PCIe clock as HCSL receiver and not LVDS receiver. May I know if it is possible to configure TX2 to output a HCSL PCIe clock ? Or can Nvidia advise how to convert its PCIe clock signal to HCSL ? Or can I provide a separate HCSL clock source to Intel FPGA, not using TX2 PCIe clock source ? Please provide details of the TX2 PCIe clock specification to clarify.

Hui Peng

HCSL signal range is typically between 0 and 800 mV and Tegra PEX_CLK typical output is within this range.

There is common mode termination resistor that can be enabled/disabled so the input termination on the receiver side is not necessary. Tegra default is to enable the common mode resistor with a typical value of 50 Ohm. Common mode resistor may need to be disabled depend on if the receiver has internal termination or not.

Do you have termination enabled on the FPGA device if it is supported? If not, you may need to include it externally.

Ideally, it shouldn’t matter what kind of source (LVPECL / LVDS / CML / HCSL) is used to generate the REFCLK as long as the REFCLK itself conforms to the PCIe spec and in Tegra’s case, it does.