BELK-TN-013: Managing ethernet and USB physical transceivers reset

Info Box Applies to Bora Applies to BORA Xpress Applies to BORA Lite Applies to BORA Technical Notes Applies to BORA Xpress Technical Notes Applies to BORA Lite Technical Notes

This technical note was validated against specific versions of hardware and software. What is described here may not work with other versions.

History

Version	Date	BELK/BXELK version	Notes
1.0.0	Apr 2022	4.1.5	First release

Introduction

This technical note provides some information for managing the reset of Ethernet and USB physical transceivers used in BORA, BORA Xpress and BORA Lite DAVE Embedded Systems' SOMs.

Reference

The BORA and BORA Xpress ethernet and USB PHY reset signals are actually managed by the two MIO pins dedicated to this functionality. The MIO pins are documented in the Power and Reset wiki page for PS MIO51 501 and PS MIO50 501 pins.

Hardware reset using MIO pins

The default configuration for the two PHY reset signals is managed - by default - in the BORA and BORA Xpress SOMS using the two dedicated MIO pins.

These MIO pins are connected to the PHY reset signals and the reset pulse can be generated using a proper software routine.

U-Boot PHY reset

In the BELK BSP, the following U-Boot routine contains the reset pulse generation:

int board_init(void)

kernel PHY reset

The Linux kernel is assumed to find the two physical transceivers already reset. So, the reset is not implemented at driver nor userspace levels.

ethernet

The ethernet PHY kernel driver, i.e. drivers/net/ethernet/cadence/macb.c supports a device tree entry for generating the hardware reset pulse.

It is enough to add the following entries in order to let the kernel to reset the PHY too:

&gem0 {
   status = "okay";
   phy-mode = "rgmii-id";
   phy-handle = <&phy0>;
   phy-reset-gpio = <&gpio0 51 1>;
   phy-reset-duration = <100>;
   phy-reset-active-low;
...
...

USB

For issuing a reset pulse to the USB PHY, a dedicated kernel hacking has to be implemented: the USB PHY driver to be modified is the ULPI drivers/usb/phy/phy-ulpi.c kernel driver.

An example of a USB driver with the hardware reset capability can be found on drivers/usb/phy/phy-generic.c. The reset management, via device tree property reset-gpios can be used as a reference for implementing the hacking in the ULPI driver.

Software reset

The ethernet PHY - in general - supports a software reset via the BCMR register. The MAC kernel driver issues a software reset during the phy attach to the ethernet device:

ret = phy_write(phydev, MII_BMCR, BMCR_RESET);

in the genphy_soft_reset function.