[PATCH v2 5/5] board: apalis_imx6: Add KSZ9131 phy skew settings

sbabic at denx.de
2020-05-01T16:32:51Z

> This patch adds skew register settings for KSZ9131. It checks first
> which phy is on the board and then applies the correct skew settings.
> Skew settings calculation for the KSZ9131:
> The i.MX6 SoC has an output skew tolerance of -100ps to 900ps. All
> PCB traces where routed exactly the same length so we can calculate
> the skew settings without taking the length into consideration. The
> traces are all length matched.
> RXC skew (PHY to MAC):
> - We use the 2ns DLL controlled delay on the PHY
> - We do not use the skew registers
> This results in the following values:
> RXC
> PHY fixed Delay 2000ps
> PHY Added Delay 0ps
> T_setup_R min 2.00ns
> T_setup_R typ 2.00ns
> T_setup_R max 2.00ns
> T_hold_R min 1.60ns
> T_hold_R typ 2.00ns
> T_hold_R max 2.40ns
> That means we are well within RGMII specs.
> TXC skew (MAC to PHY):
> - We use the 2ns DLL controlled delay on the PHY
> - We then subtract ~0.6ns with TXD[0:3] and TXC clock pad skew
> register in a resulting ~1.4ns delay.
> This results in the following values under consideration of the
> tolerances:
> TXC min TXC typ TXC max
> MAC min -100ps -100ps -100ps
> MAC max 900ps 900ps 900ps
> PHY fixed Delay 2000ps 2000ps 2000ps
> PHY added Delay -340ps -600ps -859ps
> T_setup_T min 1.56ns 1.30ns 1.04ns
> T_setup_T typ 2.06ns 1.80ns 1.54ns
> T_setup_T max 2.56ns 2.30ns 2.04ns
> T_hold_T min 1.04ns 1.30ns 1.56ns
> T_hold_T typ 1.94ns 2.20ns 2.46ns
> T_hold_T max 2.84ns 3.10ns 3.36ns
> This shows that T_hold_T min and T_setup_T min times are out of spec
> for RGMII timing. However the KSZ9131 has a minimal value for this time
> of 0.8ns which is met under all circumstances.
> Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Applied to u-boot-imx, master, thanks !

Best regards,
Stefano Babic

--
=====================================================================
DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-53 Fax: +49-8142-66989-80 Email: sbabic at denx.de
=====================================================================