topic Re: S32K358 - GMAC Clock Configuration in S32K

S32K358 - GMAC Clock Configuration

sathishkumar_sunmugavel — Sun, 12 Oct 2025 07:22:22 GMT

Hi Everyone,

I have a few questions regarding the GMAC clocking configuration.

In my setup, PTB3 is configured as an output pin for RGMII_TXCLK, and PTC16 is configured as an input pin for RGMII_RXCLK. This configuration is confirmed from the attached Excel sheet.

According to the pin descriptions:

PTB3 can function as either an input or output for GMAC0_MII_RMII_RGMII_TX_CLK

PTC16 can function as an input for GMAC0_MII_RGMII_RX_CLK.

I am using the RGMII interface, which requires a 125 MHz clock input for both EMAC_CLK_RX and EMAC_CLK_TX. From my understanding, only EMAC_MII_RMII_TX_CLK can be used as the clock input.

If this understanding is incorrect, please clarify how these two clocks should be provided.

EMAC_MII_RMII_TX_CLK → MC_CGM_MUX7 → EMAC_CLK_RX
EMAC_MII_RMII_TX_CLK → MC_CGM_MUX8 → EMAC_CLK_TX

Up to this point, everything is clear. However, the table below is a bit confusing as it lists the source, destination clock, and port details.

The IBE bit is set for all the listed ports — could you please explain what this indicates?
PTC16 appears to be used between GMAC_MII_RGMII_RX_CLK and GMAC_TS_CLK but I used PTC16 as RGMII Rx clock pin — could you clarify this connection?
Based on my understanding, PTB3 and PTC16 can be used as the RGMII Tx/Rx clock pins connected to the PHY. Additionally, the clock source EMAC_MII_RMII_TX_CLK is internally connected to EMAC_CLK_RX and EMAC_CLK_TX through MUX7 and MUX8, respectively. However, the information in the table seems inconsistent with this understanding — could you please clarify?

Best Regards,
Sathish.

Re: S32K358 - GMAC Clock Configuration

sathishkumar_sunmugavel — Sun, 12 Oct 2025 08:55:31 GMT

There is no PHY in my setup. It's MAC-MAC connection.

1. In that case, Do I need to consider this PHY_INTF_SEL signal input to GMAC module?
2. Can I use PLL_AUX_CLK with 125MHz for both CLK_TX_I and CLK_RX_I?
3. CLK_TX_I and CLK_RX_I are connected by internal muxing, and PTB3 should be configured as output from GMAC for RGMII TX_CLK and PTC16 should be configured as input to GMAC RGMII RX_CLK. Is this understanding, correct?

Re: S32K358 - GMAC Clock Configuration

PavelL — Mon, 13 Oct 2025 11:24:10 GMT

Hello @sathishkumar_sunmugavel ,

Please find answers to your questions below. FYI, you use S32K3 RM Rev 6, the latest one is Rev. 11.

I run successfully RGMII 100Mbps on S32K358EVB-Q289, with external TJA1103 on SABRE connector.

1. Here's screenshot of Pins:

2. Clocks can be tricky a little bit. RGMII TX_CLK shall be an output, RGMII RX_CLK shall be an input. There's an internal divider 2, so TX_CLK need to be doubled.

For GMAC 1Gbps change the clocks accordingly: 25MHz -> 125MHz ; 50MHz -> 250MHz

3. And the last point, it's necessary to set up clock muxes also in your code, before gmac init. I do it usually as the very first rows in the code:

I do not use Table 116 (in RM Rev 6).

Best regards,

Pavel

Re: S32K358 - GMAC Clock Configuration

sathishkumar_sunmugavel — Tue, 14 Oct 2025 05:41:29 GMT

Hi @PavelL

Thank you so much for the valuable inputs. The issue is resolved after adding the below line.

IP_DCM_GPR->DCMRWF1|=DCM_GPR_DCMRWF1_MAC_CONF_SEL(0x01) |DCM_GPR_DCMRWF1_MAC_TX_RMII_CLK_LPBCK_EN_MASK;

But could you please confirm why this tx clock loop back is required?

Best Regards,
Sathish.

Re: S32K358 - GMAC Clock Configuration

PavelL — Tue, 14 Oct 2025 06:48:19 GMT

Hello @sathishkumar_sunmugavel ,

This is related to the GMAC IP from Synopsys.

This bit allows the MAC to internally loop back its own TX clock as the input clock - which is necessary to initialize MAC in RGMII mode.

Best regards,

Pavel

Re: S32K358 - GMAC Clock Configuration

sathishkumar_sunmugavel — Mon, 03 Nov 2025 04:41:19 GMT

Hi @PavelL

Got it. Thanks!!!