SPRZ452I july 2018 – may 2023 AM6526 , AM6528 , AM6546 , AM6548
MMCSD: Negative Current from UHS-I PHY May Create an Over-Voltage Condition on VDDS6 and VDDS7 Which Exposes the Device to a Significant Reliability Risk
AM65x SR 1.0
The MMCSD0 UHS-I PHY and MMCSD1 UHS-I PHY receives 1.8 V bias power from device pins VDDS6 and VDDV7 respectively. Unexpected paths through the UHS-I PHY allows current to flow from VDDSHV6 to VDDS6 and VDDSHV7 to VDDS7 when the VDDSHV IO supply is operating at 3.3 V.
The UHS-I PHY typically consumes power from its VDDS bias supply. However, the unexpected current flowing from the 3.3 V VDDSHV IO supply may exceed the bias current consumed by the UHS-I PHY. When this occurs, current may flow out of the 1.8 V VDDS bias supply. This negative current may cause the voltage applied to VDDS6 and VDDS7 to increase above the recommended operating range in some operating conditions.
This issue has been observed on a system where SDIO LDO was sourcing the UHS-I PHY bias supply while its VDDSHV IO supply was 3.3 V. This occurs because SDIO LDO was not designed to sink current. Therefore, it is not able to shunt any negative current to VSS. Negative current causes the VDDS bias supply to increase above the recommended bias supply voltage where the UHS-I PHY enters a non-functional state that can only be cleared when the respective MMCSD subsystem is reset.
The negative current is a function of device operating temperature, device process variation, and UHS-I PHY output toggle rate.
This issue can also occur when the IOs associated with MMCSD0 UHS-I PHY and MMCSD1 UHS-I PHY are operating at 3.3 V while configured to one of the other MUXMODES defined in the Datasheet Pin Multiplexing Table (when IOMUX_ENABLE bit in the respective MMCSD0_SS_PHY_CTRL_1_REG or MMCSD1_SS_PHY_CTRL_1_REG register is set to 1 to enable alternate MUXMODES).
Further analysis has indicated a significant reliability risk when operating VDDSHV6 and VDDSHV7 at 3.3 V.
This issue does not occur when the VDDSHV IO supply is operating at 1.8 V.
There is no workaround which prevents the negative current from producing an overvoltage condition. Therefore, there is no support for operating VDDSHV6 and VDDSHV7 at 3.3 V.