General
Review and verify the following for the custom schematic design:
- Above sections, including relevant application notes and FAQ links.
- Pin attributes, signal description, and electrical specifications.
- Electrical characteristics, timing parameters and any additional available information.
- Include a series resistor (0Ω) on MMC1_CLK placed as close to processor clock output pin as possible to minimize reflections. MMC1_CLK is looped back internally on read transactions, and the series resistor minimizes possible signal reflections, which can cause false clock transitions. Use 0Ω initially and the value as required to match the PCB trace impedance.
- The MMC1 CLK, CMD, and DAT0-3 signal interfaces are implemented using SDIO buffers on pins powered from VDDSHV3 (connected to power source that changes the operating voltage level from 3.3V to 1.8V as the transfer speed transitions to one of the higher speed data transfer modes).
- The MMC1 SDCD and SDWP signal functions are implemented with LVCMOS buffers on pins powered from VDDSHV1, which operate at fixed 1.8V or 3.3V.
- The SDIO buffers are designed to support dynamic voltage switching. Dynamic voltage switching is necessary since UHS-I SD cards begins operating with 3.3V signaling and changes to 1.8V signaling when the SD card transitions to one of the higher speed data transfer modes.
Processor IO buffers are off during reset. An external pullup is recommended for any of the processor or attached device IOs that can float. Pullups are recommended on all data and command signals. Verify internal pullups are not configured when (improves noise immunity) external pullups are used.
- To meet the SD card specification, a 47kΩ pullup is recommended when internal pulls are unexpectedly enabled. The 47kΩ pullup verifies the resulting pull resistance is within the specified range.
- When UHS-I SD card interface support is required, implementing an LDO supply that switches output between 3.3V and 1.8V is required. Switching IO supply can be an external discrete implementation or internal to the PMIC. Connect the switchable voltage output to the IO supply for IO group, referencing the SD interface signals (VDDSHV3).
- When UHS-I SD card interface support is required, while the IO voltage for SD card interface is either 1.8V or 3.3V, the SD card VDD supply is connected to a fixed 3.3V source.
- When UHS-I SD card interface support is required, the 3.3V SD card power is required to be switched through a load switch to allow resetting of the SD card IO supply to 3.3V. Provision to enable the SD card load switch during reset is required.
- Provision to reset the load switch using the SD card load switch EN signal during warm reset and normal operation using processor IO is required to be provided. Use a 2-input ANDing logic.
- During boot, the ROM code checks the status of the card detect pin (SDCD). The signal is expected to be low to indicate SD card is detected (inserted).
Schematic Review
Follow the below list for the custom schematic design:
- Required bulk and decoupling capacitors are provided. Compare with the EVM schematics.
- Pull values used for the data, command and clock signals. Compare with the relevant EVM schematics.
- Series resistor value and placement on the clock output signal near to the processor.
- When UHS-I SD card interface support is required, verify IO supply for IO group rail switching (3.3V/1.8V) and the SD card power switching, power switch reset circuits are included.
- Supply rail connected to the SD card power supply (use SYS voltage).
- Implementation of reset logic for resetting the SD card power control load switch. Provision for slew rate control of the SD card supply control power switch is provided.
- Supply rails connected follow the ROC.
- Required external ESD protection are provided for the SD interface signals.
Additional
- The logic state of the MMC1_SDCD and MMC1_SDWP inputs to the host are not required to change when a UHS-I SD card changes the IO operating voltage. Maintaining a valid logic state is not possible if the signals propagate through an input buffer of a dual-voltage SDIO cell that changes voltage. The signal functions are assigned to IOs that do not change voltage dynamically. Signals only connect to switches in the SD card connector, so there is no reason for the signals to change voltage when the SD card signals change operating voltage. The MMC1_SDCD and MMC1_SDWP signals are required to connect to the SD card connector switches and pull high with external pull resistors connected to the VDDSHV1. The other MMC1 SD card signals with pullups are required to have pulls powered by the VDDSHV3 source that dynamically changes voltage.
- The MMC2_SDCD and MMC2_SDWP pins are referenced to the same IO supply for IO group the other MMC2 pins. Connecting an UHS-I SD card to MMC2 requires avoiding the use of the control for the MMC2_SDCD and MMC2_SDWP signal functions. For SD card use case, the signal functions needs to implemented using one of the other pin multiplexing options that uses an IO cell powered from a fixed voltage source. The MMC2 assignments differ because MMC2 was originally intended for use with on-board fixed voltage SDIO devices, such as Wi-Fi® or Bluetooth® transceivers.
- SD card power switch, along with the power switch supply EN pin reset logic, and the processor IO supply for IO group supply switching circuit is required to support UHS-I SD cards which begins communication using 3.3V IO level and later change to 1.8V IO level when changing to one of the faster data transfer speeds.
Cycling power to the SD card is the only way to put the SD card back into 3.3V mode because SD cards do not have a reset pin. The processor IO supply for IO group supply is expected to power off and on and switch voltage at the same time as the SD card. The circuits and the software driver operating the signals sourcing the circuits verifies that both devices are off, or on and operating at the same IO voltage at the same time.
- Add a series resistor 100Ω on the SDCD pin since processor IO connects directly to the ground when the SD card is inserted.