General
Review and verify the following for the custom
schematic design:
- The sections above, 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 dampen
reflections. MMC1_CLK is looped back internally on read transactions, and the
resistor can be needed to eliminates possible signal reflections, which can
cause false clock transitions. Use 0Ω initially and adjust as required to match
the PCB trace impedance.
- The MMC1 CLK, CMD, and DAT0..3
signal functions are implemented with SDIO buffers on pins powered from VDDSHV5
(power source that changes the operating voltage from 3.3V to 1.8V as the
transfer speed increases).
- The MMC1 SDCD and SDWP signal
functions are implemented with LVCMOS buffers on pins powered from VDDSHV0,
which operate at fixed 1.8V or 3.3V.
- The SDIO buffers are designed to
support dynamic voltage change. Dynamic voltage scaling 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 required for any of the
processor or attached device IOs that can float. Pullups are needed 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 speed support is
required, implementing an LDO supply that switches 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
group, referencing the SD interface signals (VDDSHV5).
- When UHS-I speed 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 speed support is
required, the 3.3V SD card is required to switch 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.
- Provide provision to reset the
load switch using the SD card load switch EN signal during cold reset, warm
reset and normal operation using processor IO is required to be provided. An
option is a 3-input ANDing logic.
- During boot, the ROM code checks
the status of the card detect pin (SDCD, pin P23). The signal is expected as low
to indicate card is detected.
Schematic Review
Follow the list below for the custom schematic
design:
- Required bulk and decoupling
capacitors are provided. Compare with the SK schematics.
- Pull values used for the data,
command and clock signals. Compare with the relevant SK.
- Series resistor value and
placement on the clock output signal near to the processor.
- When UHS-I speed support is
required, verify the IO supply rail switching and the SD card power switching
circuits are added.
- 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 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 must not 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.
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 VDDSHV0. The other MMC1 SD card signals with pullups are required to have
pulls powered by the VDDSHV5 source that dynamically changes voltage.
- The MMC2_SDCD and MMC2_SDWP pins are referenced
to the same IO supply group the other MMC2 pins. However, the recommendation is
not to use the control for the MMC2_SDCD and MMC2_SDWP signal functions if users
are trying to connect an UHS-I SD card to MMC2. For SD card use case, the signal
functions needs to be implemented using one of the other pin multiplexing
options that uses an IO cell powered from a fixed voltage source. The MMC2
assignments are different because we only expected MMC2 to be used with on-board
fixed voltage SDIO devices similar to Wi-Fi or Bluetooth® transceivers.
- An SD card power switch, the power switch supply
EN pin reset logic, and the host IO power supply circuit are required to support
UHS-I SD cards, which begins communications using 3.3V signal levels and later
changes to 1.8V signal levels 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 since SD
cards do not have a reset pin. The host IO power supply must power off and
on, and change 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.
- To optimize the ANDing logic, use a 2 input AND
gate with RESETSTATz and the processor IO as inputs.
- Add a series resistor 100Ω to the SDCD pin
because the processor IO connects directly to the ground when the SD card is
inserted.