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 resistor
minimize 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 VDDSHV5 (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 VDDSHV0,
which operate at fixed 1.8V or 3.3V.
- The SDIO buffers are designed to support dynamic
voltage change. Dynamic voltage change 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 speed 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 (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 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.
- 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 to use
3-input ANDing logic.
- During boot, the ROM code checks the status of
the card detect pin (SDCD, pin P23). 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 SK schematics.
- Pull values used for the data, command and clock
signals. Compare with the relevant SK schematics.
- Series resistor value and
placement on the clock output signal near to the processor.
- When UHS-I speed 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 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 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 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
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 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 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 must 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.
- To optimize the ANDing logic, use a dual input
AND gate with RESETSTATz and the processor IO as inputs
- Add a series resistor 100Ω on the SDCD pin since
processor IO connects directly to the ground when the SD card is inserted