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 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 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.
- 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. Using the control for the
MMC2_SDCD and MMC2_SDWP signal functions is not recommended when trying to
connect an UHS-I SD card to MMC2. 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 are
different because we only expected MMC2 to be used with on-board fixed voltage
SDIO devices similar to Wi-Fi® or
Bluetooth®
transceivers
- SD card power switch, along with the power switch
supply EN pin reset logic, and the host IO power supply circuit is required to
support UHS-I SD cards which begins communications using 3.3V signal levels and
later change to 1.8V signal levels when changing to one of the faster data
transfer speeds.
The only way
to put an SD card back into 3.3V mode is by cycling power to the SD card, as
SD cards do not have a reset pin. The host IO power supply must power off/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 processor IO connects
directly to the ground when the SD card is inserted