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 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 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 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 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 to be
low to indicate SD card is detected (inserted).
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 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 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 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. 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 communication using 3.3V IO level and later changes
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 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 dual input
AND gate with RESETSTATz and the processor IO as inputs.
- Add a series resistor 100Ω to the SDCD pin since
the processor IO connects directly to the ground when the SD card is
inserted.