Quectel EC21 Series User Manual

LTE Standard Module Series
EC21_Series_Hardware_Design
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In SD card interface design, in order to ensure good communication performance with SD card, the
following design principles should be complied with:

SD_INS_DET must be connected.

The voltage range of SD card power supply VDD_3V is 2.7–3.6 V and sufficient current up to 0.8 A
should be provided. As the maximum output current of VDD_SDIO is 50 mA which can only be used
for SDIO pull-up resistors, an externally power supply is needed for SD card.

To avoid jitter of bus, resistors R7–R11 are needed to pull up the SDIO to VDD_SDIO. The value of
these resistors is among 10–100 kΩ and the recommended value is 100 kΩ. VDD_SDIO should be
used as the pull-up power.

In order to adjust signal quality, it is recommended to add 0 Ω resistors R1–R6 in series between the
module and the SD card. The bypass capacitors C1–C6 are reserved and not mounted by default.
All resistors and bypass capacitors should be placed close to the module.

In order to offer good ESD protection, it is recommended to add a TVS diode on SD card pins near
the SD card connector with junction capacitance less than 15 pF.

Keep SDIO signals far away from other sensitive circuits/signals such as RF circuits, analog signals,
etc., as well as noisy signals such as clock signals, DC-DC signals, etc.

It is important to route the SDIO signal traces with total grounding. The impedance of SDIO data
trace is 50 Ω (±10 %).

Make sure the adjacent trace spacing is two times of the trace width and the load capacitance of
SDIO bus should be less than 15 pF.

It is recommended to keep the trace length difference between SDC2_CLK and SDC2_DATA[0:3]/
SDC2_CMD less than 1 mm and the total routing length less than 50 mm. The total trace length
inside the module is 27 mm, so the exterior total trace length should be less than 23 mm.
3.14. Interfaces for WLAN & Bluetooth Applications
EC21 series supports a low-power SDIO 3.0 interface for WLAN and UART/PCM interfaces for Bluetooth
function.
The following table shows the pin definition of WLAN & Bluetooth application interfaces.
Table 16: Pin Definition of Interfaces for WLAN & Bluetooth Applications
Pin Name Pin No. I/O Description Comment
WLAN Application Interface
SDC1_DATA3 129 DIO WLAN SDIO data bit 3
1.8 V power domain.
If unused, keep them open.
SDC1_DATA2 130 DIO WLAN SDIO data bit 2

Quectel EC21 Series Specifications

General

Brand	Quectel
Model	EC21 Series
Category	Control Unit
Language	English