An SD card consists of three basic elements from a block diagram level.
How SPI, the non-proprietary SD card protocol, affects the layout.
Some final thoughts on placement and pours to enhance circuit performance.
An SD card reader schematic has quickly become standard in most electronics
Secure Digital (SD) cards evolved out of MMC cards in the late 90s, with the major difference being measures adopted to prevent piracy to attract primarily music suppliers. Today, the memory card format, which is much better known by its acronym, is widespread across a variety of industries as a cheap, portable, and large storage solution in a compact form.
Considering its ubiquity across a wide swath of devices, designers may wish to incorporate the memory card format into their design. SD card reader schematics, while relatively straightforward, do require knowledge of the best practices of high-speed design to support the rapid transfer speeds associated with current generation SD standards.
What Is in an SD Card Reader Schematic?
From a layout design perspective, the schematic for an SD card reader does not require many state-of-the-art tools or tricks. Instead, SD cards are governed by a few simple features:
A voltage regulator to drop an input from (typically) 5V down to 3.3V. Newer, larger capacity SD technology iterations may make use of 1.8V power (while being fully backward compatible with the more common 3.3V rating) for greater half-duplex speeds or full-duplex transfer. The lower potential impacts battery life for portable devices, greatly reducing the energy requirements on these elevated transfer rate settings.
A microcontroller or card reader chip is used in conjunction with SPI (other proprietary SD protocols are available as well) to enable communication between the SD card and any interfacing system(s).
Different connectors are used to transfer data between devices; a common choice is USB, but other formats may be used as well.
Solely looking at the SD card, SPI communication protocol is extremely lenient towards the layout designer. Neither length nor impedance needs to be matched, meaning that blanket best signal integrity practices are the only factor in the quality of the data lines. Ensure that data lines are routed to avoid aggressor or fast-switching nodes that could induce current on a nearby trace, provide short and direct return pathing, and avoid routing over split planes.
Although there’s no need for impedance-matched traces, it’s a good idea to touch on what the stackup might look like for an SD card board. By itself or with minimal supporting circuitry, an SD card can easily be handled on a 2-layer board to reduce cost without compromising on the quality of the signals or power/return delivery. However, if the SD card accompanies even a small amount of HDI circuitry, it is often a good idea to consider a 4-layer stackup to promote better performance with two dedicated signal layers and a plane layer for power and ground, respectively.
Wrapping Up Layout With Placement, Routing, and Pours
As is common in most circuits, there exists a network of passive components that contribute to the overall robustness, and SD card readers are no exception. Placement of components is more than simply grouping the near and connected components on the schematic: the distance between components, primarily due to changes in inductance created by a longer trace, can negatively impact the performance of a particular sub-circuit. Here are a few notable placement examples where proximity between components can make a huge difference:
Terminating resistors may come into play if they aren’t already incorporated on-die and are used to slow SPI signals down to prevent issues like crosstalk in tightly-routed data lines. In these cases, placement should occur at the driver side of the signal.
Bypass capacitors need to be arranged in a radial pattern with the smallest capacitance components at the shortest distance to the appropriate power pins, ideally routed directly to the pin with a via transition to the power plane immediately afterward. While on the subject, power and ground traces should be as large as possible to avoid creating chokepoints for the current; size these traces to the width of the pin so long as this doesn’t infringe on any clearance rules.
Though the crystal oscillator technically belongs to the microcontroller, it still merits mention. Treat it similarly to bypass capacitors, with the added wrinkle that it should be isolated from surrounding traces (especially high-speed lines that could couple).
The layout can conclude with the plane design for the SD card reader circuit, which should be relatively simple, assuming the card reader captures the core functionality of the board. Power pins are likely to be highly isolated before and after the voltage regulator, and designers can configure circuit blocks and their associated pinouts such that there is no crossing between the two copper pours. Ground should have no issue flooding over its layer with minimal choking of the pour due to via placement. Speaking of the ground layer, its ideal placement in a four-layer board would be at layer two to provide the shortest possible return paths for the topside components (which should be most, if not all for an SD card reader circuit). For a 2-layer board that does not have a dedicated ground layer, consider flooding signal layers with ground after routing is finished.
Boost Your Layout With Cadence Software
Although the SD card reader schematic may be a relatively simple layout task compared to most circuits, designers should take every opportunity to optimize the final performance. Accounting for simple, yet overlooked contributions (like reducing loop sizes by fanning out of the short side of the pad or doubling the via count on power and ground to reduce impedance) allows designers to wring the most out of a design and is excellent practice for implementing enhancements during layout for any schematic.
Whether your board starts or ends with an SD card reader, Cadence’s selection of PCB Design and Analysis Software offers an unparalleled feature list and performance at any point of board development. Designers can enjoy the speed and simplicity of OrCAD PCB Designer to ease the challenge of even the densest layouts with powerful functionality.
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