Silicon Laboratories RS9116 Instruction Manual

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AN1344: RS9116 QMS Board Layout Guidelines
Version 1.0
August 24, 2021

AN1344: RS9116 QMS Board Layout Guidelines
Version 1.0
silabs.com | Building a more connected world. 2| Page
Table of Contents
1Introduction................................................................................................................................................................3
2Placement Guidelines ...............................................................................................................................................4
3RF Layout Guidelines................................................................................................................................................6
4Crystal Layout Guidelines ........................................................................................................................................8
5SDIO/SPI Layout Guidelines.....................................................................................................................................9
6USB Layout Guidelines...........................................................................................................................................10
7UART Layout Guidelines.........................................................................................................................................11
8Power Supply Layout Guidelines...........................................................................................................................12
9GND Pour Layout Guidelines .................................................................................................................................16

AN1344: RS9116 QMS Board Layout Guidelines
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1 Introduction
This Application Note provides PCB layout guidelines for designing a PCB using the RS9116 QMS SoC. These guidelines
cover parts placement, routing of various critical traces like RF, host interfaces routing like SDIO/SPI, USB, UART, power
routing, and GND pour. QMS SoC placement and routing can be done within a 4-layer PCB stack-up. However, the designer
can choose a higher number of layers, based on product needs. This Application Note describes placement and routing
considering a 4-layer stack-up.
The recommended layer stack-up and placement/routing considering a 4-layer stack-up is listed below. The exact PCB
thickness can be according to the layer stack-up provided by the PCB manufacturer. It is recommended to use a PCB
thickness close to 1.6 mm, but the designer can choose a suitable thickness based on product needs.
•Layer 1: Parts Placement, RF circuitry, Interface signals (SDIO/SPI, USB, UART), Crystal
•Layer 2: GND
•Layer 3: Power supply star routing
•Layer 4: Power supply star routing and few GPIOs

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2 Placement Guidelines
The designer can choose a suitable antenna as per the product needs, like a chip antenna, on-board PCB trace, external
PCB trace, dipole, etc. The design described in this Application Note uses a chip antenna.
Here are the recommendations for the placement of QMS SoC and its associated parts. Parts are placed on the same side
as the QMS SoC in this document. However, the designer can choose to place them on either side of PCB for achieving
best placement and routing. The steps below can be sequentially followed to achieve optimal placement.
1. To get the best RF performance from QMS SoC and antenna, it is recommended to place antenna circuitry near
the PCB edge. Antenna part guidelines must be followed for its placement and routing.
2. Place RF components, antenna and QMS SoC on the same side of PCB, so that there are no vias on RF traces.
3. RF front-end components and QMS SoC should be placed at minimal distance from RF ports (on QMS) to the
antenna, so that RF traces lengths are as short as possible.
4. Decoupling capacitors are recommended to be placed on the intended power pins. Decaps must be placed within
5 mm distance from the module pins.
5. VINBCKDC capacitor should be placed within 10 mm distance from the module pin.
6. Buck inductor should be placed within 5 mm distance from VOUTBCKDC pin. Buck capacitor should be placed
within 1 mm distance from the Buck inductor.
7. The series resistor on Clock signal of SPI or SDIO interface must be placed close to the source of this clock
signal. It must not be placed near the module end of the signal.

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Considering the above recommendations, the QMS SoC and its circuitry can be placed on a single side of the PCB as
shown below. This takes less than 25 mm x 20 mm of board space approximately, until RF antenna tuning network. The
designer can choose to place the components better than what is shown below to suit the product needs.
Figure 1: QMS SoC and Components Placement
SP3T
Switch
Band
Pass Filter
CHIP Antenna
VOUTBCKDC
Inductor
VOUTBCKDC
Cap
VINBCKDC
Cap
RS9116-
QMS
40MHz
XTAL
RF Front end
network
Antenna Tuning
Network

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3 RF Layout Guidelines
RF parts placement and trace routing affect the RF performance of the SoC and circuitry. Great care must be taken to
ensure best design practices and the guidelines below are followed.
1. The RF trace should have a characteristic impedance of 50 Ω. Any standard 50 ΩRF trace (micro-strip or coplanar
wave guide) may be used. The width of the 50 Ωline depends on the PCB stack, e.g., the dielectric of the PCB,
dielectric constant of the material, thickness of the dielectric and other factors. Consult the PCB fabrication unit to
get these factors right.
2. A thicker trace allows more manufacturing tolerance while keeping a 50 Ωimpedance.
3. Keep the length of the RF traces as short as possible.
4. Route RF traces on the top layer as much as possible and use a continuous reference ground plane underneath
them.
5. Maintain at least one trace-width of space between nearby GND pour and the RF trace.
6. Add GND stitches around RF traces.
7. Do not route any digital or analog signal traces between the RF traces and the reference ground.
8. Etch the GND copper underneath the antenna in all layers.
9. Follow the antenna placement as per the antenna vendor layout guidelines.
10. To evaluate transmit and receive performance like Tx power and EVM, Rx sensitivity and the like, an RF connector
would be required. A suggestion is to place a ‘microwave coaxial connector with switch’ between RF_OUT and the
antenna.
11. The RF signal after the BPF may be directly connected to an on-board chip antenna or terminated in an RF
connector of any form factor for enabling the use of external antennas.
12. Use ground pour on the top and bottom layers in the RF area with plenty of ground stitch vias. Use stitching in a
staggered pattern with a minimum via spacing of 32 mil (via to via, 25-mil offset) and a maximum spacing of 100 to
150 mil. After ground flooding, terminate shapes and corners with vias to tie to other planes for improved EMI
performance.
Considering the above recommendations and the placement guidelines described in the earlier section, a possible RF
circuitry routing in Layer 1 is shown below. Major parts and traces are also highlighted below.
Figure 2: RF Circuitry Routing in Layer 1
RF_TX
RF_RX
RF_BTTX
Chip Antenna
SP3T
Switch
BPF
Stitched
Ground vias
around the RF
trace

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The image below shows the overlap of RF routing on Top layer and the GND plane in the second layer. As can be observed,
the entire RF circuitry has the second layer as GND reference. GND vias around RF circuitry are stitched directly to the
GND plane.
Figure 3: RF Circuitry Overlap with GND Layer
Antenna Keep out
area. Etch the
Ground plane
underneath the
antenna in all layers,
as per Antenna
vendor layout
guidelines.

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4 Crystal Layout Guidelines
Crystal placement and traces routing affect the crystal performance, and hence the start-up of the SoC after power on. The
designer must ensure best design practices and guidelines below are followed.
1. Route the XTAL_IN and XTAL _OUT traces in top layer.
2. Place the XTAL as close to the SOC as possible and route these traces with shorter length.
3. Do not route the other traces underneath the XTAL lines.
4. All the non-crystal circuit traces and vias must be outside the crystal circuit area.
5. The XTAL GND pin should have a direct via to the reference GND plane.
6. It is recommended to have the GND pour around XTAL and its traces.
7. Follow the crystal part’s layout guidelines.
The image below shows the crystal placement and routing in Layer 1, as an example.
Figure 4: 40 MHz Crystal Routing in Layer 1
XTAL_IN
XTAL_OUT

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5 SDIO/SPI Layout Guidelines
SDIO and SPI are high speed interfaces where-in clock signals can reach up to 100 MHz. High speed design guidelines
like below must be followed for the signals in these interfaces. SDIO/SPI signals include SDIO_CLK/SPI_CLK,
SDIO_CMD/SPI_CSN, SDIO_D0/SPI_MOSI, SDIO_D1/SPI_MISO, SDIO_D2/SPI_INTR, SDIO_D3.
1. The characteristic impedance of the SDIO/SPI lines should be 50 Ω.
2. Match the lengths of all SDIO/SPI lines within 100 mils tolerance.
3. Keep the SDIO_CLK/SPI_CLK trace away from nearby traces with minimum 2x distance.
4. Do not route the parallel trace above or underneath the SDIO_CLK/SPI_CLK trace.
5. Keep SDIO/SPI traces away from all clock lines and noisy power supply components such as the switcher inductors.
Avoid crossing over power supplies or ground discontinuities. SDIO/SPI traces should have a solid ground on the
layer adjacent to them.
6. Do not leave any stubs on the SDIO/SPI traces.
The image below shows the SDIO/SPI traces routing in Layer 1, as an example. The series resistor on SDIO_CLK/SPI_CLK
is placed closer to the source of this clock, rather than placing closer to QMS SoC.
Figure 5: SDIO/SPI Signals Routing in Layer 1
SDIO_D3
SDIO_D2/SPI_INTR
SDIO_CLK/SPI_CLK
SDIO_D1/SPI_MISO
SDIO_CMD/SPI_CSN
33E Resistor
SDIO_D0/SPI_MOSI

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6 USB Layout Guidelines
USB signals can reach speeds of 480 Mbps. Guidelines for the differential signals USB_DP and USB_DM must be followed.
1. It is highly recommended that the two USB differential signals (USB_DP and USB_DN) be routed in parallel with a
spacing (say, a) that achieves 90 Ω of differential impedances and 45 Ω for each trace.
2. To minimize crosstalk between the two USB differential signals (USB_DP and USB_DN) and other signal traces
routed close to them, it is recommended that a minimum spacing of 3xa be maintained for low-speed non-periodic
signals and a minimum spacing of 7xa be maintained for high-speed periodic signals.
3. It is recommended that the total trace length of the signals between RS9116 part and USB connector (or USB host
part) be less than 450 mm.
4. If the USB high-speed signals are routed on the Top layer, best results will be achieved if Layer 2 is a continuous
Ground plane. Furthermore, there must be only one ground plane under high-speed signals and avoid the high-
speed signals crossing from GND plane to another ground plane.
5. Do not route the USB differential lines close to edge of the board.
The image below shows USB differential traces routing in Layer 1, as an example.
Figure 6: USB Signals Routing in Layer 1
USB_DP
USB_DM