PCB LAYOUT AUTHORITY

This blog presents the most common design issues affecting signal integrity in high-speed digital hardware design. These include impedance control, terminations, ground/power planes, signal routing and crosstalk. Armed with the knowledge presented here, a digital designer will be able to recognize potential signal integrity problems at the earliest design stage. Also, they will be able to apply techniques presented to prevent these issues affecting the performance of their design.

Friday, September 16, 2011

SuperSpeed USB 3 Design Guide

Introduction

SuperSpeed USB (USB 3.0) delivering data rates up to 5Gbps which is ten times faster than Hi-Speed USB (USB 2.0) with optimized power efficiency. At these high transmission rates, signal integrity issues become increasingly restrictive on PCB trace and cable lengths, and on design implementation and features. Poor signal quality can significantly impact system performance and reliability.

SuperSpeed (USB 3.0) ReDriver in Source Application
Superspeed is a dual channel (TX± and RX±), single lane USB3.0 redriver which use in source application such as Notebooks, Desktops, Docking Station, Backplane and Cabling. Each channel offers selectable equalization setting to compensate the different input trace loss. The block diagram below shows the application on notebook with docking.

SuperSpeed USB Layout Guideline

A. Decoupling capacitor of VDD

It is recommended to put 0.1uF decoupling capacitor at each VDD pin of IC. Below is a layout reference of decoupling capacitor placement on a board. Four decoupling capacitors circled in pink below are located next to the four VDD pins (pins 6, 10, 16 and 20) of the IC.

B. PCB layers

Recommend to use at least four layers PCB for SuperSpeed USB design. Every data signal trace should be routed entirely over the ground plane on an adjacent layer.

C. Routing around the USB connector

On the host design, USB receptacle connector is used on the PCB. For the Vbus trace, it’s suggested to insert a ferrite bead. For the shielding of USB connector (shielding of USB cables), AC isolation to the ground (such as proper value of inductor, instead of connecting the cable shield directly to the PCB ground plane).

For the SuperSpeed signal trace, the impedance should be maintained. Avoiding any stubs and removing any routing that cause signal discontinuity and severe EMC noise issue. Also, do not put any metal between all SuperSpeed signal pair pins on every layer when using receptacles with pins stabbing the PCB.

Crosstalk between the signal trace
There are 3 pairs of signal (SSTX± /SSRX±/ D±) for USB3.0 and these signal pairs will cause three typical type near-end crosstalk:

SSTX± to D± in RX mode
SSTX± to SSRX±
D± to SSRX± in TX mode

In order to minimize the crosstalk issue, the routing of the signal trace between SSTX±/ SSRX± and D± pairs should not be closed to each other.

SuperSpeed signal trace impedance

The layout around USB3.0 receptacle connector was routed as one or more large metal planes in specific layer (such as GND layer). In order to maintain the differential impedance of any SuperSpeed signal trace, make sure there is no metal between pins for any differential pair.

Stub on SuperSpeed trace

The pin on USB3.0 receptacle connector become an open stub if the SS signal trace pair is designed on the top layer which will cause the signal discontinuity issue.

D. Routing around the USB Controller

As high speed signal is sensitive to power signal, therefore the routing on power and ground design of USB controller need to be careful. Same as section (A), the decoupling cap is need for each power pin and it should be place as close as the power pad of USB controller. As USB controller contains both analog and digital section, analog power and digital power is required. In order to avoid the interference from the digital signal cause the malfunction on the analog circuit, the routing between analog power and digital signal trace should be placed as far as possible (including the signal trace). For the same voltage level’s analog power and digital power, a ferrite bead should be added in between for noise filtering.

Friday, August 26, 2011

Capacitive Touch Sensing Layout Guidelines --- > Part 1

Capacitive Sensing

Capacitive sensing is the art of measuring a relatively very small variation of capacitance in a noisy environment.

To illustrate the principle of capacitive sensing we will use the typical simplest button implementation below but the same basic laws apply to more complex capacitive structures like sliders or wheels.

Figure shows cut view and top view of a typical capacitive button implementation. The sensor connected is a simple round copper area on top layer of the PCB. It is usually surrounded by ground for noise immunity (see § 2.3). For obvious reasons (design, isolation, robustness …) the PCB is stacked behind an overlay which usually consists in the housing of the complete system (notebook, TV, monitor, cell phone, etc) .

When no conductive object, like a finger, is close the sensor only sees an inherent capacity value CEnv created by its electrical field’s interaction with the environment, especially with ground areas. When a conductive object, like a finger, approaches the sensor the electrical field around the sensor will be modified and the total capacitance seen by the sensor increased by the finger capacitance CFinger.

The challenge of capacitive sensing is to detect this relatively small variation of CSensor (CFinger usually contributes for a few percent only) and differentiate it from environmental noise.
For this purpose, Capacitive products integrate an auto offset compensation mechanism which dynamically removes the CEnv component to extract and process CFinger only. CFinger, like any capacitance can be expressed in the formula below :

A is the common area between the two electrodes hence the common area between the finger and the sensor, typically 1cm2 for an adult finger.

d is the distance between the two electrodes hence the overlay thickness, typically 1-3mm. Overlay thickness is a compromise between mechanical/ESD robustness (the thicker the better) and power consumption (if too thick the sensitivity may need to be increased to be able to sense CFinger properly).

εo is the free space permittivity and is equal to 8.85 10e-12 F/m (constant)

εr is the dielectric hence overlay permittivity when finger is touching. Typical permittivity of some common overlay materials is given in table below. Higher εr allows reducing power consumption and/or increasing overlay thickness.

General Guidelines

The Chip

Strong ground connection on bottom exposed pad and ground pin

VANA, VDIG, VDD decoupling capacitors must be placed as close as possible to their associated pin
Integration capacitor Cint (see Figure 1) must be placed as close as possible to the chip and as far as possible from noisy signals

Connections to CAPx Sensors

0.2 mm trace width is recommended
Minimum 0.2mm clearance between CAPx traces, recommended 0.5mm or above. CAPx are sensed serially by the chip, non-sensed CAPx are internally tied to ground hence the clearance requirement.
Connections must be as short and direct as possible.
Preferably not to be routed on top layer to reduce potential finger coupling (must only be maximized on the CAP sensors, not on the traces)
Vias number must be reduced to the minimum.
CAPx signals should be routed as far as possible from noisy signals (LEDs, etc) and ideally on different layer or isolated by ground.
Noisy signals should not be routed below CAP sensors, if needed they must be isolated with ground layer.

Ground Considerations

0.5 to 3mm recommended clearance with CAP sensors. Low values maximize noise immunity while high ones minimize power consumption.
Below 0.5mm clearance is possible but increases significantly ground coupling hence requiring higher sensitivity setting and higher consumption.
Above 3mm clearance is possible but doesn’t reduce significantly ground coupling while reducing noise immunity.
Both top and bottom layers should be filled with hatched ground (typ. 20 %) to improve noise immunity while keeping DC cap low.

Capacitive Touch Sensing Layout Guidelines --- > Part 2

Buttons

Introduction

Similarly to the mechanical buttons they intend to replace, touch buttons provide ON/OFF information i.e. respectively button touched or not touched by the finger. Each touch button is associated to its dedicated capacitive sensor.

Shape

Round is best while oval or square with round corners are also acceptable
Any other shape with acute angles is not recommended
Possibility to put a hole for reverse mount SMD LED in the middle (will reduce a little bit the sensor surface, can be compensated with higher sensitivity setting or bigger sensor)

Size

1cm diameter is recommended
Above 1.5cm diameter is useless due to finger tip surface
Below 1cm is possible at the expense of higher sensitivity setting hence higher consumption

Pitch

1.5cm is recommended as minimum
Below 1.5cm is possible but reduces user friendliness and improves the risk of side touch effects.

Examples

Slider

Introduction

Similarly to the mechanical sliders they intend to replace, touch sliders monitored by Semtech products provide of course the position information but also an ON/OFF state (i.e. respectively slider touched or not touched by the finger) as well as movement information (move-up or move-down).

Each touch slider is made by several capacitive sensors placed back to back on the PCB.
For good position resolution a slider usually requires interpolation (i.e. number of positions not limited to the number of sensors) which requires a layout ensuring that the finger always touches at least 2 sensors.

Shape

Slider

Straight shape is usually recommended for better user friendliness but other shapes are also possible
Mechanical guide on overlay improves user friendliness and robustness especially for exotic shapes

Sensors

Rectangular shape implies a lot of sensors to ensure interpolation (max half surface of finger per sensor)
Chevron shape is recommended as it provides good interpolation with relatively low number of sensors

Size

Slider

1cm recommended width
Above 1.5cm width is useless due to finger tip surface
Below 1cm width is possible at the expense of higher sensitivity setting hence higher consumption
The number of sensors depends on the length required and resolution targeted

Sensors

The smaller the better, typically below 0.5cm2 recommended (half surface of finger)
Bigger is possible but requires more complex layout (more interpolation required to ensure good resolution)

Pitch

The smaller the better to maximize interpolation
Ground clearance recommendations apply (see §2.3)

Example

Wheel

Introduction

A wheel can be seen as a slider with round shape, as such it has similar layout constraints and also provides position, ON/OFF, and movement information.
Each touch slider is made by several capacitive sensors placed back to back on the PCB.
Similarly to a slider, a wheel usually also requires interpolation but because of its “infinite length” nature, the position precision requirement is usually not as critical as for a slider. (movement detection may be more important)

Shape

Wheel

Round is usually recommended for better user friendliness but other shapes are also possible
Mechanical guide on overlay improves user friendliness and robustness especially for exotic shapes

Sensors

Rectangular shape implies a lot of sensors to ensure interpolation (max half surface of finger per sensor)
Chevron shape is the best but may be complex to design inside a wheel
Whirl shape is recommended as it gives a good compromise between the number of sensors required and layout complexity

Size

Wheel

1cm recommended width
Above 1.5cm width is useless due to finger tip surface
Below 1cm is possible at the expense of higher sensitivity setting hence higher consumption
The number of sensors depends on the wheel diameter required and the resolution targeted

Sensors

The smaller the better, typically 1cm2 recommended for whirl shape (see below)
Bigger is possible but requires more complex layout (more interpolation required)

Pitch