Which USB Cable is Right for Me?

The Universal Serial Bus (USB) is evolving into a truly standard serial interface capable of both high-speed data transmission and impressive fast charging of devices like phones and laptops. Unfortunately, the need for legacy compatibility on most of the world’s computers and laptops combined with the requirement for faster and faster serial data transfer has created a very confusing specification for USB cable socket styles, data rates and charging speeds, leading to many wondering which USB cable is right for me?

To address that confusion, RDIq is a power focused USB product portfolio, designed to make it simple to upgrade traditional USB chargers to the latest fast charge equivalents. Most of us just want a cable that will charge our phones faster than they currently charge, so we’ve made it easy to decide which cable you need on the charger front. Figuring out the data side leads to some confusing technical discussions, but we’ll cover that in the latter part of this note. For charger cable folks, here’s all you need to know.

If you have an iPhone, regardless of whether it’s an older iPhone 7 or it’s a fast charge supported iPhone 8 and above, you need an RDIq USB C to Lightning cable. You’ve heard some USB cables can cause your charging equipment to blow up or go on fire, right? Purchasing an iPhone cable with a USB Type C connector on one end is the key to eliminating that risk. See ‘Evolving USB Socket Types’ later in this article. You will also need a USB charger that supports fast charge technology. In the case of an iPhone, that should always be a charger designed for USB PD (Power Delivery) because doing so will keep your charger compatible with Apple’s chosen fast charge delivery system. The last, very important step is to make sure the cable is approved by Apple. Every piece of hardware and software designed to operate on an Apple product must pass the rigid Made for iProducts quality inspection, commonly known as MFi certification. The box or invoice the cable is shipped with should display the MFi certificate.

For iPhone 7 owners, USB PD fast charge technology reverts to legacy USB support for the iPhone 7, but with a nice advantage. Your phone will charge at almost twice the speed of your Apple supplied cable and charger because it employs the higher current iPad charge specs all Apple batteries are designed to accept safely. Using USB PD in this default state will charge an iPhone 7 to 50% in about 35 minutes.

If you have any other make of smartphone, chose a cable with USB C connectors on both ends. A good rule of thumb is if your phone does not have a USB C connector it will most likely not support fast charge technologies. There are exceptions to this rule, but since we’re trying to simplify your cable choice in this part of the discussion, go with a Type C to Type C cable, buy a charger that supports Qualcomm QC 3.0 or QC 4.0 and you will guarantee the fastest charge available for your device. Plugging your QC 3.0 supported Pixel or Samsung phone into a USB PD charge socket will result in a slightly lower charge rate per the discussion above on the iPhone 7. Devices designed for the new QC 4.0 standard will charge equally quickly on a USB PD or a QC 4.0 charger.

If it’s so easy why all the complications?

Deciding which usb cable you need is complicated, not because the technologies are complicated but because the USB Implementers Forum, the folks who control USB specifications, created too many marketing names for the many variants the advances in high speed serial data imposed on their specifications.

On the face of it there’s a logical progression of USB data specs from the original USB 1.0 to the current USB 3.2 version which is in fact rather easy to understand from a USB Power perspective. Table 1 shows the progression, and we should note only USB 3.2 supports fast charge, indicated by the 5V-20V charge voltage range compared to every other revision.

Table 1: USB Specifications and Supported Power Output

USB Revision	Voltage Range	Current Capability	Max Power Output
USB 3.2	5V-20V	0.5A, 0.9A. 1.5A, 3A, 5A	100W
USB 3.1	5V	0.5A, 0.9A	4.5W
USB 3.0	5V	0.5A	2.5W
USB 2.0	5V	0.5A	2.5W
USB 1.0	5V	0.5A	2.5W

USB 3.2 replaced USB 3.1 in September 2017, but instead of creating a clean break, the USB-IF decided to roll all level 3 versions into 3.2. Historically USB revisions have been made based on new technology that allows serial data transmission at higher speeds, but now the new 3.2 rev encompassed the slower data rates of the other two level 3 revisions.

This isn’t the first time the USB-IF has done this; three USB level 1 revisions were resolved into USB 2.0. The first confusing step though was the initial move from USB 3.0 to USB 3.1 where USB 3.0 suddenly became USB 3.1 Gen1 and USB 3.1 became USB 3.1 Gen2.

Blame Moore’s Law for exponentially changing USB data rate capability but be sure to blame the marketing geniuses at the USB-IF for the next piece of wisdom. The fastest version of USB 3.2 is 20Gbps and it is known as USB 3.2 Gen 2×2. This is twice the data rate of USB 3.1 and achieves that mighty jump by creating two parallel data highways, hence the 2×2 moniker. The sophisticated USB Type C socket and cable was developed to enable this step change in data rate, but why on earth take a great engineering achievement and make it virtually impossible to understand by allowing marketers to name and rename previously difficult to follow versions of USB 3 into the multiple versions we have now.

There should be a pause for effect here…

• USB 3.0 was changed to USB 3.1 Gen 1 but is now called USB 3.2 Gen 1. It is marketed under the name SuperSpeed USB and has a maximum data rate of 5Gbps.
• USB 3.1 was changed to USB 3.1 Gen 2 but is now called USB 3.2 Gen 2. It is marketed under SuperSpeed USB 10Gbps.
• Flagship USB 3.2 became USB 3.2 2×2 and is marketed under SuperSpeed 20Gbps.
• It gets worse. Intel joined the party with their 40Gbps Thunderbolt system, capable of transmitting 40Gbps on a dual highway cable. The cable looks like a standard USB C to USB C, but it is not. Transmission line effects at the higher data rate means a standard C to C cable will fail some of the data bit checks during operation and will not work in any application that operates close to the maximum speed available.
• USB C to USB C cables that have been designed for Thunderbolt applications have small Thunderbolt logos on one or both connectors. Users need to make sure that logo is present if the Thunderbolt specification is required.
• On the good news front, the USB-IF announced the all-encompassing USB 4.0 revision for deployment late 2020. USB 4 will eliminate the rev 3 confusion and provide competition to Intel’s Thunderbolt standard. We can but hope the rev 3 marketers have moved on and will not get the job of clarifying the spec and its uses to we mere users this time around!

Evolving USB Socket Types

If the USB standard had only catered to data transmission there would be no confusion over which USB socket to choose for each application. The slower cables and sockets just wouldn’t work with the faster apps and that would be the end of it. USB however also serves power to the load and it’s this feature which has caused fires and destruction of computer equipment because users (and ultimately manufacturers) have not read the fine print on the USB-IF specifications when it comes to how the new, higher power USB devices negotiate battery charge voltage and current.

There are two socket Types in question here; USB A and USB C. The USB B socket is a multi-version confusing story all on its own, but it does not have a role in this part of the story. USB A is the original 4-channel socket and is in billions and billions of legacy computers and associated hardware. The practice of connecting smartphones to computers to sync and update apps like iTunes via USB made it very attractive to allow charging of the phone, and therefore any other USB device from the computer socket. However, new USB PD and QC compliant devices have the ability to place higher voltage and current demands on the USB lines than the legacy systems can tolerate. The result in several famous cases has been destruction of computer motherboards and small fires with cables.

This has caused a great deal of concern in the market, but because the solutions provided are seen as complicated or wordy, many users haven’t read them or are still confused about the action they should take to avoid the risk.

The simple solution is one of elimination; do not charge a modern smartphone from a USB A computer or laptop socket using anything other than the cable provided by the manufacturer to do so. If the cable fails and you must connect the phone to the computer, buy the appropriate cable from the smartphone manufacturer. If that’s not an option, the USB 3.2 specification includes a cable spec designed to ensure any USB A to USB C cable has a mechanism to make sure the smart power side of the connection cannot request more power than can be safely handled by the computer. Instead of relying on both sides negotiating the power level per the USB PD specification, a hard wired 50k resistance is included in the chip at the C end of the cable as shown in Figure 1 which tells the smart side it can only ask for legacy USB power.

This flexible arrangement is the very cause of USB cables causing computer destruction and fires. By creating a USB C to A version with the resistor requirement, it left the door wide open for unscrupulous manufacturers to make the cable with standard C sockets, enabling fast charge voltages and currents in unsafe applications.

There are of course legitimate uses of the fast charge USB C to A cable without the 50k insert. Most QC chargers, especially the dual channel types, have at least one USB A socket and provide safe fast charge when used with a fast charge adapter. However, if the QC device is going to be connected to a PC, use the manufacturers original cable to be completely certain the device won’t ask for more power than can be safely served.

These problems are going away. New laptops and tablets have USB C connections and allow USB-PD charge. Qualcomm’s QC 4.0 is now compatible with USB PD allowing users with a mix of PD and QC compatible devices to buy a single charger type for all their devices. The PD spec actually encourages phones and other USB PD compliant hardware to be connected to a laptop and allows power to be transferred in both directions, so any battery with charge can help out any battery without charge.

Until these two platforms are ubiquitous, the best advice we can offer the billions and billions of USB A computer owners who want to charge via their PC is don’t connect a smart power device to your computer with anything other than the manufacturer’s cable. Better still, eliminate the problem by purchasing a charger from the RDIq range of fast chargers and get the fastest charge available with zero risk of fires and other catastrophes for your PC.

RDIq is a trademark of RDI Incorporated and provides USB Fast Chargers for all USB applications.