The mind behind the wireless mouse

Carl Brasek, WirelessUSB Marketing Manager of Cypress Semiconductor's Personal Communications Division, looks at the key RF requirements for a wireless PC human interface device.

Carl Brasek, WirelessUSB Marketing Manager of Cypress Semiconductor's Personal Communications Division, looks at the key RF requirements for a wireless PC human interface device.

If you were able to get inside the mind of a PC HID (human interface device) manufacturer, these are the key RF requirements you would consider for going wireless: cost, range, power, latency, interference, colocation and ease of use.

This article will review each of these factors in the context of providing designers with a template for evaluating wireless technology for their PC HID devices.

The PC HID market is extremely cost sensitive.

Every $0.005 added to the bill of materials is counted, and assembly costs are equally scrutinised.

Careful consideration is therefore given to the quality and quantity of components that get added to the mouse to enable it to be wireless.

Can the HID developer still use the same cost effective application microcontroller or will they have to upgrade to a higher performance microcontroller in order to process the digital logic for the RF device?.

Will the wireless protocol require memory beyond what is already provided in the microcontroller or will there be a requirement for external Flash?.

What about crystals, antennas, RF filters and front ends?.

How many additional components or layers will be added to the board, and how will these impact manufacturing and assembly costs?.

Although there is a tangible premium for wireless, an RF adder for a mouse includes more than what ends up on a mouse PCB.

There is an additional wireless dongle to consider with nearly identical RF components that wasn't required before.

The cost to go wireless also includes some new accessories like batteries for the mouse (the dongle is usually powered by the USB or PS/2 bus) and in most cases a CD with software drivers that also wasn't a cost consideration for a wired mouse.

The rapidly emerging PC multimedia environment with larger sharper monitors is enabling the consumer to sit further and further away from the computer system than in the traditional desktop environment.

The range, therefore, must enable reliable wireless connectivity beyond the 5m length of a USB cable (or 3m unshielded cable).

27MHz wireless devices, common to this space, have a range of 1-2m.

With 2.4GHz technology, a wireless device's operating range can be easily extended beyond 10m.

In home theatre environments, large plasma screens are enabling people to sit further from the action.

A good 2.4GHz technology eliminates line-of-sight requirements of audio-visual components used in conjunction with large plasma displays, so users don't have to open the TV cabinet doors for the IR remote to change the channel.

At the board level, power is measured as current consumption but in the system level it is translated into battery life.

The longevity of battery life in a wireless mouse is directly proportional to end-user satisfaction.

Having to change out batteries every two months will adversely affect the end-users' willingness to continue to invest in their wireless mouse.

When evaluating power requirements, HID designers must not only consider hardware components like the LED sensor, the application microcontroller and the wireless ICs they must also consider the wireless protocol being employed.

A wireless protocol that is constantly "on air" will burn through batteries a lot quicker than a wireless protocol that is event-driven or otherwise not transmitting.

Regarding the power source, a mouse fitting in the palm of your hand, and requiring fluid motion underneath that same hand will not react quickly nor feel incredibly lithe with more than two AA alkaline batteries weighing it down.

So there is also a challenge as to the amount of power you can give a mouse.

Good end-to-end wireless system latency directly translates to acceptable end-user performance.

If there is a noticeable lag between a left button click on the mouse and a pop up window on the screen or an unwieldy mouse pointer that stutters across the screen then the consumer will return back to the wired world with a strong distaste for wireless.

Typical wired systems have total system latency readings of between 12 and 20ms.

This is the amount of time it takes for a signal to travel from the attached device to the PC's CPU.

With wireless, additional latency is added as the signal must also travel from the radio device to the wireless dongle.

Going wireless, you want to make sure that added latency is kept to a minimum.

A wireless latency "adder" as low as 4ms is achievable with certain wireless technologies and goes largely undetected by the end user.

The ability to resist interference from other technologies sharing the same frequency band is extremely important as this also impacts end-user performance.

Robustness to interference in the 2.4GHz world means the ability to reliably coexist with 802.11b/g, Bluetooth, WirelessUSB and a host of cordless phones and microwave ovens.

With the wireless peripheral market beginning to take off, wireless will also take flight from the home and into the campus, office and commercial settings.

Channel availability, and an intelligent coding scheme to ensure cohabitation of several wireless devices of similar make is extremely important.

In a cubicle environment, it would not be uncommon at a corner intersection to have four wireless dongles within a couple metres of each other.

If your coworker in one of the adjoining cubes begins deleting your files with their wireless mouse, that could spell disaster.

The best way to solve interference issues and enable multiple devices to operate in close proximity is to use a radio device that employs both direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) transmission schemes.

DSSS ensures data robustness, and FHSS allows the wireless signal to "hop" to new channels once interference becomes too great.

Ease of use is looked at from both a design perspective and from an end-user perspective.

If PC HID designers spends months developing drivers for their wireless products or are forced in to using complicated tricks with antenna layout in order to bolster range then this will inevitably extend the development cycle.

Likewise, from a consumer perspective, if there are several steps to installation required for a wireless mouse, an end user will likely become baffled resulting in the customer service phones ringing off the hook with confused callers.

Look for a wireless technology that is largely plug-and-play.

If you don't have to deal with writing device drivers, creating device profiles or software stack, you can greatly reduce your product's time-to-market.

Wireless technology has made some progress in penetrating the PC-HID market.

It is estimated that by 2007, 25% of PC-HID devices shipped will be wireless.

Paying a little attention to each of these criteria will help ensure that you are well on your way to designing a successful wireless product.

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