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Designing low-cost wireless game controllers

Posted: 22 Apr 2004     Print Version  Bookmark and Share

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IIC-China/ESC-China 2004 7 Conference Proceedings2

Designing low-cost wirelessDesigning low-cost wirelessDesigning low-cost wirelessDesigning low-cost wirelessDesigning low-cost wireless

game controllersgame controllersgame controllersgame controllersgame controllers

Steve Moore

Director of Worldwide Applications

Micro Linear Corporation

to understand the radio systems requirements and imple-

mentation issues necessary for the best overall design.

Gaming Systems and Controller Overview

While nearly all children seem to be comfortable and fa-

miliar with video game systems very few radio engineers

tend to be. Basically these systems (see Figure 1) integrate

massive amounts of computing power with even more im-

pressive video processing to build a state-of-the-art multi-

media platform that, when combined with software that

NASA engineers would be proud of, provides for a truly

realistic gaming experience. The user controls the game

through a multitude of buttons (see Figure 2) which are pres-

sure sensitive and instantaneous. In addition the game con-

Abstract

Even as the general economy suffers, the consumer gaming

industry continues to grow. Within this industry there is a

strong demand for a low-cost wireless controller to elimi-

nate the unsightly cabling found strung across so many liv-

ing room floors. Existing wireless controllers, while func-

tional, are still too expensive and there is a strong push to

develop a wireless controller at (or near) the cost of an ex-

isting wired controller. In addition, the gaming industry is

already discussing next generation features (such as full-

duplex voice/audio) which will add further requirements to

consider when designing a new wireless controller for such

systems as the Sony PS2, Microsoft Xbox, or Nintendo

GameCube. Developing a wireless game controller requires

designing a good radio, integrating it with a low-cost

microcontroller, and writing the embedded firmware. This

paper will discuss the detailed systems design considerations

using a low-cost transceiver IC from Micro Linear (the

ML2724) and a low-cost PIC microcontroller (the PIC 18F)

from Microchip running a simple protocol implemented in

the embedded firmware.

Introduction

Even in a slow economy consumers still want to be enter-

tained. One of the more popular means of entertainment is

the video game industry. Gaming systems like the Sony PS2,

Microsoft Xbox, or Nintendo GameCube combined to ship

over 30 million consoles in the year 2002. At an average

ASP of $200 this makes this a $6 billion dollar industry!

After- market sales of controllers is estimated to be around

30 million units but the wireless penetration has remained

low due to the cost differential between wired and wireless

controllers (approximately $25 vs $59 USD average selling

price). Many manufacturers are rushing to meet this demand

for a low-cost, high-performance radio controller and need

Figure 1. PS2, Xbox, and Gamecube Systems

Figure 2. Controller Components

IIC-China/ESC-China 2004 7 Conference Proceedings 3

sole sends feedback to the controller to make it rumble dur-

ing extreme situations of the game (e.g. a car wreck). What

all this means to the controller designer is that it's not just a

simple `detect a few button presses' hardware design. Pres-

sure sensitive buttons mean an ADC port for each button,

instantaneous response means unnoticeable latency (even

by the most critical gamers), and console-to-controller com-

munications require a two-way radio link.

While a wired controller simply plugs into the game

console, a wireless controller requires a separate `dongle'

which plugs into the console and acts as the wired-to-wire-

less converter. On the wired side (of the dongle) it must

communicate with the console using the (proprietary) con-

sole communications protocol and on the wireless side it

acts as a 2-way transceiver communicating with the wire-

less controller using a protocol optimized for a wireless

channel. The specifics of the wireless implementation (both

physical layer and protocol) are non-standard and imple-

mented slightly differently by each manufacturer.

The general requirements that any wireless controller

should meet include;

7 Freq: 900MHz US or 2.4GHz Worldwide (typically)

7 Range: >10m

7 Throughput: under 30kbps per user

7 Power Consumption: > 50hrs on 3 AA batteries

7 Reliability: good (even with WLAN, Bluetooth, or

microwave oven interference)

7 Latency: low (imperceptible)

7 Cost: low (similar to wired controller)

Wireless Controller System Parameters

Designing a wireless controller involves many trade-offs in

pursuit of the best performing product at the lowest pos-

sible cost. While the wired functionality and electrical in-

terface are very well defined the wireless design is essen-

tially open to the experience level of the designer. First gen-

eration wireless controllers achieved basic functionality but

their performance was less than optimum and therefore

weren't widely accepted by hard-core game players. Recent

wireless controllers are achieving much higher levels of per-

formance (and reliability) but have still not met the cost tar-

gets necessary to stimulate widespread acceptance. Since sili-

con costs are still a large percentage of the overall BOM the

goal is to increase the level of integration in the radio trans-

ceiver chip (so the total design uses the fewest number of

components). The ML2724 achieves this goal and has been

used by numerous manufacturers of wireless controllers.

Before starting a wireless game controller design the

system's engineer must first address the following issues.

7 Frequency

Even though the consumer doesn't know (or care) which

frequency the product uses, the decision should be weighed

carefully by the designer. Local regulatory issues must al-

ways be met although most manufacturers prefer 2.4GHz

because it is the closest thing to a truely international band.

Certainly 2.4GHz isn't the only available band and there

can be benefits using other (lower) frequencies but offering

a single product worldwide has immense marketing attrac-

tion which overrides the technical concerns. The primary

concern is the possibility of interference. IEEE 802.11

WLAN and Bluetooth both operate in this band so any wire-

less game controller must be able to operate in their pres-

ence.

7 Range

Luckily the range of a wireless game controller is limited

by the viewing distance to a typical TV - which is about 10

meters. Any longer range can only cause potential interfer-

ence with other systems. This range is commonly obtained

using a Tx power of about 0dBm into a reasonably effi-

cient, small antenna. While some controllers may work at

slightly longer range than others, range has not been a no-

ticeable differentiator.

7 Power Consumption

Wireless controllers operate with anywhere from two to four

AA batteries. The primary drain on the battery has histori-

cally been from the dual-vibration motors used for player

feedback. To improve battery life some controllers include

switches to adjust the vibration level (or to turn it completely

off) depending on user preference. To further improve bat-

tery life it is important to go into sleep mode as often as

possible. Since the ML2724 transmits packets at a burst rate

of 1.5Mbps it allows the controller to stay in sleep mode

most of the time which means battery life is limited prima-

rily by the use of the vibration motors and/or leakage current.

7 Multi-Player Radio Access

Two different wireless techniques exist which allow mul-

tiple players to access the same game console - FDMA and

TDMA. In FDMA (frequency division multiple access) each

player (and console receiver) is allocated his own frequency

channel. This is the simplest implementation but requires

one console receiver for each wireless controller. Nearly all

of today's wireless controllers use this approach with some

having the built-in intelligence to switch frequency chan-

nels when there is interference (although most just include

a manual switch). Alternatively a TDMA (time division

multiple access) approach can be used where each player is

assigned a fixed time slot to transmit data but every con-

troller uses the same frequency. This approach requires a

higher data rate wireless link but allows a single console

receiver to work with 4 (or more) controllers allowing a

much lower-cost `multi-user' solution. The ML2724's

1.5Mbps data rate is much higher than existing transceivers

and is an ideal match when implementing a TDMA system

for next generation wireless controllers.

7 Throughput

Figure 3 outlines a simple way of estimating controller

throughput requirements. Today's controllers use analog

joystick controls and analog (pressure sensitive) push-but-

tons which must be sampled using an 8-bit analog-to-digi-

tal converter. While there are many different combinations

of switches, buttons, etc. on each brand of controller - this

simple exercise shows that controller throughput for a single

user to be in the 25-30kbps rate which for a 4-player TDMA

system would be around 100-120kbps.

IIC-China/ESC-China 2004 7 Conference Proceedings4

7 Interference

To operate reliably means to operate in the presence of other

wireless systems and potential interferers. Both IEEE 802.11

and Bluetooth operate at 2.4Ghz (as well as microwave ov-

ens and cordless phones) so the systems designer must plan

for a potentially noisy radio channel and add ways to com-

bat it. Historically radio designers have used spread spec-

trum techniques (either frequency hopping or direct se-

quence), adaptive channel selection (change frequency chan-

nels if interfered with), and good old fashioned multiple

retries (where you send it again until it gets through). The

trick is to implement the least amount of countermeasures

necessary so the customer is satisfied with performance

without increasing the cost. Complex direct sequence or fre-

quency hopping algorithms require much faster micro-con-

trollers which translates to higher power and cost. Most of

today's controllers tend to implement a combination of

multiple retries along with simple adaptive channel selec-

tion (either automatic or manually selected).

7 Latency

It was widely known that the first attempts at wireless game

controllers had a noticeable delay between when a button

was pressed and the associated reaction on the screen. While

latency is still a critical requirement it seems to have been

eliminated using much higher datarate transceivers and im-

proved protocols. It's generally accepted that a response time

of 10mS (Sony actually polls at a 16mS rate) should have

latency delays that are imperceptible to even the most ad-

vanced gamer.

Radio Design

Implementing the hardware and software designs for a wire-

less controller is becoming much easier as a result of in-

creased transceiver integration and easy-to-use development

tools. Figure 4 shows the block diagram of the Micro Lin-

ear ML2724 2.4GHz transceiver. It is a low-cost, highly

integrated FSK transceiver requiring very few external com-

ponents. Integrating nearly all the critical radio components

on-chip translates to lower cost, smaller size, and less of the

`black magic'associated with RF design. When choosing a

radio transceiver the system designer should make sure it

has the following features.

7 High enough throughput to handle present and future

requirements

7 All the critical RF circuitry (i.e. VCO resonator, Rx

IF filtering, etc.) are on-chip

7 No manually tuned components or manufacturing

calibration required

7 Easy interface to commonly available microcontrollers

7 Capability of going into a power-saving sleep mode

The ML2724 is ideal because it includes all critical RF

and IF circuitry on chip (i.e. no external SAW bandpass fil-

ter). In addition it achieves the optimum receive sensitivity

by auto-aligning its on-chip selectivity filters between each

Rx-to-Tx transition (with no user intervention required).

Using common 2-level FSK modulation with on-chip Tx

pre-modulation filtering means that most of the complex

system design is already done and with both Rx and Tx on

the same chip the RF link is already optimized.

After the transceiver IC is chosen the designer needs to

investigate how a suitable antenna will be implemented.

While embedded antennas can sometimes be complex and

mysterious it is usually common to find very simple anten-

nas used in these types of applications. Due to the low-cost

constraint and relatively short range requirement rarely will

designers use anything more complex than an `electrically

short'length of wire or even simpler, a long PCB trace. With

a little patience and basic test equipment a good RF de-

signer can tune up these types of designs without too much

trouble and basically accept the fact they are both ineffi-

cient (i.e. more power dissipated in the resistive losses of

the wire than actually radiated) and not always omni-direc-

Figure 3. Estimating Wireless Controller Throughput

Figure 4. ML2724 2.4GHz Transceiver

Figure 5. Wireless Game Controller Radio Board

IIC-China/ESC-China 2004 7 Conference Proceedings 5

tional (i.e. they may radiated in some directions more than

others). Many good books and articles exist on practical

embedded antenna design but as a practical matter most ra-

dio designers give the poor antenna whatever space is left

after everything else has been squeezed in! Figure 5 shows

how the antenna was implemented using a curving microstrip

trace with associated components to match it to the Rx in-

put impedance.

Using the block diagram shown in Figure 6 we can esti-

mate the free space range assuming the following typical

RF component specifications.

Using standard cascaded analysis we calculate the to-

tal front-end NF to be around 9dB. For 2FSK we require a

SNR of about 10dB for 10-3 BER. Therefore the mini-

mum receive sensitivity level can be calculated (assuming

transceiver occupied bandwidth of 2MHz) as;

Pr = kT + SNR + NF + BW

= -174dBm/Hz + 10 + 9 + 10log(2MHz)

= -91dBm @ 10-3 BER

Transmit power of the transceiver IC is approximately

0dBm and with 3dB of loss between the IC and the antenna

yields a final transmit power of -3dBm. Maximum allow-

able path loss can then be calculated as;

PL = Pt + Gt - Pr where Pt=-3dBm, Gt=0dBi, and Pr=-

91dBm

PL = (4d/)2

where d=distance and =wavelength=12cm

which at 2.4GHz calculates to a theoretical free space

distance d ~ 250m. Knowing that indoor propagation is

much less than free space we should still have plenty of

margin meeting a 10m spec.

The next part of the design is the microcontroller design

and firmware development. The block diagram in Figure 6

shows how simple it is to interface the transceiver radio to a

common low-cost microcontroller. The function of the

microcontroller is basically to control the transceiver IC and

to monitor and respond to inputs from the user.

7 Microcontroller interface to the transceiver

o Enabling the transceiver and selecting either Rx

or Tx mode

o Tuning to the desired frequency channel

o Synchronizing and recovering the incoming

received data

o Establishing and maintaining a link with the

console radio

o Sending digital data to the transceiver transmitter

7 Microcontroller interface to the user

o Continuous monitoring of analog buttons

(multi-channel ADC)

o Reading the desired freq channel

(manual channel select)

o Driving vibration motors

(user selectable intensity level)

o Setting and recovering from sleep mode

(when there are no key presses)

Figure 7 shows a basic firmware flowchart. Since this is

an asynchronous system packets can be received at any time.

Therefore the system continually monitors the received sig-

nals to see if a legitimate packet is recognized. Since re-

ceived data will be relatively infrequent the system will

spend most of its time scanning the controller buttons and

transmitting the digitized value of the key that was pressed.

The final part of the design is to implement a simple

communication protocol for this wireless system. For most

basic systems it is as simple as including 2 bytes for a pre-

amble training sequence (basically calibrating out any re-

ceiver DC offsets) followed by 3 bytes for a start-of-packet

sequence (for timing and sychronization) followed by the

actual data payload and (optionally) 2 bytes for CRC error

detection. For more robust systems there needs to be an

additional byte reserved for addressing and management/

control. This would be necessary (for instance) if it was

desirable to automatically change frequencies when the

packet errors are too high or to just simply require acknowl-

edgment of every packet.

Figure 6. Microcontroller Interfacing

Figure 7. Firmware Flowchart

IIC-China/ESC-China 2004 7 Conference Proceedings6

Where;

CP = Conditioning Preamble (2bytes)

SP = UART Synchronization Preamble (4bytes)

*) optional - not required if using software UART

SOP = Start of Packet Sequence (3bytes)

Addr = Source/Destination Address (1byte)

Payload = Payload Data (0-128bytes typical)

CS = Checksum (2bytes)

The other key requirement of wireless game controllers

is long battery life. Common methods of increasing battery

life include;

7 Running the microcontroller at the lowest clock speed

possible

7 Running all electronics at the lowest voltage possible

and using efficient DC regulators

7 Automatically powering down if no key-presses are

made within a certain amount of time

7 Going into sleep mode between packet transmissions

The last item actually requires sending the data at a faster

rate than necessary and then turning off until the next packet

is required. For example - the ML2724 sends data at a rate

of 1.536Mbps which would take about 177 uS to send the

34 byte packet calculated in Figure 3. If we round this up to

200uS and assume we need to send updates every 10mS

then we can theoretically run at a 2% duty cycle and spend

most of the time powered down! Running the ML2724 in

this burst mode maximizes battery life without any notice-

able degradation to the user.

Finally, today's wireless game controllers are under ex-

treme cost pressures. Reducing the cost of the radio is a com-

bination of reducing the transceiver IC cost while at the same

time integrating as much of the discrete circuitry as possible.

Additionally, any costs associated with poor yields, produc-

tion tuning, calibration, etc. must be eliminated.

The Micro Linear ML2724 is a highly integrated 2.4GHz

transceiver which includes both a low-IF receiver and di-

rect VCO (closed loop) modulated FSK transmitter in a

single 32pin package. The ML2724 integrates the voltage

regulators, frequency synthesizer (including the VCO tank

circuit), image rejection mixer, IF selectivity filter and FSK

discriminator on a single BiCMOS integrated circuit. The

only external components are low-cost bypass capacitors

and a simple R-C PLL loop filter. One of the major benefits

of the ML2724 is that it includes on-chip, automatic align-

ment of critical filtering (both Tx and Rx) and also inte-

grates the highly-sensitive VCO resonator circuitry elimi-

nating any production level alignment and storage of cali-

bration parameters.

Development Tools

The other important aspect of design is having good devel-

opment tools. To facilitate the development of wireless ap-

plications, like game controllers, Micro Linear created the

ML2724SK. It consists of two fully functional wireless sta-

tions which can be set up to establish a simple point-to-

point link but can also be used to evaluate the performance

of the ML2724 transceiver. A simple, generic protocol was

written which runs on a low-cost PIC microcontroller. All

design material (schematic, gerbers, and firmware) is in-

cluded to help customers get up and running in the shortest

amount of time. Designers can optimize both hardware and/

or software for their specific application and run performance

testing (for example range tests, interference rejection, etc)

before finalizing the actual design.

Summary

As the wireless gaming industry continues to grow, so does

the demand for a low-cost wireless controller. Existing de-

signs have met the basic performance needs of hard-core

gamers but the high potential volumes are dependent on

reducing overall wireless controller cost. To reduce con-

troller radio cost requires a highly integrated radio trans-

ceiver like the Micro Linear ML2724 capable of working

with today's simple 8-bit microcontrollers.The ML2724 also

increases battery life by utilizing its fast 1.5Mbps data rate

to run in burst mode allowing the controller to be in a power

down state most of the time. To facilitate customer devel-

opment of wireless game controllers Micro Linear has re-

leased the ML2724SK with all associated design informa-

tion/materials.

About the author

Steve Moore

Micro Linear Corporation

2050 Concourse Drive, San Jose, CA 95131, USA

Phone: (+1-408) 428 6541

Fax: (+1-408) 434 9897

E-mail: moore.steve@microlinear.com

Steve Moore has over 24 years experience in the design and

applications of RF and wireless products. He received his

BSEE degree from UC Berkeley and MS degree in Engi-

neering Management from Santa Clara University. He has

had both design and managerial roles at Watkins-Johnson

Company,Trimble Navigation,WirelessAccess, SiRF, Sym-

bol, and is presently the Director of Worldwide Applica-

tions at Micro Linear. Mr. Moore has had the privilege of

working on a wide array of RF and wireless products in-

cluding microwave subsystems for defense applications,

GPS receivers and ICs, 2-way pager products, 802.11

WLAN products, and most recently with low-cost, highly-

integrated transceivers for proprietary short-range applica-

tions like cordless phones, wireless audio, and other wire-

less consumer products. Last year, Mr. Moore spoke at the

Wireless Systems Design Conference in San Jose, Califor-

nia, USA. He was also featured in the Wireless Systems

Design magazine (July/Aug 2003 Issue).





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