The nRF24xx family has already been incorporated in a number of new and existing wireless global applications such as sensor, remote control, mouse, game controllers, tire pressure sensors and intelligent sports equipment applications.

Low cost wireless at 2.4GHz

John Leonard | Nordic VLSI ASA

Low cost wireless communication at 2.4GHz in consumer and industrial products

The nRF24xx family has already been incorporated in a number of new and existing wireless global applications such as sensor, remote control, mouse, game controllers, tire pressure sensors and intelligent sports equipment applications.

by John Leonard, Field Application Engineer with Nordic VLSI ASA, Wireless Communication


With the new 0.18µm CMOS platform for 2.4GHz wireless communication, nRF24XX™,  Nordic VLSI ASA has been able to release a family of four new RF devices in less than 6 months. The new platform sets new standards for wireless communication in the global 2.4GHz ISM frequency band with extremely small packages, few external components, low current drain, and operation down to 1,9V over the industrial temperature range -40°C to +85°C. The new RF circuits have very competitive features ranging from protocol pre-processing - ShockBurst™ as well as an 8051MCU core, 9-channel 12 bit ADC, PWM, timers and a dual receive - all resulting in a very low Bill of Material cost.. The nRF24XX family is supplied in packages from 4x4mm to 6x6mm, with a crystal and a resistor required externally to make up a complete radio.

0.18µ CMOS Brings new levels of functionality

Nordic VLSI made the decision to migrate to the 0.18µm pure CMOS fabrication process for the nRF24xx family, a move which competitors in the industry are struggling to match. Nordic had previous project experience with this technology and its clear benefits were brought to bear during the device's concept. The level of integration now employed means operational voltages of 1.9v are achievable and current demands are reduced accordingly, typically 13mA at full output power and a mere 12µA in stand-by mode, equalling or bettering any comparable device available. The nRF24xx devices have been designed to fit effortlessly into a close-proximity environment and with 128 (1Mbs each) channels programmable and a dynamic range even exceeding that of the sensitivity range (>95dBm @ 250kbs & >85dBm @ 1Mbs) they are extremely robust in a close proximity situation.

Device uniqueness with ShockBurst & DuoCeiver

The real beauty of this fabrication evolution is the opportunities it has opened up for implementation of added analogue and digital functionality 'on-chip'. A number of unique features have been incorporated into the available real estate that has become available on the silicon. Two radical core functionality developments have been made in the nRF24xx devices: ShockBurst & DuoCeiver™.

Fig. 1 Operation of ShockBurst & DuoCeiver

 ShockBurst incorporates an integrated FIFO register for temporary storage of data in the pre-transmission and post-reception stages. This FIFO is controlled by an SPI interface, thus enabling an MCU to clock data in/out at a speed that is appropriate to its capacity. ShockBurst is enhanced further with on-chip auto-address resolution and CRC implemented in hardware. This functionality has the great advantage of removing message verification from the MCU's list of duties, leaving the micro to remove data only when the nRF2401 informs it that an appropriate packet has arrived.

This is a key enabler in the integration of typical 2.4GHz band datarates (1Mbs) and ultra low-cost MCU's which may be operating at say, 20kbs. It allows the applications typically using less than 1Mbs to transmit at 1Mbs rates with packet bursts, thus drastically reducing current consumption and minimising time-on-air by factors of 100 or more. DuoCeiver allows a single receiver architecture to receive full data rates of up to 1Mbs simultaneously on each channel.  The DuoCeiver function operates without the need to use any multiplexing schemes, and enables full, simultaneous reception of 1Mbs datarates on dual channels. 

Fig. 2 Block diagram of the nRF2401 and required external components.

Broadening the application scope

The nRF2401 is currently causing a storm in the wireless keyboard & mouse arena with its feature set and performance. This is not such a surprise as during the conceptual stage this market was defined as a key target for the device's implementation. The ShockBurst and DuoCeiver features are a partial reflection of this, however, the embedded functions are finding interesting and novel applications in other areas also. The DuoCeiver and auto-address resolution ability can be used in applications that require the ability to individually locate and communicate from node to node, and also need to broadcast messages to all nodes in the network. This is very simply implemented on the nRF2401 with the assigning of a common address ID to the second channel. This is a key feature for the 'intelligent-home' type of application. Within this market many OEM's have grown tired of the wait for the final arrival of Zigbee, and other such standardised devices which can also be a straightjacket in development of products. The Zigbee standard is currently undergoing ratification and with master node protocol stacks of 64K or more is appearing too top-heavy for many application developers. In this style of application Bluetooth solutions are often regarded as too complex, expensive and current demanding thus putting them out of this current market and place their appeal more as a gateway to a proprietary network than as the network topology itself. The Nordic nRF2401 could operate from a 280mAh button-cell operating with a typical 1sec update of 256bits of data for longer than 5 yrs. Application areas as diverse as mobile intelligent sports kit, smart-home, tyre-pressure, automation positioning systems and MP3 systems are seeing project development with the nRF24xx due to it's low power requirements and exclusive feature set.

Global products in the 2.4GHz Environment

With the utilisation of the 433 & 868MHz ISM band having been around for some years now some developers are looking to the 2.4GHz band for new and existing application areas. The 2.4GHz band also has the added appeal of being globally available unlike ISM bands at 433/868/915MHz etc. This enables singular development costs as opposed to having several hardware platforms to maintain and reduces the logistical problems that arise when addressing differing geographical regulations for the other ISM bands.

 The 2.4GHz band has been seen as a 'local' communications medium with typical ranges of 15-30m indoors. The nRF2401 achieves >220m line-of-sight at 0dBm, and with the addition of a simple PA device up to 100mA is allowed by regulatory bodies with the implementation of a frequency-hopping scheme. The nRF2401 is fully capable of performing these tasks with channel switching times of only 200µS.

One of the major challenges seen by engineers in this band is the ability of their products to communicate in a band which is widely recognised as containing both W-LAN and Bluetooth also.

Fig. 3  2.4 GHz ISM Band and Co-existence

Nordic VLSI engineers have found that the interference that is created by these technologies is minimal and only in the most extreme situations is communication compromised. W-LAN whilst operating within the same band is a sophisticated technology utilising CSMA/CD for medium utilisation before transmissions. The vast majority of W-LAN also implements the Direct Sequence Spread Spectrum (DSSS) technique of modulation. This method of modulating the signal uses 'chipping' codes in the modulation which not only spreads the output power over a number of channels (typically 22) but also inherently resists narrowband interference, therefore the lower local channel power is beneficial to the nRF2401 and nRF2401 transmissions are attenuated by the W-LAN receiver.

The nRF24xx modulation and channel scheme is the same as those for Bluetooth  but tests have shown that given the width of the frequency band and the very short nature of transmission collisions have not been a major problem even in a heavily populated Bluetooth  environment (>8 devices around nRF24xx). 

Layout headaches are over

Historically RF design-in has left many developers fearful over stories of ultra critical attention to layout considerations, component criticality and effects of noise and interference from other parts of the system design. This has for a large part been true, for all RF designs. The level of on-chip integration has until now fallen short of areas such as VCO inductors, IF filtering and PLL filtering - all of which are extremely critical areas for any radio's operation. The placement of VCO and PLL components externally meant they were susceptible to noise, and inappropriate placement or choice of component could render a radio completely inoperable. The nRF24xx family has practically eliminated this issue with the on-chip integration of all inductors and filter networks. Indeed now the only external components required are a crystal reference and a reference resistor. This is a crucial step forward in RF design-in for general application developers who may not have extensive in-house RF design expertise. A full set of layout designs exists for the nRF24xx family including integrated printed circuit antennas. Wireless nRF24xx USB, wireless mouse and hands-free headsets are among a range of reference designs being released to developers using thenRF24xx to simplify the design task yet further.

Fig. 4  Small & Simple layout for nRF2401 device

Fig. 4  Wireless USB Keyboard & Mouse solution with nRF24xx

It is all about reducing the Bill of Material

As has been shown in the previous sections the innovative built in features of the nRF2401 transceiver and the nRF2402 transmitter has a great effect on the workload which the external microcontroller has to handle. Furthermore the two components have integrated all the filters, inductors and capacitors seen with normal wireless components -how can the integration be moved one step up from this? Clearly with 0.18u CMOS there is great advantages in including digital logic, as this can be done with little increase in size. The latest offering from Nordic VLSI, the nRF24E1 and nRF24E2, 2.4GHz transceiver and transmitter reflect this. The new, leading 2.4GHz components come with an embedded 8051 MCU, a 9-channel 12 bit ADC, Pulse Width Modulator (PWM), RC oscillator, digital I/O, timers, UART and other peripherals, providing the user with a complete System On a Chip running at 2.4GHz, delivered in a 6x6mm 36 pin QFN package.

Each component inside the nRF24E1/E2 is in itself leading with respect to performance and current consumption. The on chip 8051 MCU consumes 3mA @ 16MHz, and the ADC runs on as little as 0.9mA. The system as a whole can be put into a 2uA power down mode with wakeup on external pin or from the on chip wake up timer or by a watch dog reset.

Fig. 5  Block diagram of the nRF24E1 and required external components.

The nRF24xx family has already been incorporated in a number of new and existing wireless global applications such as sensor, remote control, mouse, game controllers, tire pressure sensors and intelligent sports equipment applications. The level of integration has seen the device being incorporated into a growing number of industrial designs also due to the very small footprint for a complete radio and it's EMI robustness. The low power level requirements, unique feature set coupled with the appeal of a global frequency band has made the nRF24xx devices a first in many applications. The nRF24Ex family has taken the level of integration yet further and provide a complete system on a device for the applications mentioned. The system cost and complexity is reduced significantly compared to any other technology or frequency available in the market today. An overview of currently available nRF24XX components is included below.

 

nRF2401

nRF2402

nRF24E1

nRF24E2

Type

Transceiver

Transmitter

Transceiver

Transmitter

Frequency range

2400-2524MHz

2400-2524MHz

2400-2524MHz

2400-2524MHz

Voltage range

1.9-3.6V

1.9-3.6V

1.9-3.6V

1.9-3.6V

Temp range

-40°C to +85°C

-40°C to +85°C

-40°C to +85°C

-40°C to +85°C

DuoCeiver™

Yes

No

Yes

No

ShockBurst™

Yes

Yes

Yes

Yes

Package

24 pin QFN 5x5mm

16 pin QFN 4x4mm

36 pin QFN 6x6mm

36 pin QFN 6x6mm

On chip voltage reg

Yes

Yes

Yes

Yes

8051 MCU

No

No

Yes

Yes

9 channel 100 ksps ADC

No

No

Yes

Yes

PWM

No

No

Yes

Yes

Prog Digital I/O

No

No

Yes

yes

Battery monitor

No

No

Yes

Yes

On chip RC Osc

No

No

Yes

Yes

On chip PWM

No

No

Yes

Yes

On chip timers

No

No

Yes

Yes

RRP 10K units US$

 3.5

2.5

4.25

3.25

Fig. 6  Feature Set Comparison Table for nRF24xx Products


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