Single chip.4ghz wireless mouse using nrf4ex *((5$/ With the nrf4e and nrf4e in this document called nrf4ex from Nordic VLSI ASA is it now possible to design a button axis wireless mouse for the.4ghz ISM band as a single chip solution. The nrf4ex series has an embedded 805 microcontroller a 9 channel ADC and the same RF part as the popular nrf40/nrf40. The nrf4ex series can run at voltages from.9v to.6v making it especially suited for battery applications. This white paper will show how a button axis wireless mouse can be realized by using the nrf4ex. 7+('(6* X-Optics Y-Optics Z-Optics Left button Wheel button Right button nrf4ex Figure nrf4ex in a wireless button axis mouse design 7KH5)SDUW As shown in the schematics Figure the RF part is based on the nrf4ex reference design that can be downloaded from www.nvlsi.no. The design has a 6MHz crystal and an external EPROM for firmware storage. The firmware will use the ShockBurst technology to transmit the packets from the mouse. The ShockBurst technology is designed to minimize the current consumption per transmitted bit in order to extend the battery lifetime. Read more about the ShockBurst technology in the nrf4ex datasheet and in a white paper that can be downloaded from www.nvlsi.no. :LUHOHVVPRXVHEDVLFV A wireless mouse has the same basic functionality as a standard mouse. The only difference is that the wireless mouse provides its data to the host PC by air instead of using a cable. This means that the way of collecting the movement and button information is the same for a wireless mouse as for an old fashion wired mouse. This white paper uses a ball mouse as an example. This mouse has a switch for each button and a ball and rollers to measure movement. The rollers are rolling pinwheels that
trough optics will give a significant pattern for each movement. Since a wireless mouse uses battery power it should be put a lot of concern in saving as much power as possible. Updates should therefore be transmitted as rare as possible to the PC (60-00 times/second when being moved/clicked.) :LUHOHVVPRXVHRSWLFV Mouse optics is connected so that the IR-LED shines trough a pinwheel onto two phototransistors as shown in Figure. The pinwheel turns on the mouse ball rollers. The light that goes trough the pinwheel hits two phototransistors connected in a source follower configuration. Light from the IR-LED will cause the phototransistor to be turned on. The pinwheel is designed to block the infrared light such that the phototransistors are turned on and off in a quadrate output pattern. Each change in the output pattern represents a count of mouse movement. Comparing the current and the last state of the optics gives information about the direction. P N P N Output T Output T Figure Mouse optics the view from the IR LED :LUHOHVVPRXVHEXWWRQV A mouse button is a standard switch and every switch is connected directly to a GPIO pin of the nrf4ex device. The button GPIOs are configured as inputs and are pulled up by external pull-up resistors. The switches should be debounced in firmware for 5-5ms. This white paper design has three buttons: left middle and right. 6FKHPDWLFV Figure shows how the optics and the buttons can be connected to the standard nrf4ex layout with an EPROM in order to make the.4ghz wireless mouse.
ž Š œ Ž Žœœ žœž ÆÆ PT D LED X-Axis R0 RES PT R9 X-U X-U X-D X-D D LED Y-Axis R RES D LED Z-Axis R RES PT PT4 PT5 PT6 R8 R7 R6 Z-U R5 Z-D R4 Y-U Y-D Y-U Y-D Z-U Z-D X-U X-D Y-U Y-D Z-U LED Driver Z-D Left Button Middle Button Right Button R4 R5 0K 0K U CS SO WP 4 VSS 5XX0 R6 0K VCC HOLD SCK SI 8 7 6 5 C5 nf C4 0nF C6 0nF 4 5 6 7 8 9 AIN0 D DIO0 DIO DIO DIO DIO4 DIO5 6 DIN0 5 AIN 4 AIN TEST VSS 0 AIN 9 AIN4 8 AREF IREF nrf4e AIN5 AIN6 AIN7 VSS VSS_PA Error : FUGL.bmp file not found. ANT ANT DIO6 DIO7 DIO8 DIO9 D VSS XC XC _PA 0 4 5 6 7 8 U nrf4e X C9 C0 nf nf 6 MHz R M R k 7 6 5 4 0 9 C7 0nF C.nF L.6nH L nh C8 nf C4 pf C.0pF C.5pF C.0pF Ant PCB Antenna C 5pF C 5pF MT LED Driver LED Driver S Left S Middle S Right Left Button Middle Button Right Button Left Button Middle Button Right Button Figure Schematics of the button axis.4ghz wireless mouse
%$77(5</)(70( This chapter contains calculations of current consumption and an example on how the power saving protocol can be implemented. RZHUVDYLQJ The most current consuming device inside a wireless mouse is not the RF part of the design but the IR diodes used in the optics. It is therefore important to have the LEDs light for as short time as possible and to keep them off for as long time as possible and still get the functionality we want from the mouse. As a way to achieve this we define three different states for how we pulse the LEDs. State ; this is the state where the mouse is on the move and we need the most accurate measurements. In this state the t ledon 0us and t ledoff 00us. Movements done in this state are accumulated and transmitted to the PC each 0 th millisecond. State ; this is the state where the mouse has been recently used but is laying still now. In this state the t ledon 0us and t ledoff 5000us. The user will not be aware of the short delay when the mouse is being moved again. When being in state it should take no more than 5 ms of no activity before going to state. Going from state to state should happen at once when a movement is detected. State ; this is the state the mouse will turn to when it has not been used for a while. The t ledon 0us and the t ledoff 00000us. Being in this mode means that the user has not used the mouse for a long time and will not beware the delay from the mouse is moved to the cursor moves. Once a movement has been detected the mouse should go into state immediately. From being in state to entering state it should take - minutes. &DOFXODWLQJWKHFXUUHQWFRQVXPSWLRQ Each IR LED uses 0mA when lit. The nrf4ex uses ma when active ua when in power down and an additional 05mA when transmitting a ShockBurst packet. Let us assume that the nrf4ex is in standby when the IR LEDs are off except when the nrf4ex shall transmit a ShockBurst packet. Then the average current consumption for each state can be calculated by Equation. ( RQ WRQ RII WRII ) ( W + W ) Equation Average current consumption RQ RII
For state and state the average current consumption can be found directly: (( OHGRQ 4([RQ ) WRQ 4 ( W + W ) W ) (000X$ 0XV + X$ 5000XV) 500XV ([SZUGQ RII 6WDWH 5 RQ RII Equation Average current consumption in state X$ (( 4 6WDWH 5 OHGRQ 4([RQ ) WRQ ([SZUGQ WRII ) (000X$ 0XV + X$ 00000XV) ( WRQ + WRII ) 0000XV Equation Average current consumption in state For state we also have to add the current consumption for the ShockBurst transmissions. As agreed earlier a transmission has to be done every 0 th millisecond when the mouse is being moved. Let us assume that a mouse packet contains 4 bits. This means that the nrf40 will use 4us to transmit this packet at Mbit/s. In addition there is a start up time of 0us. This means that a ShockBurst transmission will cost 05mA in 6us in addition to the ma used by the microcontroller part. Putting this into the equation gives: X$ (( OHGRQ ) WOHGRQ ) 5)RQ W 5)RQ + ( W + W ) W + W 6WDWH 4([RQ OHGRQ OHGRII 5)RQ 5)RII 0P$ 0XV 05P$ 6XV + + P$ 4770X$ 0XV 0PV Equation 4 Average current consumption in state
%DWWHU\OLIHWLPHH[DPSOH Calculating the expected battery lifetime requires statistical material of how a typical user uses the mouse. This example is not based on a statistical model and is only intended as an example on how to calculate the battery lifetime. Let us assume that the battery capacity is 000mAh and that a typical user will set the mouse in state for about 0min/day in state for about 0min/day and in state for the rest of the day. This will give an overall current consumption of: W W W 6WDWH 6WDWH 6WDWH 6WDWH 6WDWH 6WDWH RYHUDOO 4K 4770X$ 0min+ 5X$ 0 min+ 5X$ 00 min 7X$ 440 min Equation 5 Overall average current consumption The battery lifetime is given by: (%DWWHU\ 000P$K WEDWWHU\OLIH 700K. 5\HDUV 7X$ Equation 6 Calculating the battery lifetime Using a 4 MHz crystal instead of a 6MHz crystal will reduce the current consumption of the nrf4ex when operating in active mode from ma to 6mA. Even if this causes the speed on the RF link to go down from Mbit/s to 50Kbit/s it will cause a gain in battery lifetime. Doing the same calculations with these numbers gives: W W W 6WDWH 6WDWH 6WDWH 6WDWH 6WDWH 6WDWH RYHUDOO 4K 76X$ 0min+ 46X$ 0 min+ 5X$ 00 min 440 min 58X$ Equation 7 Overall average current consumption with a 4MHz crystal (%DWWHU\ 000P$K WEDWWHU\OLIH 740K \HDUV 58X$ Equation 8 Battery lifetime with a 4MHz crystal
/$%/7<'6&/$0(5 Nordic VLSI ASA reserves the right to make changes without further notice to the product to improve reliability function or design. Nordic VLSI does not assume any liability arising out of the application or use of any product or circuits described herein. /)(6857$/&$76 These products are not designed for use in life support appliances devices or systems where malfunction of these products can reasonably be expected to result in personal injury. Nordic VLSI ASA customers using or selling these products for use in such applications do so at their own risk and agree to fully indemnify Nordic VLSI ASA for any damages resulting from such improper use or sale. White paper. Revision Date : 7.05.00. All rights reserved. Reproduction in whole or in part is prohibited without the prior written permission of the copyright holder.
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RUGLF9/6:RUOG:LGH'LVWULEXWRU )RU<RXUQHDUHVWGHDOHUSOHDVHVHHKWWSZZZQYOVLQR 0DLQIILFH Vestre Rosten 8 N-7075 Tiller Norway Phone: +47 7 89 89 00 Fax: +47 7 89 89 89 9LVLWWKHRUGLF9/6$6$ZHEVLWHDWKWWSZZZQYOVLQR