The document describes the design of a small Bluetooth Low Energy button that connects to smartphones. Key aspects include using a CC2541 Bluetooth chip, CR1632 battery, and small surface mount components. Firmware was developed on the CC2541 using IAR Workbench. An iOS app was created using Xcode to test and characterize the button's performance. Power consumption tests showed the effective current was over 1mA in data transmission mode. The button can achieve a clicking frequency close to that of a touchscreen.

1. bodong.li@kaust.edu.sa
Product Description
A physical button that connects to
smart phones to enable functions on
the phone side.
Design Focus
Small
Fast response
Long battery life
The Button

2. Component Selection
Components Parts Remarks
Bluetooth LE TI CC2541 Cost-effective, MCU embedded
Battery CR1632 Small, ø16×3.2mm
3V, 140mAh (Marked power)
Buck convertor TI TPS62730 Enable BLE low power mode
Tactile Switch SKQYAAE010 SMD, Compact, High-speed
Antenna Puls W3008 Ceramic SMD, 1.7 dBi gain
LED LED RED 0402 Small forward voltage (1.8 V)
Passive components SMD 0402 Save PCB space

3. Design Breakdown
Power supply
Buck convertor
Power
management
3~2.2 V
Button
Sensing and Display
CC2541
Bluetooth
MCU (8051)
LED
2.1 V
Antenna
+
Matching
Network
Communication
GPIO write
TX
RX
GPIO read

4. Schematic

5. PCB
Layout
Top
Bottom
GND
PWR
4 layer PCB
Top side:
•CC2541
•TPS62730
•Switch
•Antenna
•Crystal
•LED
•Passive SMD
Bottom side:
•Battery
•Debug port
*Preliminary design,
refinement required

6. Mechanical Concept Design
20 mm
8mm

7. Peripheral
Smart phoneButton
Central device
scan
Establish connection
turn on key
press notification
advertise
read P0_0 ?
1
0
send notification
characteristic
change
N
Y
create event
GATT clientGATT server
Flash LED (P1_0)
System Flow Chart

8. Firmware Development
• The firmware of the button function is programed in the 8051 MCU of
CC2540, which is included in the mini Keyfob module (shown below) from TI
CC2540 mini development kit.
• Programming environment: IAR Embedded Workbench for 8051.
• The right Key and LED of the mini Keyfob module are enabled.
• The “SimpleBLEPeripheral” project provided by TI is used as a reference.

9. Code Implementation
2. Declare HandleKeys function
1. Enable “simplekeys” service and register Key event to be
processed in “SimpleBLEPeripheral_ProcessEvent” task
function
simpleBLEPeripheral.c
simpleBLEPeripheral.c
SimpleBLEPeripheral_Init
3. Process Key event in “simpleBLEPeripheral_ProcessOSALMsg”
simpleBLEPeripheral.c
S..Peripheral_ProcessEvent
S..Peripheral_ProcessOSALMsg
4. Response with enable/disable advertising1
by updating GAP
and send notification2
through GATT
simpleBLEPeripheral.c
S..Peripheral_ProcessEvent
S..Peripheral_ProcessOSALMsg
S..Peripheral_HandleKeys
Code Location
…
1
2

10. Software Development
• The debugging and testing is performed on an Iphone 5.
• Programming environment: Xcode 5.
• An App is developed with button connection, button test, and flappy bird
game functions.
• Bluetooth communication is handled by BLE Controller object, when BLE
notification is sent to the Iphone, the updated key information is passed to
BLE Connection View Controller through delegation.
BLE Connection View Flappy Bird View
• BLE Connection View includes BLE
initialization view and button test
view.
• Source Code of CC2540 Keyfob
IOS demo and Spritybird app are
used as references.

11. App Flow AppDelegate
object
AppDelegate
object
BLE Connection
View Controller
object
BLE Connection
View Controller
object
Flappy Bird
View Controller
object
Flappy Bird
View Controller
object
BLE Controller
object
BLE Controller
object
Flappy Bird
View Controller
View
Flappy Bird
View Controller
View
BLE Connection
View Controller
View
BLE Connection
View Controller
View
UIButtonUIButton
Scene
object
Scene
object
BirdNodeBirdNode
SKSprite
Node
SKSprite
Node
UILabelUILabel
UISwitchUISwitch
UIImageUIImage

12. Button Connection Interface
BLE Button
Start scanning
Key status
indicator
Connection status
indicator
Button Connection Interface handles the Bluetooth scanning and connection
status indication.

13. Button Test Interface
Real time updated data
Minimum time interval
between clicks/touches
Maximum frequency
(mean of 5)
Screen touch dataBLE button click data
Button Test Interface implements a function to get the button clicking time
interval for studying the usability of the BLE button as well as providing data for
power consumption test.

14. Button Characterization
• The power consumption of the BLE button is
characterized using a 34410A high precision
multimeter and an external power supply.
• The firmware is modified to control one key
and one LED only. As there are other
peripheral devices connected on the Keyfob
such as buzzer, accelerometer which
introduce leakage current to the module, the
measured power consumption only reflects
the relative power performance of the BLE key
press application.
• In the following test, the multimeter is
configured with 0.6 ms sample interval time,
50000 current data of the button performing
different tasks within 30 seconds is recorded
and analyzed.

15. Test Result Connection interval: 100 ms
Slave latency: 0, RF power: 0 dB

16. Clicking Frequency Comparison
Calculated
Ieffective
1.17 mA
1.30 mA
2
The button can reach a clicking frequency of 7 Hz, close to 9 Hz
maximum frequency of the screen touching.

17. Conclusion
• For simple BLE applications, using embedded MCU reduces circuit complexity.
In this case, battery size becomes the major constrain for the further
miniaturization of the device. 16 mm diameter CR1632 could be an option
when the power consumption is properly managed.
• For more complicated system on button cell sized board, 6/8 layers PCB
design needs to be considered.
• By using the plastic button, ceramic chip antenna can be placed on the top
side of the board, which saves the overall board space.
• CC2541 Low power mode saves additional 20% energy.
• Red LED using as a indicator, powered directly by the MCU, avoiding
additional transistors.
• In data transmission mode, the effective current consumption of the BLE with
button function is above 1 mA.
• The BLE button can be used for action intense applications.

18. References
CC2541-TPS62730EM Reference Design
CC2541 Programming Guide
CC2541 Mini Development Kit Sample Code
https://github.com/kirualex/SprityBird (Sprity Bird Source Code)
http://processors.wiki.ti.com/index.php/Category:KeyFobDemo (Keyfob Source Code)
Software used
Circuit and PCB design: Diptrace Freeware Edition
Firmware development: IAR Embedded Workbench
Software development: Xcode 5.1.1
Mechanical concept design: SketchUp
http://youtu.be/aoQFClP3_eY
A Demo can be found in the link below.

19. Time Line*
CC2540 received
Design
completed
July 14
July 16
July 27
July 9
Order placed for
CC2540 MINI DK
Firmware
completed
July 19
Apple developer
license received
July 23
Software
completed
Characterization
&Demo
July 30
* Completed during spare time

20. Thank you!