• $2.65 CC2540 module (RF-BM-S02) from Aliexpress (11.2 x 15.1 x 1.74 mm, onboard USB support)
• $1.83 Arduino Nano from Aliexpress (to program the chip)
• $0.92 Panasonic CR1216 lithium 3v battery (12.5 × 1.6 mm), 34 mAh
• $0.00 Cherry MX Blue keychain (20 x 11 mm) from GeekKeys (actually $3.50 but I got it for free)

• IAR Embedded Workbench for 8051: http://iar.com (8.30.2)
• CC2540/CC2541 SDK: http://www.ti.com/tool/BLE-STACK-ARCHIVE (BLE-STACK V1.4.0)
• CCLoader: https://github.com/RedBearLab/CCLoader (CCLoader.exe, CCLoader.ino)
• Hex to bin converter: http://hex2bin.sourceforge.net (hex2bin.exe)

Build or download precompiled firmware from the repository (HIDEmuKbd.hex), use CCLoader and Arduino Nano to flash it.

Build and upload CCLoader sketch (CCLoader.ino) to Arduino Nano using Arduino IDE.

Hook up CC2540 to Arduino Nano digital pins D4-D6 as shown in the table (don't forget about 3.3V VCC and GND):

Convert firmware.hex to firmware.bin using hex to bin converter.

hex2bin.exe firmware.hex

Run CCLoader client on PC. E.g. for COM6 and Arduino Nano (Device 0) use:

CCLoader.exe 6 firmware.bin 0

That's it, the cc2540 module should accept the new firmware and start working right away (after bluetooth pairing).

The keyboard button is attached to P0_1 (P01) and GND pins. It sends WinKey keycode, you may modify the code to send any key or a key sequence of your choice. The battery (3V) is attached to VCC and GND pins.

This keyboard uses modified HIDEmuKbd sample from the CC2540 SDK.

Copy provided files to the corresponding BLE-CC254x-1.4.0 SDK folders.

Run IAR, open .eww, hit Make. You may also try precompiled firmware from the repository.

• Error[e16]: Segment ISTACK (size: 0xc0 align: 0) is too long for segment definition. At least 0xe more bytes needed. The problem occurred while processing the segment

You got to set number of virtual registers to 8 (in Project - Options - General Options).

This section is pure experimental and it's not related to the keyboard above, you may skip it.

As you know, CC2540 supports USB (CC2541 doesn't), and can be used as USB-HID, USB Serial or USB Mass Storage Device.

• HIDAdvRemoteDongle sample for the USB HID device.
• HostTestApp sample for the BTool-compatible serial device.
• util/UBL sample for the USB Mass Storage device

To run USB firmware and to detect USB on this module, U+ needs to be pulled to +3.3v via 1.5k resistor.

Sadly, USB Mass Storage doesn't work (yet) on this RF-BM-S02 module. It sometimes shows removable drive and even shows DEFAULT.CFG file but never opens it. Not sure if it is software or hardware problem (looks like it opens slightly faster on a low speed USB 2.0). The module uses CC254XF256 so memory should be enough. Probably software problem, because USB HID works just fine.

You don't really need official CC2540 SDK hardware (Keyfob and USB Dongle) but you could need schematics and defines.

CC2540 buttons and LEDs for Keyfob and Dongle development kits (defined in hal_board_cfg.h):

Note that P0_0 pin is not available on RF-BM-S02

See the full album here: https://imgur.com/a/OkwEb

• https://hackaday.com/2017/05/20/the-tiniest-mechanical-keyboard-ever/
• https://github.com/rampadc/cc254x-hidKbdM (experimental HID firmware for CC254x)
• https://imgur.com/a/KWmz6 (Turning HM-10, HM-11 into Bluetooth HID modules)