Lemon Wireless
The Lemon Wireless is an nRF52840-based microcontroller for split keyboards with a USB-C port, power switch, JST battery connector, a VIK connector, and two FPC connectors for connecting column-flex PCBs. If you use column-flex PCBs and VIK-compatible peripherals, it’s possible to build a wireless split keyboard without soldering any wires!
If you haven’t already, I suggest you check out the Lemon Landing page to learn more.
What are Column Flex PCBs?
Column Flex PCBs affix hotswap sockets, diodes, RGB Leds, and copper traces to a flexible polyimide backing. All the wiring is already done for you, and the polyimide is flexible enough that you can shift every socket up to 20mm in any direction.
There are two kinds of column flex PCBs. The fullly interconnected one I’ve pictured above is the Pumpkin PCB, which I plan to sell soon. All sockets come connected to each other. You need to be very careful with it, because if you break a connection you will either need to bridge the gap with wire or buy a whole new PCB. TheBigSkree also sells connected-column ones, where every row in a column is connected, but you will need a second flexible PCB to wire the columns together.
What’s VIK?
VIK is a standard for how keyboard modules connect to one another. It combines SPI, I2C, power, and GPIO into a single connector. The idea is that the Cosmos benefits from VIK modules for other keyboards, and any VIK modules created specifically for Cosmos will benefit the wider keyboard community,
Pinout
Go to the landing page :)
Power
The Lemon Wireless can run on both USB power and a 4.2/3.7V Lithium Polymer (LiPo/LiPoly) or Lithium Ion (LiIon) rechargable battery. If you connect both at the same time, the microcontroller will draw from USB power while charging the battery.
Warning
Connecting a battery with incorrect JST connector polarity can permanently damage the board. There’s no standard for the polarity, so batteries ordered from places like Amazon may be wired incorrectly. You can fix a reversed connector by removing and rearranging the crimped leads in the JST housing. Then double check the wire coloring matches the silkscreen on the board (red goes to +, black goes to -).
To avoid this risk entirely, it’s safest to buy batteries from Adafruit. Their batteries are correctly wired and buying from them helps support their open-source contributions—such as the bootloader used by this board.
These are the only two kinds of batteries supported. Plugging in a higher voltage battery (e.g. a multicell one like 7.4V) or alkaline/nonrechargeable batteries will damage the board.
ZMK Example
The best example of using ZMK on the Lemon microcontroller will for now probably be the ZMK implementation of my peaMK software. Currently, all this program does is print the matrix position of a key you press, but you can disable this feature by editing boards/shields/peamk/peamk.keymap. There are a few noteworthy aspects to this program:
- The structure is the same as a ZMK userspace repository, so you can follow the ZMK install instructions then copy these files over to your GitHub repository.
- It uses Sadek Baroudi’s VIK module which makes configuring VIK modules very easy. For example, to add a Cirque trackpad you only need to add this to your
build.yml:--- include: - board: cosmos_lemon_wireless shield: <YOUR_KEYBOARD> vik_cirque_spi
Arduino Core
I haven’t contributed this board upstream yet since I don’t know how popular this option is going to be. Therefore, if you do program the Lemon with Arduino then please tell me so I can prioritize this!
Expand Arduino Instructions (Caution: Not Easy)
You will need to install Adafruit’s nRF52 core via git by following the instructions here, all the way down to and including Advanced Option: Manually Install the BSP via ‘git’. Then, open the cloned Adafruit_nRF52_Arduino folder and add these lines to the end of boards.txt:
# -----------------------------------
# Cosmos Lemon Wireless
# -----------------------------------
cosmos_lemon_wireless.name=Cosmos Lemon Wireless
cosmos_lemon_wireless.vid.0=0x1915
cosmos_lemon_wireless.pid.0=0x6c77
# Upload
cosmos_lemon_wireless.bootloader.tool=bootburn
cosmos_lemon_wireless.upload.tool=nrfutil
cosmos_lemon_wireless.upload.protocol=nrfutil
cosmos_lemon_wireless.upload.use_1200bps_touch=true
cosmos_lemon_wireless.upload.wait_for_upload_port=true
cosmos_lemon_wireless.upload.maximum_size=815104
cosmos_lemon_wireless.upload.maximum_data_size=237568
# Build
cosmos_lemon_wireless.build.mcu=cortex-m4
cosmos_lemon_wireless.build.f_cpu=64000000
cosmos_lemon_wireless.build.board=NRF52840_FEATHER
cosmos_lemon_wireless.build.core=nRF5
cosmos_lemon_wireless.build.variant=cosmos_lemon_wireless
cosmos_lemon_wireless.build.usb_manufacturer="Cosmos"
cosmos_lemon_wireless.build.usb_product="Lemon Wireless"
cosmos_lemon_wireless.build.extra_flags=-DNRF52840_XXAA {build.flags.usb}
cosmos_lemon_wireless.build.ldscript=nrf52840_s140_v6.ld
cosmos_lemon_wireless.build.openocdscript=scripts/openocd/daplink_nrf52.cfg
cosmos_lemon_wireless.build.vid=0x1915
cosmos_lemon_wireless.build.pid=0x6c77
# Menu: SoftDevice
cosmos_lemon_wireless.menu.softdevice.s140v6=S140 6.1.1
cosmos_lemon_wireless.menu.softdevice.s140v6.build.sd_name=s140
cosmos_lemon_wireless.menu.softdevice.s140v6.build.sd_version=6.1.1
cosmos_lemon_wireless.menu.softdevice.s140v6.build.sd_fwid=0x00B6
# Menu: Debug Level
cosmos_lemon_wireless.menu.debug.l0=Level 0 (Release)
cosmos_lemon_wireless.menu.debug.l0.build.debug_flags=-DCFG_DEBUG=0
cosmos_lemon_wireless.menu.debug.l1=Level 1 (Error Message)
cosmos_lemon_wireless.menu.debug.l1.build.debug_flags=-DCFG_DEBUG=1
cosmos_lemon_wireless.menu.debug.l2=Level 2 (Full Debug)
cosmos_lemon_wireless.menu.debug.l2.build.debug_flags=-DCFG_DEBUG=2
cosmos_lemon_wireless.menu.debug.l3=Level 3 (Segger SystemView)
cosmos_lemon_wireless.menu.debug.l3.build.debug_flags=-DCFG_DEBUG=3
cosmos_lemon_wireless.menu.debug.l3.build.sysview_flags=-DCFG_SYSVIEW=1
# Menu: Debug Port
cosmos_lemon_wireless.menu.debug_output.serial=Serial
cosmos_lemon_wireless.menu.debug_output.serial.build.logger_flags=-DCFG_LOGGER=0
cosmos_lemon_wireless.menu.debug_output.serial1=Serial1
cosmos_lemon_wireless.menu.debug_output.serial1.build.logger_flags=-DCFG_LOGGER=1 -DCFG_TUSB_DEBUG=CFG_DEBUG
cosmos_lemon_wireless.menu.debug_output.rtt=Segger RTT
cosmos_lemon_wireless.menu.debug_output.rtt.build.logger_flags=-DCFG_LOGGER=2 -DCFG_TUSB_DEBUG=CFG_DEBUG -DSEGGER_RTT_MODE_DEFAULT=SEGGER_RTT_MODE_BLOCK_IF_FIFO_FULL
There should be a variants folder in the same directory as boards.txt. Inside the variants folder, create a directory called cosmos_lemon_wireless. Then add these two files
#include "variant.h"
#include "wiring_constants.h"
#include "wiring_digital.h"
#include "nrf.h"
const uint32_t g_ADigitalPinMap[] =
{
(32+13), // D0
(32+15),
(2),
(29),
(31),
(26),
(4),
(6),
(8),
(32+9),
(12), // D10
(10),
(9),
(32+0), // D13
(24),
(22),
(20),
(18),
(15),
(13), // D19
(14), // Useless pin
(17), // useless pin
};
void initVariant() { }
#ifndef _VARIANT_LEMONWIRELESS_
#define _VARIANT_LEMONWIRELESS_
/** Master clock frequency */
#define VARIANT_MCK (64000000ul)
#define USE_LFXO // Board uses 32khz crystal for LF
// define USE_LFRC // Board uses RC for LF
/*----------------------------------------------------------------------------
* Headers
*----------------------------------------------------------------------------*/
#include "WVariant.h"
#ifdef __cplusplus
extern "C"
{
#endif // __cplusplus
// Number of pins defined in PinDescription array
#define PINS_COUNT (22)
#define NUM_DIGITAL_PINS (22)
#define NUM_ANALOG_INPUTS (6) // A6 is used for battery, A7 is analog reference
#define NUM_ANALOG_OUTPUTS (0)
#define LED_STATE_ON 1
// LEDs
/* #define PIN_LED1 (3) */
/* #define PIN_LED2 (4) */
/* #define PIN_NEOPIXEL_POWER (34) */
#define PIN_NEOPIXEL (13)
#define NEOPIXEL_NUM 1
#define PIN_LED1 (20)
#define PIN_LED2 (21)
#define LED_BUILTIN PIN_LED1
#define LED_CONN PIN_LED2
#define LED_RED PIN_LED1
#define LED_BLUE PIN_LED2
#define ADC_RESOLUTION 14
// Other pins
/*
* Serial interfaces
*/
#define PIN_SERIAL1_RX (14)
#define PIN_SERIAL1_TX (15)
/*
* SPI Interfaces
*/
#define SPI_INTERFACES_COUNT 1
#define PIN_SPI_MISO (7)
#define PIN_SPI_MOSI (10)
#define PIN_SPI_SCK (9)
static const uint8_t SS = (5);
static const uint8_t MOSI = PIN_SPI_MOSI ;
static const uint8_t MISO = PIN_SPI_MISO ;
static const uint8_t SCK = PIN_SPI_SCK ;
/*
* Wire Interfaces
*/
#define WIRE_INTERFACES_COUNT 1
#define PIN_WIRE_SDA (22)
#define PIN_WIRE_SCL (23)
// QSPI Pins
#define PIN_QSPI_SCK 27
#define PIN_QSPI_CS 28
#define PIN_QSPI_IO0 29
#define PIN_QSPI_IO1 30
#define PIN_QSPI_IO2 31
#define PIN_QSPI_IO3 32
// On-board QSPI Flash
#define EXTERNAL_FLASH_DEVICES GD25Q16C
#define EXTERNAL_FLASH_USE_QSPI
#ifdef __cplusplus
}
#endif
/*----------------------------------------------------------------------------
* Arduino objects - C++ only
*----------------------------------------------------------------------------*/
#endif
After modifying these files, restart Arduino IDE. You should now see Cosmos Lemon Wireless listed as the last option when choosing a board through Tools -> Board -> Adafruit nRF52 Boards -> Cosmos Lemon Wireless.
Further Documentation
Documentation for older boards, schematics and layout, and more can be found on the Wireless Lemon’s GitHub page.