Keychron K4 Reverse Engineering

This is partial and may or may not be completed. Use this information at your own discretion and risk.

This is for K4 RGB v1 and v2 ANSI.

TODO List - QMK support

• Ability to safely flash bootloader
• Ability to revert to factory firmware
• Confirm Chibios functionality
• Get QMK Working
• Get LEDs working
• Confirm bluetooth functionality

Chips

• Main MCU - HFD2201KBA - appears to be based on/clone of SONIX SN32F24xB

• Bt module CYPRESS CYW20730A2KFBG

• Charge IC - TPU5000

Key Matrix & MCU

Keyboard Layout

MCU-Diagram - Keyboard matrix diagram on the MCU

---	col	C0	C1	C2	C3	C4	C5	C6	C7	C8	C9	C10	C11	C12	C13	C14	C15	C16	C17	C18
row	pin	25	26	27	28	29	30	31	32	34	35	36	37	38	39	40	41	42	43	44
R0	64
R1	63
R2	62
R3	61
R4	60
R5	59

MCU-Diagram - LED matrix

g	b	r	---	---	---	col	C0	C1	C2	C3	C4	C5	C6	C7	C8	C9	C10	C11	C12	C13	C14	C15	C16	C17	C18
ch1	ch2	ch3	pin	pin	pin	pin	25	26	27	28	29	30	31	32	34	35	36	37	38	39	40	41	42	43	44
Q13	Q7	Q1	01	02	04	---	--	--	--	--	--	--	--	--	--	--	---	---	---	---	---	---	---	---	---
Q14	Q8	Q2	05	06	07	---	--	--	--	--	--	--	--	--	--	--	---	---	---	---	---	---	---	---	---
Q15	Q9	Q3	08	09	10	---	--	--	--	--	--	--	--	--	--	--	---	---	---	---	---	---	---	---	---
Q16	Q10	Q4	11	12	13	---	--	--	--	--	--	--	--	--	--	--	---	---	---	---	---	---	---	---	---
Q17	Q11	Q5	14	15	47	---	--	--	--	--	--	--	--	--	--	--	---	---	---	---	---	---	---	---	---
Q18	Q12	Q6	50	49	48	---	--	--	--	--	--	--	--	--	--	--	---	---	---	---	---	---	---	---	---

MCU-Diagram - mac/win and bt/off/cable dip switches

• Bluetooth / O / Cable Mode: pin 57
• Win - Android / Mac - iOS Mode: pin 58

MCU-Diagram - Status LED indicators - K4 v2 only

• Caps Lock: pin 46
• Num Lock: pin 51

MCU Pinout - SN32F248BF

Bluetooth module

seems to be wired like the Blitzwolf BW-KB1(https://github.com/IslamAlam/blitzwolf-bw-kb-1)