CSE7766 — Single-Phase Energy Monitoring IC with UART

Overview

The CSE7766 is a single-phase electricity metering IC manufactured by China Silergy (CSE). Embedded inside popular commercial Wi-Fi smart plugs—most notably the Sonoff S31 and Sonoff POW R2—it measures RMS AC voltage, RMS AC current, active power (W), and cumulative energy consumption.

Selected by manufacturers as the successor to pulse-counting ICs like the HLW8012, the CSE7766 offloads pulse-timing calculations from the microcontroller. It streams pre-calculated 24-byte telemetry data frames continuously over a 4,800 baud TTL UART interface. It is natively supported in Tasmota, ESPHome, and Home Assistant.

Quick reference


Operating voltage (`VDD`)	4.5 V to 5.5 V DC (5.0 V nominal)
Interface	TTL UART (4,800 bps, 8-E-1: 8 data bits, Even parity, 1 stop bit)
Baud rate	Fixed 4,800 bps
Measurement parameters	$V_{RMS}$ , $I_{RMS}$ , Active Power ( $P$ ), Energy Accumulator ( $E$ )
Telemetry frame frequency	Automatically streams 1 frame every ~1.0 second
Measurement accuracy	Class 0.5 ( $\pm 0.5\%$ )
Operating current	$3.5\text{ mA}$ typical

Pinout (SOP-8 Package)

text

             ┌───┴───┐
          VDD ─┤ 1    8├─ GND
          V1P ─┤ 2    7├─ NC
          V1N ─┤ 3    6├─ RX
           V2P ─┤ 4    5├─ TX
             └───────┘

Pin	Name	Type	Description
1	`VDD`	Power	Core supply (+4.5 V to +5.5 V DC)
2	`V1P`	Analog Input	Positive current sense input (from $1\ \text{m}\Omega$ shunt)
3	`V1N`	Analog Input	Negative current sense input
4	`V2P`	Analog Input	Voltage sense input (from resistor divider; $V_{2N}$ connected to GND)
5	`TX`	Digital Output	UART Transmit output pin (4,800 bps)
6	`RX`	Digital Input	UART Receive input pin
8	`GND`	Power	Ground reference (0 V)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Supply Voltage	$V_{DD}$	4.5	5.0	5.5	V	DC
Supply Current	$I_{DD}$	—	3.5	5.0	mA	Active operation
Current Channel Input Range	$V_{V1P-V1N}$	-43.7	—	+43.7	mV Peak	Differential shunt voltage
Voltage Channel Input Range	$V_{V2P}$	-43.7	—	+43.7	mV Peak	Single-ended voltage
Measurement Accuracy	$E_{meter}$	-0.5%	$\pm 0.1\%$	+0.5%	—	Active power range 1000:1
Internal Voltage Ref	$V_{REF}$	1.20	1.25	1.30	V	Low temperature coefficient

Telemetry Frame Structure

The CSE7766 streams 24-byte binary frames every second over 4,800 baud serial (8-E-1):

Header Bytes (0–1): 0x55 0x5A (or 0x5A 0x5A).
Voltage Coefficients & Data (Bytes 2–7): 24-bit Voltage coefficient ( $K_V$ ) and 24-bit Voltage cycle count ( $T_V$ ).
Current Coefficients & Data (Bytes 8–13): 24-bit Current coefficient ( $K_I$ ) and 24-bit Current cycle count ( $T_I$ ).
Power Coefficients & Data (Bytes 14–19): 24-bit Power coefficient ( $K_P$ ) and 24-bit Power cycle count ( $T_P$ ).
Status & Pulse Count (Bytes 20–22): Error flags and cumulative energy pulse counter.
Checksum Byte (23): 8-bit sum of bytes 2 through 22.

$V_{RMS}\text{ (V)} = \frac{K_V}{T_V} \quad \text{Volts}$

$I_{RMS}\text{ (A)} = \frac{K_I}{T_I} \quad \text{Amperes}$

$P\text{ (W)} = \frac{K_P}{T_P} \quad \text{Watts}$

Wiring (Sonoff S31 / POW R2 Internal PCB)

CSE7766 Pin	ESP8266 / ESP32 GPIO	Notes
`VDD`	3.3V / 5V	Power rail
`GND`	GND	Common ground (tied to Neutral AC mains)
`TX`	MCU RX Pin (GPIO 3 or 13)	4800 baud 8-E-1
`RX`	MCU TX Pin (GPIO 1)	Command input

Warning

High Voltage Safety Warning:

The CSE7766 GND pin is connected directly to AC Neutral. Connecting a serial programmer to the UART pins while the smart plug is plugged into mains power will blow computer USB ports and cause electric shock. Program/debug smart plugs ONLY while unplugged from 120V/230V AC wall outlets.

Example (ESPHome Configuration)

yaml

uart:
  rx_pin: GPIO13
  baud_rate: 4800
  parity: EVEN

sensor:
  - platform: cse7766
    voltage:
      name: "Sonoff S31 Voltage"
    current:
      name: "Sonoff S31 Current"
    power:
      name: "Sonoff S31 Power"
    energy:
      name: "Sonoff S31 Energy"

Common mistakes

Setting incorrect serial parity: The CSE7766 uses Even parity (8-E-1), not standard No-parity (8-N-1). Setting NONE parity results in frame checksum validation failures.
Using 9600 or 115200 baud: The CSE7766 UART clock is fixed at 4800 baud.

Notes

CSE7766 vs HLW8012: CSE7766 sends pre-calculated telemetry over UART (8-E-1); HLW8012 outputs variable frequency pulse trains (CF/CF1).

Interface	Optocoupled TTL UART (4800 bps)
Supply	4.5–5.5 V
Package	8-pin SOP / Smart Plug Internal PCB