PCF8574 I2C LCD backpack: specs and pinout
A small board that turns a parallel HD44780-style character LCD into an I2C device by using a PCF8574 8-bit I2C I/O expander. The values below describe the PCF8574 chip; the backpack's own wiring is not covered by an official document.
Specs
| Spec | Value | Source |
|---|---|---|
| PCF8574 supply voltage (VCC) | 2.5 V to 6 V | source 5.3 Recommended Operating Conditions (p.4) |
| PCF8574 high-level input voltage (VIH) | 0.7 x VCC min | source 5.3 Recommended Operating Conditions (p.4) |
| PCF8574 I2C address | 0100 A2 A1 A0 (0x20 to 0x27); the PCF8574A is a different part with its own address range, so check the chip marking on your backpack | source Section 7.3.2 Interface Definition (p.13); Section 7.3.3 Address Reference (p.14); Table 4-1 address pins (p.3) |
| PCF8574 I2C clock frequency | 100 kHz max | source 5.6 I2C Interface Timing Requirements (p.5) |
| PCF8574 I/O port | 8-bit quasi-bidirectional (P0-P7); IOL 25 mA max, IOH -1 mA max | source Description (p.1); 5.3 (p.4) |
| PCF8574 standby current | 10 uA max | source Features (p.1) |
| PCF8574 operating temperature | -40 to 85 C | source 5.3 Recommended Operating Conditions (p.4) |
Power
| Value | Source | |
|---|---|---|
| Supply | Chip: 2.5-6 V VCC (module supply not documented) | source Features (p.1); 5.3 (p.4) |
| Current | Chip standby 10 uA max; module current not documented | source Features (p.1); 5.3 (p.4) |
Chip pins
These are the pins of the chip on the module, from the chip datasheet. The module's own header may label or expose them differently.
Only the four chip signals that any I2C module brings out are listed, under the chip's own names. The module board (regulator, pull-ups, address pads, LCD pin mapping, which of PCF8574 or PCF8574A is fitted) is not covered by an official document.
| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| SDA | I2C serial data (chip pin name) | VIH 0.7 x VCC | Chip signal; module header order is not documented | source Table 4-1 Pin Functions (p.3) |
| SCL | I2C serial clock (chip pin name) | VIH 0.7 x VCC | Chip signal; module header order is not documented | source Table 4-1 Pin Functions (p.3) |
| VCC | Supply (chip pin name) | 2.5-6 V | Module header label not documented | source Table 4-1 Pin Functions (p.3); 5.3 (p.4) |
| GND | Ground (chip pin name) | 0 V | Module header label not documented | source Table 4-1 Pin Functions (p.3) |
Build with it
Project ideas for the PCF8574 I2C LCD backpack. Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.
- Weather readout: The Uno runs the backpack at 5 V logic. The backpack is soldered to the header of the 16x2 character LCD.
- Light level readout: A voltage divider of the LDR and a resistor feeds the Nano analog input; the backpack saves display pins. Learning project.
- Clock display: The clock and the backpack share the same two I2C wires on a 5 V board; check that their two addresses differ.
Common mistakes
- Driving a 5 V-powered backpack from a 3.3 V GPIO: at VCC = 5 V the PCF8574 needs VIH of at least 0.7 x VCC (3.5 V), above a 3.3 V high; module pull-ups are not documented, so use a 5 V board or confirm the module's behaviour (source: link, 5.3 p.4)
- Using the wrong I2C address: the PCF8574 address is 0100 A2 A1 A0, i.e. 0x20 to 0x27; the PCF8574A is a different part with its own address range, so check the chip marking on your backpack (source: link, section 7.3.2 p.13 and section 7.3.3 p.14)
- Running I2C faster than 100 kHz: that is the maximum clock the chip sheet lists (source: link, 5.6 p.5)
Not covered by the official source
- Chip facts only: the module is not documented, and the HD44780 LCD pin mapping on the backpack is unconfirmed.
- The PCF8574A is a different part with its own address range that is not covered by the TI PCF8574 sheet.
Sources
- link (TI PCF8574 datasheet SCPS068K, revised Sept 2024; second-source maker)