# 20x4 I2C LCD (PCF8574): specs and pinout

> I2C backpack with a PCF8574 expander that drives an HD44780-compatible 20x4 character LCD over a 4-bit parallel bus.

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I2C backpack with a PCF8574 expander that drives an HD44780-compatible 20x4 character LCD over a 4-bit parallel bus.

Every value cites its sourceChip-level pinout

[Buy on Compoden](https://compoden.com/products/20x4-i2c-character-lcd-display-module-lcd2004-pcf8574-backpack-pre-soldered)

## Specs

| Spec | Value | Source |
|---|---|---|
| PCF8574 supply VCC | 2.5 V to 6 V | source Features (p.1) |
| PCF8574 absolute maximum VCC | -0.5 V to 6.5 V | source Section 5.1 Absolute Maximum Ratings (p.4) |
| PCF8574 standby current | 10 uA max | source Features (p.1) |
| PCF8574 operating supply current ICC | 40 uA (operating mode, fSCL = 100 kHz, VCC 2.5-6 V; listed value) | source Electrical Characteristics (p.5) |
| PCF8574 I2C clock fSCL | 100 kHz | source Section 5.6 I2C timing (p.5) |
| PCF8574 input high VIH | 0.7 x VCC | source Electrical Characteristics (p.4) |
| PCF8574 input low VIL | 0.3 x VCC | source Electrical Characteristics (p.4) |
| PCF8574 slave address | 0100 A2 A1 A0 (binary; A2-A0 set by address pins) | source Slave address (p.10) |
| HD44780 logic supply VCC | 2.7 V to 5.5 V | source Features (p.1); pin table (p.8) |
| HD44780 LCD drive voltage (VCC - V5) | 11 V max | source Pin table, power supply for LCD drive (p.8) |
| HD44780 display RAM (DDRAM) | 80 x 8-bit (80 characters max) | source Features (p.1) |
| HD44780 power-on busy time | busy for 10 ms after VCC rises to 4.5 V | source Initialization (p.23) |

## Power

| | Value | Source |
|---|---|---|
| Supply | PCF8574 VCC 2.5-6 V; HD44780 VCC 2.7-5.5 V (chip ratings) | source PCF8574 Electrical Characteristics (p.5) |
| Current | PCF8574 ICC 40 uA (operating mode, fSCL 100 kHz); LCD backlight current not in datasheet | source PCF8574 Electrical Characteristics (p.5) |

## 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. Module header labels come from the Compoden listing, not an official module schematic.

| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| PCF8574 VCC | Voltage supply | 2.5-6 V; absolute max 6.5 V | | source Table 4-1 Pin Functions (p.3); Section 5.1 Absolute Maximum Ratings (p.4) |
| PCF8574 GND (VSS) | Ground | 0 V | | source Table 4-1 Pin Functions (p.3) |
| PCF8574 SDA | Serial data line, I/O; connect to VCC through a pullup resistor | VIH min 0.7 x VCC, VIL max 0.3 x VCC; Input absolute max -0.5 V to VCC + 0.5 V | At VCC = 5 V, VIH is 3.5 V; a 3.3 V host may not meet it. Datasheet states no 5 V tolerance when VCC is lower. | source Table 4-1 Pin Functions (p.3); Section 5.1 Absolute Maximum Ratings (p.4) |
| PCF8574 SCL | Serial clock line, input; connect to VCC through a pullup resistor | VIH min 0.7 x VCC, VIL max 0.3 x VCC; Input absolute max -0.5 V to VCC + 0.5 V | Same note as SDA. Datasheet timing is given at 100 kHz. | source Table 4-1 Pin Functions (p.3); Section 5.1 Absolute Maximum Ratings (p.4) |
| PCF8574 A0, A1, A2 | Address inputs 0 to 2; connect directly to VCC or ground | Input absolute max -0.5 V to VCC + 0.5 V | Address 0100 A2 A1 A0 (binary). On the module these are the A0/A1/A2 solder jumpers (listing). | source Table 4-1 Pin Functions (p.3) |
| PCF8574 INT | Interrupt output; connect to VCC through a pullup resistor | | Not used on the listing's four-pin header. | source Table 4-1 Pin Functions (p.3) |
| PCF8574 P0-P7 | P-port input/output, push-pull | Output absolute max -0.5 V to VCC + 0.5 V | How P0-P7 are wired to the LCD on the backpack is not in any official source. | source Table 4-1 Pin Functions (p.3) |
| HD44780 VCC / GND | LCD controller logic supply / ground | VCC 2.7-5.5 V, GND 0 V; VCC - V5 = 11 V max | | source Pin Functions (p.8) |
| HD44780 RS | Register select: 0 instruction register (write) / busy flag + address counter (read); 1 data register | | | source Pin Functions (p.8) |
| HD44780 R/W | Read/write select: 0 write, 1 read | | | source Pin Functions (p.8) |
| HD44780 E | Starts data read/write | | | source Pin Functions (p.8) |
| HD44780 DB4-DB7 | Four high-order bidirectional data bus pins (4-bit operation); DB7 can be the busy flag | | DB0-DB3 are not used in 4-bit operation. | source Pin Functions (p.8) |

## Common mistakes

- Mixing logic levels: SDA and SCL thresholds are 0.7 x VCC (high) and 0.3 x VCC (low) of the expander supply, so the bus must match the MCU's level ([source](https://www.ti.com/lit/ds/symlink/pcf8574.pdf))
- Leaving the address pins floating: A2-A0 must be tied to VCC or ground to set the I2C address ([source](https://www.ti.com/lit/ds/symlink/pcf8574.pdf))
- Running the bus faster than 100 kHz: the datasheet I2C timing is given at fSCL = 100 kHz ([source](https://www.ti.com/lit/ds/symlink/pcf8574.pdf))

## What we could not confirm

- Display size and LCD panel specs are not in the HD44780 or PCF8574 datasheets.
- PCF8574 'A' variant (0x38-0x3F) is not described in the TI PCF8574 datasheet; only the 0100 A2-A0 base is given.
- The PCF8574 operating current column in the datasheet table is ambiguous; check the PDF.
- Backlight and contrast potentiometer values are module-level; not in datasheets.
- Module-level 5 V logic claim (level shifter on 3.3 V boards) is not in datasheets.
- Pin rows are the PCF8574 (I2C expander) and HD44780 chip pins. The 4-pin module header is VCC, GND, SDA, SCL (listing); the backpack's P0-P7 to RS/RW/E/D4-D7 mapping, contrast trimpot and backlight jumper are board facts with no official source.
- Listing says operating voltage and logic level 5 V. The PCF8574 input high level is 0.7 x VCC (3.5 V at 5 V) and the absolute maximum on SDA/SCL is VCC + 0.5 V, so a 3.3 V host on a 5 V-powered backpack is not guaranteed and a 5 V pull-up on a 3.3 V host's pins exceeds its rating; use a level shifter (listing agrees).

## Sources

- [link](https://www.ti.com/lit/ds/symlink/pcf8574.pdf) (TI PCF8574, SCPS068K, revised Sept 2024)
- Hitachi HD44780U (ADE-207-272(Z), Rev 0.0): copy hosted by a hosted copy: [link](https://www.sparkfun.com/datasheets/LCD/HD44780.pdf)
- Module pinout and LCD panel size come from the listing; no official module datasheet

Building with the 20x4 I2C LCD (PCF8574)? [Describe your project on Soldr](https://app.soldr.ai): it assigns pins from the board's published pinout, checks power and logic levels, and shows how to wire every part.