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sensor:pcf857x [2026/05/14 15:51] – vamsansensor:pcf857x [2026/05/15 01:03] (current) – [PCF857x Vcc and Vdd] vamsan
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 The current limits of the PCF857x are heavily lopsided because of its quasi-bidirectional architecture. It handles current entirely differently depending on whether you are sinking current (outputting 0/LOW) or sourcing it (outputting 1/HIGH). The current limits of the PCF857x are heavily lopsided because of its quasi-bidirectional architecture. It handles current entirely differently depending on whether you are sinking current (outputting 0/LOW) or sourcing it (outputting 1/HIGH).
  
 +The [[arduino:display_lcd#i_c_lcd_adapter|I²C LCD adapter]] utilizes the PCF8574.  
  
- +==== PCF857x Pinout ====
-**Main Control & Power Header** +
   * **VCC:** Power input. Connects to **3.3V or 5V** to match your microcontroller's logic levels.   * **VCC:** Power input. Connects to **3.3V or 5V** to match your microcontroller's logic levels.
 +  * **VDD:** Power input. Connect the external hardware power to **3.3V or 5V**.
   * **GND:** Common ground reference.   * **GND:** Common ground reference.
   * **SDA:** Serial Data line for I²C communication.   * **SDA:** Serial Data line for I²C communication.
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   * **Port 1** (PCF8575 only, P10 to P17): The second group of 8 quasi-bidirectional GPIO pins.   * **Port 1** (PCF8575 only, P10 to P17): The second group of 8 quasi-bidirectional GPIO pins.
  
 +==== PCF857x Reverse side of the module ====
 +{{ :sensor:pcf8575_2.png |PCF857x I/O Expander chip/modul reverse side}}
 +
 +==== PCF857x Vcc and Vdd ====
 +^Pin^Function^Typical Target^Voltage Role|
 +^VCC|Microcontroller (Logic) Side|Connect to your MCU's power pin (e.g., 3.3V or 5V).|Sets the operating reference voltage for the I²C communications line (SDA/SCL).|
 +^VDD|Peripheral (Device) Side|Connect to the external hardware power (e.g., 5V).|Directly sets the high voltage logic level of the 16 target output/input pins (P00–P17).|
 +
 +**The Crucial Condition: Check the Solder Jumper**
 +
 +For this dual-voltage (3.3V/5V) configuration to work safely, the onboard VCC-VDD solder jumper must be open (disconnected).
 +  * If **the jumper is open**, the module’s built-in level shifters are enabled. The I²C lines (SDA/SCL) will communicate securely at 3.3V with your microcontroller, while the 16 I/O pins (P00–P17) will operate at 5V, since the expander logic runs at the VDD level.
 +  * If **the jumper is bridged**, VCC and VDD are connected together. Applying 3.3V to VCC and 5V to VDD can create a short circuit, leading to overheating and <color red>possible damage to your microcontroller</color>.
 +
 +==== PCF857x module's addressing ====
 +To assign a specific 7-bit hex address, use a soldering iron to connect the center pad of each group to either GND or VCC.
 +
 +|<100%>|
 +^A2 Status^A1 Status^A0 Status^Resulting Binary Offset^Hex I²C Address|
 +^GND (Open)|GND (Open)|GND (Open)|000|0x20 (Default)|
 +^GND (Open)|GND (Open)|VCC (Bridged)|001|0x21|
 +^GND (Open)|VCC (Bridged)|GND (Open)|010|0x22|
 +^GND (Open)|VCC (Bridged)|VCC (Bridged)|011|0x23|
 +^VCC (Bridged)|GND (Open)|GND (Open)|100|0x24|
 +^VCC (Bridged)|GND (Open)|VCC (Bridged)|101|0x25|
 +^VCC (Bridged)|VCC (Bridged)|GND (Open)|110|0x26|
 +^VCC (Bridged)|VCC (Bridged)|VCC (Bridged)|111|0x27|
 +
 +==== PCF857x using I/O pin ====
 **Sinking Current (Output LOW / Connecting to Ground)** **Sinking Current (Output LOW / Connecting to Ground)**
  
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   * **Maximum per individual pin:** Only 100 µA (0.1 mA).   * **Maximum per individual pin:** Only 100 µA (0.1 mA).
   * **The Limit:** This current is too weak to directly light up an LED or actuate a standard relay trigger.   * **The Limit:** This current is too weak to directly light up an LED or actuate a standard relay trigger.
 +
 +**How to Correctly Wire Components**
 +
 +  * **Correct (Sinking):** Connect your load's positive wire to VCC, and the negative side (through a current-limiting resistor) directly to the PCF8575 I/O pin. Setting the pin LOW completes the path to ground and activates the load.
 +  * **Incorrect (Sourcing):** Connecting your load's positive wire to the PCF8575 I/O pin and the negative side to Ground will fail, as the pin cannot supply sufficient current.
 +
 +{{ :sensor:pcf857x_wiring_1.png |PCF8575 I/O How to Correctly Wire Components}}
 +
 +{{page>:tarhal}}
 +
 +==== PCF857x wiring with relay modules ====
 +Connecting optocoupled relay modules to the PCF8575 is a common practice, but it requires strict electrical care. Because of the chip's asymmetrical quasi-bidirectional ports, a mistake in how you trigger the relay will prevent it from working entirely.
 +
 +**The Triggering Rule: Active LOW Only**
 +
 +  * **Why Active HIGH fails:** Standard optocoupled multi-channel relay modules require 2-5 mA of logic current on their IN pins to light the internal infrared LED and engage the circuit. Because the PCF8575 can source only 0.1 mA when outputting HIGH, it cannot trigger an Active HIGH configuration.
 +  * **Why Active LOW works:** When configured as Active LOW, the PCF8575 acts as a ground connection (0V), cleanly sinking the current from the relay module. The PCF8575 can safely sink up to 20 mA per pin, easily meeting the relay's current requirements.
 +
 +For example, the [[actor:relay_moduls|HL-54 (4-channel 3.3V optocoupled relay module)]] can be directly connected to the PCF8575. Because the HL-54 is hardwired as an Active LOW module, it is perfectly suited to the PCF8575's current limits.
 +
 +===== I²C topics on lamaPLC =====
 +{{topic>i2c}}
 +
 +\\
 +\\
 +{{tag>communication i2c PCF857x PCF8574 PCF8574A PCF8575 I/O_Expander I/O_Extension I2C NXP Texas_Instruments}}
 +\\
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