The **PCF8564A** is a CMOS Real-Time Clock (RTC) and calendar optimized for low power consumption. It communicates with microcontrollers via a two-line bidirectional I2C-bus. --- ## 1. Core Technical Specifications The following table highlights the primary electrical and operational characteristics of the PCF8564A: | Feature | Specification | | :--- | :--- | | **Supply Voltage (Vdd)** | 1.8 V to 5.5 V | | **Operating Current** | Typ. 250 nA (at Vdd = 3.0V, 25°C) | | **Interface** | I2C-bus (up to 400 kHz) | | **Clock Output** | Programmable: 32.768 kHz, 1.024 kHz, 32 Hz, 1 Hz | | **Package Options** | TSSOP8, SO8, VSSOP8 | | **Internal Oscillator** | Requires external 32.768 kHz crystal | --- ## 2. Key Internal Components The PCF8564A consists of several functional blocks that handle timekeeping and communication: ### A. The Oscillator and Divider The device requires a standard **32.768 kHz quartz crystal**. An internal divider chain reduces this frequency to 1 Hz, which increments the time counters. It includes internal integrated capacitors for the crystal, though fine-tuning may be required depending on the PCB layout. ### B. Register Map The device contains 16 8-bit registers used for: * **Control/Status:** Managing the interface and alarm flags. * **Time:** Seconds, Minutes, Hours. * **Date:** Days, Weekdays, Months, Years. * **Alarm:** Setting specific times for an interrupt trigger. * **CLKOUT:** Frequency selection for the output pin. ### C. The I2C Interface * **SDA (Serial Data):** Bidirectional line for data transfer. * **SCL (Serial Clock):** Input line for synchronization. * **Addressing:** The fixed I2C slave address is usually `1010001` (binary). --- ## 3. Pin Configuration | Pin Number | Name | Function | | :--- | :--- | :--- | | 1 | **OSCI** | Oscillator input | | 2 | **OSCO** | Oscillator output | | 3 | **INT** | Interrupt output (open-drain; active LOW) | | 4 | **VSS** | Ground (0 V) | | 5 | **SDA** | I2C serial data I/O | | 6 | **SCL** | I2C serial clock input | | 7 | **CLKOUT** | Programmable clock output (open-drain) | | 8 | **VDD** | Supply voltage | --- ## 4. Notable Features 1. **Low Backup Current:** Because it operates at nano-ampere levels, it is ideal for battery-backed systems (using a CR2032 or Supercapacitor). 2. **Alarm and Timer:** Includes a programmable alarm and a countdown timer that can trigger the `INT` pin to wake up a "sleeping" microcontroller. 3. **Century Flag:** It includes a bit to track century transitions, simplifying long-term calendar calculations. 4. **Open-Drain Outputs:** The `INT` and `CLKOUT` pins are open-drain, meaning they require external pull-up resistors to the VDD rail. ---