# Overview The USB-RTC is a simple real-time clock that can supply devices without one (e.g. Raspberry Pi) with the current time and date via USB. The CR1220, CR1225 or LIR1220 (recommended) backup battery keeps the clock running even without an external power supply. The built-in 32.768 kHz crystal ensures a reasonable accuracy of the clock. The on board CH330N (or CH340N) USB-to-serial adapter can also be used as a UPDI programmer, so that no external programming device is required. This makes the USB-RTC also suitable as a development board for RTC applications based on the new tinyAVR or megaAVR microcontrollers. - Firmware (Github): https://github.com/wagiminator/ATtiny814-USB-RTC   # Hardware The wiring is pretty simple:  The power path control ensures that the battery is disconnected as soon as the device is supplied with power via USB. By using a MOSFET, the battery can supply the ATtiny with almost no voltage drop when no power is available via USB. The CH330N can be replaced by a CH340N. With the toggle switch the user can select UART mode for data transfer or UPDI mode for programming the device. C3/C4 are calculated for a crystal with a load capacitance (CL) of 12.5pF (C3 = C4 = 2 * CL - 7pF). # Software ## RTC Implementation In order to make the RTC tick, the external 32.768 kHz crystal must first be selected as the clock source. Then the Periodic Interrupt Timer (PIT) is started with a prescaler of 32768, which triggers an interrupt exactly once per second and wakes the ATtiny from sleep. The clock is then advanced by one second in the main routine. For more information on the RTC refer to [Microchip Technical Brief TB3213](https://ww1.microchip.com/downloads/en/Appnotes/TB3213-Getting-Started-with-RTC-DS90003213.pdf). ```c // Setup external 32.768 kHz crystal and periodic interrupt timer (PIT) void RTC_init(void) { _PROTECTED_WRITE(CLKCTRL_XOSC32KCTRLA, CLKCTRL_ENABLE_bm); // enable crystal RTC.CLKSEL = RTC_CLKSEL_TOSC32K_gc; // select external 32K crystal RTC.PITINTCTRL = RTC_PI_bm; // enable periodic interrupt RTC.PITCTRLA = RTC_PERIOD_CYC32768_gc // set period to 1 second | RTC_PITEN_bm; // enable PIT } // Interrupt service routine for PIT (wake up from sleep every second) ISR(RTC_PIT_vect) { RTC.PITINTFLAGS = RTC_PI_bm; // clear interrupt flag } ``` ## Time and Date The time and date functions surprisingly take care of the time and date. When the firmware is uploaded, the time is automatically set to the current time (compilation time). The time and date are updated every second, triggered by the Periodic Interrupt Timer (PIT). Leap years are taken into account and weekdays are calculated. For more information on the relevant calculations, see [Microchip Application Note AN2543](https://ww1.microchip.com/downloads/en/Appnotes/AN2543-Temperature-Logger-with-ATtiny817-and-SD-Card-v2-00002543C.pdf) and [Sakamoto's method on wikipedia](https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Sakamoto's_methods). ```c // Returns TRUE if year is NOT a leap year (refer to AN2543) uint8_t TIME_notLeapYear(void) { if (!(t.year % 100)) return (t.year % 400); return (t.year % 4); } // Returns day of the week (Sakamoto's method, refer to wikipedia) uint8_t TIME_dayOfTheWeek(void) { static uint8_t td[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4}; uint16_t y = t.year; if (t.month 4.3V (65=256*1.1V/4.3V) } ``` ## Power Saving In order to save energy, the ATtiny spends most of its time in sleep mode power down when it is running on batteries. The Periodic Interrupt Timer (PIT), which is driven by the 32.768 kHz crystal, wakes the ATtiny once per second to update the time. In addition, all pins that are not required are switched off. For more information on power saving refer to [Microchip Training Manual](https://ww1.microchip.com/downloads/en/DeviceDoc/ADC-Power-Optimization-with-tinyAVR-0-1-series-and-megaAVR-0-series.pdf). ```c // Disable unused pins to save power for (uint8_t pin=7; pin; pin--) (&PORTA.PIN0CTRL)[pin] = PORT_ISC_INPUT_DISABLE_gc; PORTB.PIN0CTRL = PORT_ISC_INPUT_DISABLE_gc; PORTB.PIN1CTRL = PORT_ISC_INPUT_DISABLE_gc; // Prepare sleep mode SLPCTRL.CTRLA = SLPCTRL_SMODE_PDOWN_gc // set sleep mode to power down | SLPCTRL_SEN_bm; // enable sleep mode ``` According to the measurements with the [Power Profiler Kit II](https://www.nordicsemi.com/Products/Development-hardware/Power-Profiler-Kit-2), an average current of 2.5µA at a voltage of 3V is consumed in battery operation. A typical CR1225 battery has a capacity of 50mAh. This results in a theoretical battery life of 20,000 hours or 833 days or 2.34 years. A rechargeable LIR1220 battery, on the other hand, only has a capacity of 8mAh, which leads to a service life of around 3,200 hours or 133 days. However, this is sufficient for most applications and since a LIR1220 can withstand higher current peaks, which leads to more stable operation, it is to be preferred overall.   ## Compiling and Uploading - Install the CR1220, CR1225 or LIR1220 (recommended) buffer battery. - Set the selector switch on the device to UPDI. - Plug the device into a USB port of your PC. ### If using the Arduino IDE - Open your Arduino IDE. - Make sure you have installed [megaTinyCore](https://github.com/SpenceKonde/megaTinyCore). - Go to **Tools -> Board -> megaTinyCore** and select **ATtiny1614/1604/814/804/414/404/214/204**. - Go to **Tools** and choose the following board options: - **Chip:** ATtiny814 or ATtiny414 or ATtiny214 - **Clock:** 5 MHz internal - **Programmer:** SerialUPDI - Leave the rest at the default settings. - Go to **Tools -> Burn Bootloader** to burn the fuses. - Open USB-RTC sketch and click **Upload**. - Set the selector switch on the device back to UART. ### If using the makefile (Linux/Mac) - Download [AVR 8-bit Toolchain](https://www.microchip.com/mplab/avr-support/avr-and-arm-toolchains-c-compilers) and extract the sub-folders (avr, bin, include, ...) to /software/tools/avr-gcc. To do this, you have to register for free with Microchip on the download site. - Open a terminal. - Navigate to the folder with the makefile and the Arduino sketch. - Run "make install" to compile, burn the fuses and upload the firmware. - Set the selector switch on the device back to UART. The time and date of the USB-RTC are automatically set to the current time (compilation time) when the firmware is uploaded. # Operating Instructions 1. Set the selector switch on the device to UART. 2. Plug the device into a USB port of your PC. 3. Open a serial monitor and set it to 9600 BAUD. # References, Links and Notes 1. [ATtiny814 Datasheet](https://ww1.microchip.com/downloads/en/DeviceDoc/ATtiny417-814-816-817-DataSheet-DS40002288A.pdf) 2. [TB3213 - Getting Started with RTC](https://ww1.microchip.com/downloads/en/Appnotes/TB3213-Getting-Started-with-RTC-DS90003213.pdf) 3. [TB3216 - Getting Started with USART](https://ww1.microchip.com/downloads/en/Appnotes/TB3216-Getting-Started-with-USART-DS90003216.pdf) 4. [AN2447 - Measure VCC/Battery Voltage](https://ww1.microchip.com/downloads/en/Appnotes/00002447A.pdf) 5. [AN2543 - Temperature Logger](https://ww1.microchip.com/downloads/en/Appnotes/AN2543-Temperature-Logger-with-ATtiny817-and-SD-Card-v2-00002543C.pdf) 6. [ADC and Power Optimization](https://ww1.microchip.com/downloads/en/DeviceDoc/ADC-Power-Optimization-with-tinyAVR-0-1-series-and-megaAVR-0-series.pdf) 7. [Determination of the Day of the Week](https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Sakamoto's_methods)  # License  This work is licensed under Creative Commons Attribution-ShareAlike 3.0 Unported License. (http://creativecommons.org/licenses/by-sa/3.0/)