HT8 MCU Internal Ultra-Low Power Consumption RTC Application Note

Transcription

1 HT8 MCU Internal Ultra-Low Power Consumption RTC Application Note D/N: AN0482E Introduction The HT66F25x0/HT67F25xx/HT69F25xx series Flash MCUs, all include an ultra-low power consumption RTC oscillator. This internal RTC has a standby power consumption of lower than 200nA at an operating voltage of 3V, therefore making the devices suitable for applications requiring low power consumption which will increase battery life. The MCU internal RTC oscillator can reduce the requirement for an external Timepiece IC cost and simplify the overall circuitry. This series of devices also include a calendar circuit, making the devices convenient for use in timer counter applications. This application note will introduce the Holtek MCU internal ultra-low power consumption RTC oscillator characteristics and application guidelines, to help users design their own ultra-low power consumption products. Functional Description Oscillators Taking the HT69F2562 as an example, this device provides three frequency oscillation sources, namely the HIRC, LIRC and LXT oscillators. Users can select their desired system frequency, f SYS, using the application program. The LIRC oscillator is used as clock source during power on, which is then used for system setup. The LXT oscillator provides a 32768Hz frequency which can be used for various MCU functional clocks however it is more commonly used for applications requiring an RTC real-time clock. Additionally, the MCU can enter the IDLE2 or SLEEP power saving modes using the application program to achieve different standby power consumption values. Refer to the Oscillators section in the datasheet for more related register description details. AN0482E V / 6 February 21, 2018

2 The clock frequency block diagram is shown below. The LXT oscillator can provide four clocks, f SUB, f LCDP, f LXT and f LXT/8. The f LXT/8 clock is used as the Watchdog Timer and Time Base functions clock source. Here the device can achieve a minimum power consumption configuration with a standby power consumption of lower than 200nA at an operating voltage of 3V. LXT Oscillator The LXT oscillation circuit consists of an external 32768Hz crystal oscillator and capacitor components. The crystal oscillator is connected between pins XT1 and XT2. Frequency accuracy adjustments are implemented using the C1 and C2 capacitors. Users should refer to the oscillator specification for capacitor configurations and oscillator frequency tests. After power on the LXTEN bit will be in a high state which enables the LXT oscillator. By examining whether the LXTF bit has been set high, users can determine whether the LXT oscillator is powered up and ready for use after which the actual 32768Hz clock source operations can be activated. The LXT oscillator circuit is shown below. Time Base Control Register The device includes a Real Time Clock function. A timer counter function can be implemented by configuring the Time Base 0 or Time Base 1 interrupt control bit, TB0E or TB1E, together with the data memory. The Time Base function clock source f PSC0 or f PSC1, originates from the internal clock source, f LXT/8. The Time Base frequency division ratio is AN0482E V / 6 February 21, 2018

3 selected by configuring the registers to choose a value within the range of f PSC0/2 11 ~f PSC0/2 18 or f PSC1/2 11 ~f PSC1/2 18. Refer to the datasheet for the related registers and the Time Base interrupt address. This series of devices contain a calendar circuit which provides a convenient method for timer counting. Refer to the datasheet for the related register configurations. Internal RTC Characteristics The LXT oscillator can operate within a voltage range of 1.8V~5.5V. The devices need to be used in conjunction with a crystal whose C L value is less than 7pF to obtain the optimum current. If C L increases in value, the oscillation current will also consequently increase. The oscillation frequency depends on C L, the external C1 and C2 capacitors and the PCB parasitic capacitors. As a result special attention must be paid to the PCB track routing distances and the wiring diameter during PCB layout. Refer to the Low Speed Crystal Oscillator Characteristics LXT section in the datasheet or the following table for the related characteristics. AN0482E V / 6 February 21, 2018

4 Negative Impedance The LXT oscillator provides a negative impedance reference. Here a value of at least 3 times the ESR value is recommended to avoid oscillation problems, such as no oscillation or ceased oscillation, during mass production. Special attention should be paid to this area when using the 32768Hz crystal oscillator. Oscillation Frequency The LXT oscillator frequency is mainly determined by the 32768Hz crystal capacitance C L. If the sum of the series equivalent capacitances of external C1/C2 and the PCB parasitic capacitance is equal to the crystal capacitance C L, then an accurate frequency of 32768Hz can be achieved. Users can adjust the oscillation frequency to determine the appropriate C1 and C2 capacitance values. It should be noted that the oscillation frequency will be lower if the chosen C1 and C2 capacitance values are greater than C L and vice versa. The following figure shows the frequency error of various C1/C2 capacitance values under different voltages. Note that checking the oscillation frequency by directly using an oscilloscope probe to observe the XT1/XT2 pins is inadvisable. The best way to check the frequency is using the I/O pins Hz Crystal Oscillator and External C1/C2 Capacitor Frequency Error Here the temperature deviation shows the 32768Hz crystal frequency offset characteristics at different temperatures. As shown in the figure below, the external C1 and C2 capacitors frequency errors are mainly affected by the 32768Hz crystal oscillation frequency offset. Note: When the temperature is in the range of 25±5, the oscillation frequency will be lower due to the external capacitors (C1/C2=7pF) and the parasitic capacitances Hz Crystal Oscillator and External C1/C2=7pF Capacitance Temperature Curve AN0482E V / 6 February 21, 2018

5 Standby Power Consumption When the LXT is in an oscillating state with the power saving clock circuit, the device can provide an ultra-low MCU standby power consumption of lower than 200nA at an operating voltage of 3V, thus achieving longer service life for battery-powered products. This means that the HT69F2562 forms an excellent choice for power sensitive applications. In addition to the frequency characteristics, users should also pay attention to the standby power consumption when selecting the crystal, C L, and external, C1 and C2 capacitors. The common C L capacitance is 7pF. When measuring the standby current, an oscilloscope probe should not be placed on any MCU pins. The pins must be properly configured to avoid unwanted current consumption resulting from floating conditions. PCB Routing Considerations During circuit design, designers should select proper C1 and C2 capacitors to match the oscillation frequency after the frequency test. Any high frequency circuits located close to the LXT circuit will influence the oscillator characteristics, therefore special care must be taken in such cases. The LXT oscillator circuit should be located as close to the XT1/XT2 pins as possible while the interconnecting lines between the C1/C2 capacitor and VSS should be as short as possible. The reference circuit for an SMD crystal oscillator is shown as follows. LXT Oscillator Layout Conclusion This application note has introduced the ultra-low standby power consumption devices, by describing the 32768Hz LXT crystal oscillator and its measurement considerations as reference information for designers. Versions and Modify Information Date Author Issue 黃啓德 First Version AN0482E V / 6 February 21, 2018

6 Reference Files Reference file: HT69F2562 Data Sheet. For more information, refer to the Holtek official website Disclaimer All information, trademarks, logos, graphics, videos, audio clips, links and other items appearing on this website ('Information') are for reference only and is subject to change at any time without prior notice and at the discretion of Holtek Semiconductor Inc. (herein after 'Holtek', 'the company', 'us', 'we' or 'our'). Whilst Holtek endeavors to ensure the accuracy of the Information on this website, no express or implied warranty is given by Holtek to the accuracy of the Information. Holtek shall bear no responsibility for any incorrectness or leakage. Holtek shall not be liable for any damages (including but not limited to computer virus, system problems or data loss) whatsoever arising in using or in connection with the use of this website by any party. There may be links in this area, which allow you to visit the websites of other companies. These websites are not controlled by Holtek. Holtek will bear no responsibility and no guarantee to whatsoever Information displayed at such sites. Hyperlinks to other websites are at your own risk. Limitation of Liability In no event shall Holtek Limited be liable to any other party for any loss or damage whatsoever or howsoever caused directly or indirectly in connection with your access to or use of this website, the content thereon or any goods, materials or services. Governing Law The Disclaimer contained in the website shall be governed by and interpreted in accordance with the laws of the Republic of China. Users will submit to the non-exclusive jurisdiction of the Republic of China courts. Update of Disclaimer Holtek reserves the right to update the Disclaimer at any time with or without prior notice, all changes are effective immediately upon posting to the website. AN0482E V / 6 February 21, 2018