HT67F86A Internal RTC Application Guidelines

Transcription

1 HT67F86A Internal RTC Application Guidelines D/N: AN0448E Introduction One special feature of the Holtek 8-bit Flash MCU, the HT67F86A, is that it includes an ultra-low power consumption RTC oscillator and an LCD driver. To meet the demands for low power consumption applications, the device s internal RTC has a standby power consumption of lower than 1µA at an operating voltage of 3V. The integrated LCD driver can drive dot matrix LCD display applications of up to 1024 pixels, which can be used with various display configurations and font settings. With the increasing trends of mobile device applications such as cellphones and tablets, in addition to being utilised in industrial control and consumer products, the ultra-low standby power consumption performance is also an especially important consideration in battery-powered applications. The MCU s internal RTC oscillator forms an excellent solution for applications which require a real time clock function as it reduces the need for an external timing IC with the resulting reductions in cost and design simplification. This application note will introduce the Holtek MCU internal RTC oscillator characteristics and application guidelines. Functional Description s The HT67F86A provides 3 frequency oscillation sources, namely the HXT, HITC and LXT oscillators. Users can select their desired system frequency, f SYS, using their application program. The LXT provides an f L frequency of 32768Hz for various MCU clocks, but more commonly for applications requiring an RTC real-time clock. Additionally, the MCU can enter the IDLE2 or SLEEP power saving modes also using the application program to implement different standby power consumption power down modes. The related registers are listed in the following table, refer to the s section in the datasheet for more detailed information. Register Bit Name SCC CKS2 CKS1 CKS0 FHS FHIDEN FSIDEN HIRCC HIRC1 HIRC0 HIRCF HIRCEN HXTC HXTM HXTF HXTEN LXTC LXTF LXTEN System Operating Mode Control Registers List AN0448E V1.00 1/7 January 16, 2017

2 The clock frequency block diagram is shown as below. High Speed FHS fh fh/2 HIRCEN HIRC fh/4 f H /8 HXTEN HXT fh Prescaler fh/16 fh/32 fsys fh/64 Low Speed LXT flxt IDLE2 SLEEP CKS2~CKS0 LXT The LXT oscillation circuit consists of an external 32768Hz crystal and resistor as well as capacitor components. The crystal is connected between pins XT1 and XT2. The frequency accuracy adjustment is implemented by the R P resistor together with the C1 and C2 capacitors. Users should refer to the oscillator specification for the resistance and capacitance configurations. After power on the LXTEN bit will be in a high state which enable the LXT oscillator. By examining if the LXTF bit has been set high, it can be determined if the LXT oscillator is powered up and ready for use. The LXT oscillator circuit is shown as below. C kHz C2 RP XT1 XT2 Internal Circuit Internal RC To internal circuits Note: 1. R P, C1 and C 2 are required. 2. Although not shown pins have a parasitic capacitance of around 7pF. External LXT Time Base Control Register The HT67F86A provides a Real Time Clock function. A time counting function can be implemented by configuring the Time Base 0 or Time Base 1 interrupt control bits, TB0E or TB1E, together with the data memory. The time base function clock source, f PSC, comes from the f LXT. The Time Base frequency division ratio is selected by configuring the registers to choose a value within the range of f PSC /2 8 ~ f PSC /2 15. Refer to the datasheet for the related registers and the TB0E and TB1E interrupt addresses. AN0448E V1.00 2/7 January 16, 2017

3 LXT f LXT IDLE2 SLEEP Low Speed 1/8 flcd LCD f SUB LVR flxt WDT flxt/4 1/4 Prescaler Time Base 0 TB0[2:0] Time Base 1 TB1[2:0] Device Clock Configuration fpsc TB0ON flxt/4 fpsc/2 8 ~ fpsc/2 15 Prescaler 0 U M X Time Base 0 Interrupt fpsc/2 8 ~ fpsc/2 15 Prescaler 1 TB1ON Time Base Interrupt TB0[2:0] M U X TB1[2:0] Time Base 1 Interrupt TB0C Register Bit Name TB0ON TB02 TB01 TB00 R/W R/W R/W R/W R/W POR Bit 7 TB0ON: Time Base 0 Enable Control 0: Disable 1: Enable Bit 6~3 Unimplemented, read as 0 Bit 2~0 TB02~TB00: Time Base 0 time-out period selection 000: 2 8 /f PSC 001: 2 9 /f PSC 010: 2 10 /f PSC 011: 2 11 /f PSC 100: 2 12 /f PSC 101: 2 13 /f PSC 110: 2 14 /f PSC 111: 2 15 /f PSC The f PSC is derived from the internal clock source f LXT /4. AN0448E V1.00 3/7 January 16, 2017

4 TB1C Register Bit Name TB1ON TB12 TB11 TB10 R/W R/W R/W R/W R/W POR Bit 7 TB1ON: Time Base 1 Enable Control 0: Disable 1: Enable Bit 6~3 Unimplemented, read as 0 Bit 2~0 TB12~TB10: Time Base 1 time-out period selection 000: 2 8 /f PSC 001: 2 9 /f PSC 010: 2 10 /f PSC 011: 2 11 /f PSC 100: 2 12 /f PSC 101: 2 13 /f PSC 110: 2 14 /f PSC 111: 2 15 /f PSC The f PSC is derived from the internal clock source f LXT /4. Internal RTC Characteristics Operating Voltage The internal RTC operating voltage depends on the LXT oscillator which has a voltage range of 2.2V~5.5V. Such applications are suitable for battery powered products which require 2~3 batteries. Refer to the Low Speed Crystal Characteristics LXT section in the datasheet or the following table for the related voltage characteristics. Symbol f LXT Duty Cycle Parameter Frequency V DD 2.2V~ 5.5V Test Condition Condition f SYS =f LXT =32.768kHz Ta=25 C Min. Typ. Max. Unit khz Duty Cycle % t START LXT Start-up Time 500 ms R NEG Negative Resistance Note 2.2V 3 ESR Ω Note: C1, C2 and R P are external components. C1=C2=15pF, R P =10MΩ, C L =7pF, ESR=30kΩ. AN0448E V1.00 4/7 January 16, 2017

5 Negative Impedance The LXT oscillator provides a negative impedance reference and for safety provides a value which is at least 3 times that of the ESR to avoid oscillation problems in mass production. Special attention should be made with regard to this when using the 32768Hz crystal. Oscillation Frequency The LXT oscillator oscillation frequency is mainly determined by the 32768Hz crystal capacitance C L. For example, if the external capacitors C1 and C2 series equivalent capacitance values together with the PCB parasitic capacitance is equal to the crystal capacitance C L, then an accurate frequency of 32768Hz can be achieved. The user can adjust the oscillation frequency to determine the appropriate C1 and C2 capacitance. It should be noted that the oscillation frequency will be lower if the chosen C1 and C2 capacitors are greater than C L and vice versa. The crystal related characteristics and the oscillation frequency curve used by the MCU should be taken into account for temperature deviation. Note that checking the oscillation frequency by directly using an oscilloscope probe is inadvisable. The best way to check the frequency is using the I/O pins. Standby Power Consumption When the LXT is in an oscillating state together and the Time Base clock circuit turned on to reduce power, it can provides a low MCU standby power consumption of lower than 1µA at an operating voltage of 3V, thus achieve longer service life for battery-powered products. This means that the HT67F86A forms an excellent choice for power sensitive applications. As well as considerations regarding the frequency characteristics, the user should also pay attention to the standby power consumption when selecting the crystal CL and external C1 and C2 capacitors. The common CL capacitance is 12.5pF. When measuring the standby current not that scope probes should not be placed on any MCU pins. Symbol I STB Standby Mode SLEEP Mode IDLE0 Mode V DD Test Condition Condition Min. Typ. Max. Unit 3V WDT off, Time Base on 5V µa 3V WDT on 5V µa 3V f SUB on V µa PCB Planning Considerations During circuit design, designers should select an appropriate feedback resistor R P, with a resistance range of between 5MΩ and 10MΩ, and capacitors C1 and C2 for a matched 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 capacitors and VSS should be as short as possible. The reference circuit is shown below. AN0448E V1.00 5/7 January 16, 2017

6 Application Range Token Card Reader Online banking, transactions using mobile devices, such as cellphones and tablets, can be carried out using a Bluetooth connected smart card reader. As these devices are normally battery powered, the HT67F86A is an excellent choice for these applications due to its low power consumption and 1024 pixel LCD driver. Consumer Products The HT67F86A internal Real Time Clock function can be applied for use in a wide range of battery powered consumer products such as electronic alarm clocks etc. Conclusion This application note summarises the internal RTC usage considerations providing reference information for designers. Reference Files Reference file: HT67F86A Datasheet. For more information, refer to the Holtek official website AN0448E V1.00 6/7 January 16, 2017

7 Version and Modification Information HT67F86A Internal RTC Application Guidelines Date Author Issue 黃啟德 First Version 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. AN0448E V1.00 7/7 January 16, 2017