Perpetual Calendar using the HT1382

Transcription

1 Perpetual Calendar using the HT1382 D/N:AN0258E Introduction The HT1382 is a low power real time clock device which includes two serial interfaces: I 2 C or 3-wire. The interface mode is selected by the selected device type. The device provides both clock and calendar information in BCD format and also includes alarm functions that can be read or setup by an external device, which is usually an MCU and which will be the master device. The calendar is accurate until the year 2099 and includes automatic leap year correction. An external 32768Hz crystal is used as the device oscillator for device timing for which is provided integrated crystal load capacitances of 12.5pF. The device includes a crystal oscillator temperature compensation function and internal power control circuitry which detects power failures and automatically switches to a battery supply when a power failure occurs. Operating Principles The HT1382 clock MCU is comprised of power control circuitry, crystal frequency dividing circuitry, I 2 C or 3-wire communication interface, TC register, control status register, alarm register, EEPOM etc. The HT1382 clock MCU internal diagram and pins are shown below: HT1382 Block Diagram 1

2 HT1382 Pin Assignment HT1382 egisters The device includes 21 registers which are used to control functions such as the TC, Status, Control, Alarm, Frequency output and five bytes of EEPOM for the clock compensation settings and stored user data. The TC and Alarm register data is stored in BCD format while other data is stored in binary format. The register map shows the address definitions for the I 2 C interface. The command byte and / bit are used for the 3-wire interface. Address egister Definition egister ange D7 D6 D5 D4 D3 D2 D1 D0 Name Data Default 00h CH 10SEC SEC Seconds 00 ~59 80h 01h 0 10MIN MIN Minutes 00 ~59 00h 02h 12/ AP H 01~12 0 HOU Hours H 00~23 12h 03h DATE DATE Date 01~31 01h 04h M MONTH Month 01~12 01h 05h DAY Day 01~07 01h 06h 10YEA YEA Year 00~99 00h 07h P ST - 80h 08h AE 0 0 EE EB AI BE 0 ST - 00h 09h IME AE LPM OEOBM FO3 FO2 FO1 FO0 INT - 00h Seconds 0Ah SECEN AL. 10SEC AL. SEC Alarm 00 ~59 00h Minutes 0Bh MINEN AL. 10MIN AL. MIN Alarm 00 ~59 00h 0Ch HEN 0 0 AL. Hours 01~12 AL.HOU 10H Alarm 00~23 00h Date 0Dh DTEN 0 AL. 10DT AL.DATE Alarm 01~31 00h 0Eh MOEN 0 0 AL. Month AL.MONTH 10M Alarm 01~12 00h Day 0Fh DAYEN AL.DAY Alarm 01~07 00h Bit / Command Byte

3 Address egister Definition D7 D6 D5 D4 D3 D2 D1 D0 egister Name ange Data Default Bit / Command Byte EEPOM data 10h DTS DT6 DT5 DT4 DT3 DT2 DT1 DT0 DT h EEPOM user data US h EEPOM user data US h EEPOM user data US h EEPOM user data US Power Control Function Normal Mode: Power supplied by VDD hen VDD < VBAT-VBATHYS and VDD < VCOMP, the power supply mode is switched to Battery Backup Mode. Battery Backup Mode: Power supplied by VBAT. Set the BE bit in the status register to 1. Users can clear this bit to zero by software or reading the status register (when AE=1.) hen VDD > VBAT+VBATHYS or VDD > VCOMP+VCOMPHYS, the power supply mode is switched to Normal Mode. Low Power Mode: hen VDD > VBAT, the power is supplied by VDD. hen VDD < VBAT, the power is supplied by VBAT. hen the LPM bit in the control register INT is set to 1, it will enter the low power mode. TC Function The TC register stores the year, week, month, day, hour, minute, second data in BCD format. The seventh bit in the Second register is the crystal enable bit. hen the bit is 1, the crystal is disabled. If 0, the crystal is enabled. The seventh bit in the Hour register is used to define the 12-hour or 24-hour mode elect bit. If the bit is 1, the TC uses a 24-hour mode. If 0, the TC uses a 12-hour mode. 3

4 IQ/FOUT Frequency Output hen the FO3~FO0 bits in the status register are set as shown in the table below, the corresponding output frequency of the IQ/FOUT Pin is follows. Fout(Hz) FO3 FO2 FO1 FO / / / / / Note: hen the OEOBM bit in the status register is set to 1, IQ/FOUT is in the battery backup mode and the frequency output and alarm function will be invalid. hen the bit is zero, IQ/FOUT is in the battery backup mode with all function effective. Alarm Function The register addresses associated with the alarm function is 0AH~0FH, which stores the alarm data of second, minute, hour, day, month and week in BCD format. hen set to 1 and the register matches the TC register, the AL bit in the status register will be set to 1 and IQ/FOUT will be enabled. Set the AL bit in the status register to 1 which can be cleared to zero by software or reading the status register (when AE=1.) hen an alarm occurs, IQ/FOUT will operate in low level trigger or pulse trigger mode. Low Level Trigger: hen the AE bit in the status register is set to 1, the IME bit is cleared to zero, the AL bit is set to 1, and the frequency output is stopped. hen the TC register matches the corresponding alarm register value, then IQ/FOUT will be 0 until the AL bit in the status register is cleared to zero. Pulse Trigger: hen the AE and IME bits in the status register are set to 1, the frequency output is stopped. hen the TC register matches the corresponding alarm register value, the AL bit in the status register will be set to 1 and IQ/FOUT will output a low pulse of 250ms width. In this mode a repeated trigger is acceptable. For example, when only the second register is set for a compare match, a low pulse will be generated every minute. 4

5 EEPOM Function hen the EE bit in the status register is zero, the EEPOM is read only. hen this bit is 1, both read and write operations are allowed. hen the EB bit in the status register is set to 1, this means that data writing to the EEPOM is not yet complete, therefore no data read/write operations are allowed to the EEPOM until the EB bit becomes zero. Communication Method The device provides two data communication methods (decided by which IC type is selected), which are I 2 C and 3-wire serial communication methods. I 2 C Serial Communication Method Start and Stop Timing 1 Bit Timing ACK Timing 1 Byte Timing 5

6 Data riting Single Byte riting Timing Successive Data riting Timing Data eading Data eading Timing 3-wire Serial Communication Method 6

7 Crystal Oscillator Temperature Compensation Function To avoid frequency offsets resulting from the influence of temperature on the crystal oscillator, the device includes a temperature compensation function. The operating steps are as follows: 1. Set the device to the normal mode. 2. Setup FO3~FO1 to allow IQ/FOUT generate a frequency. 3. Measure the IQ/FOUT frequency under different temperatures. (Evenly measure at least five temperature frequencies during the whole temperature range.) 2 f 4. Substitute the following formula: o c1 ( T c2 ) c3 or 2 f o at bt c 5. Use the values given from the above formula to calculate the following three values. Qcoefreal=c1=-a, To_real=c2=-b/(2a), XtalOffsetreal=c3=c-b2/(4a). 6. Use the three values below in the EEPOM for the MCU to execute temperature compensation. Qcoef =4096*Qcoefreal; To=To_real; XtalOffset=XtalOffsetreal; 7. The MCU will get the COMP_val from the following formula. COMP_val=Qcoef*(T-To)2-XtalOffset ( To is the transition temperature, the unit of COMP_val is ppm.) 8. Select the temperature compensation range according to the COMP_val. hen the DTS bit in the status register is set to 1, the temperature compensation range will be ppm to ppm. hen the DTS bit is zero, the temperature compensation range will be ppm to ppm. 9. Acquire the minimal compensation calibration according to the temperature compensation range. If DTS=1, the minimal calibration is 3.052ppm. If DTS=0, the minimal compensation is 1.017ppm. 10. rite the COMP_val/(the minimal compensation calibration) value to the DT5~DT0 bits in the DT register. The DT6 bit is a symbol bit, being positive when this bit =1, or negative when this bit =0. 7

8 Noise ejection Design Notes Any noise flow into the oscillator circuit will have an influence on the accuracy of the timer circuit, so some necessary precautions must be taken when designing the PCB of the TC circuit: The crystal oscillator should be located as far away from the serial or parallel bus as possible as high speed data may be present on the bus. This high speed data may influence the stability of the oscillator circuit. Add an unclosed ground wire around the crystal oscillator with one side connected to the HT1382 ground wire to reduce external noise Do not add any ground wires at the back of the PCB where the crystal oscillator is located to reduce capacitance effects on the oscillator frequency accuracy. If IQ/FOUT is to be used as a square wave output, the output line must be located distantly from the device Application Circuit 8

9 The application uses a HT4537 as the master device to control and display values from the HT1382. After power-on, the display will display the year. hen switch S3 is pressed, the display content can be switched to year, date, hour, minute, second, in a repeat cycle. hen switch S4 is pressed, the display content will be displayed in the reverse direction. hen S1 is pressed, it will enter the setup mode where the corresponding display numbers to be setup will flash. Press S1 again to switch to the desired unit to be setup, namely year, month, day, hour, minute, week in an ongoing cycle. S3 is used to increase the value while S4 is used to decrease the value. If no switch is pressed for 10 seconds then it will exit the setup mode. hen S2 is pressed, the LED D1 will be illuminated and the minute alarm function will be enabled. After one minute, the LED D2 will flash three times. hen S2 is pressed again, the LED D1 will extinguish and the minute alarm function will be disabled. S/ Flowchart Main Flowchart 9

10 Main Flow Description After the MCU is powered on, the I/O ports will be initialised, the AM will be cleared and the timer interrupt set to 5ms. Initialize the HT1382 and enable the A/D. After the initialization, the MCU will enter the main program loop, starting with display control, key scan, LED display control and acquire the present temperature so as to proceed with data calibration using the EEPOM data. Check if a time of 5ms has elapsed. If not then remain waiting, or return to a new loop when 5ms has elapsed. Crystal Oscillator Automatic Calibration Flowchart 10

11 Crystal Oscillator Automatic Calibration Program Description The program starts by checking to see if a time of 1S has elapsed. If so then it will enter the crystal oscillator automatic calibration program to read the Qcoef, To, XtalOffset values, which will have been stored previously in the EEPOM, and substitute COMP_val=Qcoef*(T-To)2-XtalOffset for COMP_val. After this write the COMP_val/(minimal compensation calibration) value to the DT5~DT0 bits in the DT register. The DT6 is a sign bit which if equal to 1 means positive or if 0 means negative. Program Example See attachment file Conclusion The simple perpetual calendar application above provides the user with an easy way of understanding the HT1382 including an explanation of the internal automatic temperature calibration function to calibrate the frequency for implementation of more accurate timings. 11