TIPS for RTC & it's accuracy:
Many of us use RTC in project.
During RTC use following point must consider
1. Proper RTC IC selection
2. It's Require external Crystal & load capacitor
3. Battery Backup & it's Charging Circuitry
4. Software Interface
5. Time setting or changes done by user & it's updation.
6. Accuracy of RTC
7. PCB designing
1.0 Proper RTC IC selection.
Generally Maxim & Philips make RTC IC is selected by many engg.But before selecting Ic make paper work of RTC requirement which include following point
a. Requirement of RTC for Time, Date , Day, Week , Month, Year & Alarm interrupt facility etc.
b. Is any Storage of parameter require if yes than how much memory & what duration even if battery remove. e.g. Birth Day reminder in mobile
c. Is Alarm Required then it's quantity in day or week etc. e.g Alarm = 2/day or 5/day etc.
d. Alarm setting resolution such as in second/minute/hour.
e. Battery voltage monitoring & it's status to external device in terms of interrupt or any ...
1.1. Operating voltage: Select 3V operating IC as 3 volt Battery easily obtain in market. But in this case your controller must operate on 3-3.3 Volt
range or logical voltage level of both IC match. For 5 volt also RTC IC work but check RTC IC lower & upper operating voltage limit in datasheet.
1.2. Package: DIP or SMD as per ur application select. But for prototype use DIP because some time IC require remove from socket if get hang due s/w
problem.
2.0 Crystal & Capacitor: RTC work totally depend upon Crystal. I read some document where they indicate to use 32 Khz (In in build controller have RTC
module) but it must be 32.768 Khz. If required accuracy in time then crystal frequency must have small PPM variation. This ppm change according to
temperature & capacitance. So high accuracy select low ppm crystal.
Calculate track capacitance & required load capacitance to Crystal. If require then place external low value capacitor as per datasheet.
3.0 Generally during development RTC work fine, show best accuracy. During production we mfg. bulk quantity of unit & store in store room. From store to
user application again some time elapse. Now when user Power unit at that time RTC Battery is discharge & some time unit not work properly as battery
is get damage. To avoid this proper Battery Selection & it's good current Capacity require. I suggest use Super Capacitor which is best.Chargeable
Battery is also another option.Isolate all other circuitry supply from RTC power. Also provide proper charging circuit so that Battery Charge fast.If Battery
voltage down then it's indication must provide so that user can change it. During power on if found that Battery down below threshold then user allow
some time to charge it & after that it must set proper Time setting. Note during this period all data must be remain in EEPROM.
e.g. Attendance machine in School where Vacation period is long. In mobile after Battery remove it ask for time setting
4.0 Software Interface :RTC is easily configurable by software. Time Display is main task which regularly required updation so RTC must provide
interrupt for second.Due to this software polling time save.
5.0 Time Change : RTC based system required some action as per RTC pre define time set by user. In this case we can use Alarm interrupt. Due to
this software does not require polling action for particular event. When user change Time then it is immediately update RTC. To do this avoid second
setting. Use only Hour & minute setting because due to user interface & other software burden we can't set RTC in terms second format easily
e.g. Set time 11:15 is done easily but 11:15:13 not easily
6.0 Accuracy: RTC accuracy testing require more time because we can't test/predict it from 2/3 hour.It required day/week even month also. So start testing
by measuring crystal frequency using Scope & attenuated probe. Observe Crystal frequency & it's deviation. Set RTC time with accurate Time of
GPS/Internet system which is more accurate. Take reading of RTC & reading at same event after say 12 hour span for 5/6 days. After this u can check
how much deviation in time.Compare it with Crystal deviation. If it is not within limit then adjust load capacitor..
7.0 PCB Design: Refer IC datasheet in which ground plan & Crystal placement recommendation is given. Because due to track size ,other circuitry noise, bus
capacitance. RTC circuit some time get hang or time deviation observe.
Many of us use RTC in project.
During RTC use following point must consider
1. Proper RTC IC selection
2. It's Require external Crystal & load capacitor
3. Battery Backup & it's Charging Circuitry
4. Software Interface
5. Time setting or changes done by user & it's updation.
6. Accuracy of RTC
7. PCB designing
1.0 Proper RTC IC selection.
Generally Maxim & Philips make RTC IC is selected by many engg.But before selecting Ic make paper work of RTC requirement which include following point
a. Requirement of RTC for Time, Date , Day, Week , Month, Year & Alarm interrupt facility etc.
b. Is any Storage of parameter require if yes than how much memory & what duration even if battery remove. e.g. Birth Day reminder in mobile
c. Is Alarm Required then it's quantity in day or week etc. e.g Alarm = 2/day or 5/day etc.
d. Alarm setting resolution such as in second/minute/hour.
e. Battery voltage monitoring & it's status to external device in terms of interrupt or any ...
1.1. Operating voltage: Select 3V operating IC as 3 volt Battery easily obtain in market. But in this case your controller must operate on 3-3.3 Volt
range or logical voltage level of both IC match. For 5 volt also RTC IC work but check RTC IC lower & upper operating voltage limit in datasheet.
1.2. Package: DIP or SMD as per ur application select. But for prototype use DIP because some time IC require remove from socket if get hang due s/w
problem.
2.0 Crystal & Capacitor: RTC work totally depend upon Crystal. I read some document where they indicate to use 32 Khz (In in build controller have RTC
module) but it must be 32.768 Khz. If required accuracy in time then crystal frequency must have small PPM variation. This ppm change according to
temperature & capacitance. So high accuracy select low ppm crystal.
Calculate track capacitance & required load capacitance to Crystal. If require then place external low value capacitor as per datasheet.
3.0 Generally during development RTC work fine, show best accuracy. During production we mfg. bulk quantity of unit & store in store room. From store to
user application again some time elapse. Now when user Power unit at that time RTC Battery is discharge & some time unit not work properly as battery
is get damage. To avoid this proper Battery Selection & it's good current Capacity require. I suggest use Super Capacitor which is best.Chargeable
Battery is also another option.Isolate all other circuitry supply from RTC power. Also provide proper charging circuit so that Battery Charge fast.If Battery
voltage down then it's indication must provide so that user can change it. During power on if found that Battery down below threshold then user allow
some time to charge it & after that it must set proper Time setting. Note during this period all data must be remain in EEPROM.
e.g. Attendance machine in School where Vacation period is long. In mobile after Battery remove it ask for time setting
4.0 Software Interface :RTC is easily configurable by software. Time Display is main task which regularly required updation so RTC must provide
interrupt for second.Due to this software polling time save.
5.0 Time Change : RTC based system required some action as per RTC pre define time set by user. In this case we can use Alarm interrupt. Due to
this software does not require polling action for particular event. When user change Time then it is immediately update RTC. To do this avoid second
setting. Use only Hour & minute setting because due to user interface & other software burden we can't set RTC in terms second format easily
e.g. Set time 11:15 is done easily but 11:15:13 not easily
6.0 Accuracy: RTC accuracy testing require more time because we can't test/predict it from 2/3 hour.It required day/week even month also. So start testing
by measuring crystal frequency using Scope & attenuated probe. Observe Crystal frequency & it's deviation. Set RTC time with accurate Time of
GPS/Internet system which is more accurate. Take reading of RTC & reading at same event after say 12 hour span for 5/6 days. After this u can check
how much deviation in time.Compare it with Crystal deviation. If it is not within limit then adjust load capacitor..
7.0 PCB Design: Refer IC datasheet in which ground plan & Crystal placement recommendation is given. Because due to track size ,other circuitry noise, bus
capacitance. RTC circuit some time get hang or time deviation observe.
