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Any RTC Favorites/Suggestions? — Parallax Forums

Any RTC Favorites/Suggestions?

Duane DegnDuane Degn Posts: 10,588
in Propeller 1
Do any of you have suggestion on a good RTC?

It looks like my boss wants to add a real time clock to the Propeller based industrial meter I've been working on (my job is mainly software).

We're thinking of using Microchip's MCP7940N (link to datasheet pdf).

We're not very concerned with the RTC's accuracy since the meter will regularly connect to external devices which can update the time.

We want to use a battery or super cap as backup power for the RTC.

Apparently this RTC requires a 32.768KHz crystal with load capacitence between 6nF and 8nF. I've found this one by Citizen.

CM200_crystals.jpg

I haven't figured out what sort of battery/battery holder (or supercap) to use yet. I just wanted to see if any of you have been down this road already have have any advice to offer.

I've always just used the inexpensive ebay RTC modules in my one off projects but this time the parts need to be available from reliable sources.

Comments

  • SapphireSapphire Posts: 496
    ISL12022MA is my RTC of choice. Has a built-in crystal.
  • jmgjmg Posts: 15,173
    There is also PCF8563 and newer variants PCF85063, PCF85263, for external Xtals.

    and the PCF2129T/PCF2127T have inbuilt Crystals, and TCXO operation for better ppm
    Costs more, but swallows the sensitive parts, lowers the BOM and gives good temperature compensation.
    ( ISL12022MA above sounds similar approach)
  • JonnyMacJonnyMac Posts: 9,104
    I built a little DS1337 add-on for an EFX-TEK board. Schematic is attached. Of course, in your app, you'd want to use pins 28 and 29 for the I2C connection. Back-up is with a 3v coin cell battery.
    pdf
    pdf
    DipTrace Schematic - hc8_rtc__2015-02-07.pdf
    18K
  • Peter JakackiPeter Jakacki Posts: 10,193
    That PCF2127T looks like a pretty good all-in-one for a good price.

    @Duane: That crystal there is an ancient monster, why not use the small 3.2x1.5mm (or less) crystals that are readily available?
  • Cluso99Cluso99 Posts: 18,069
    I have been using the expensive Maxim RTCs with inbuilt xtal s. Work fine but they were about $6 last time I bought them.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2016-09-20 14:19
    Sapphire wrote: »
    ISL12022MA is my RTC of choice. Has a built-in crystal.
    jmg wrote: »
    . . . and the PCF2129T/PCF2127T have inbuilt Crystals, and TCXO operation for better ppm
    Costs more, but swallows the sensitive parts, lowers the BOM and gives good temperature compensation.
    ( ISL12022MA above sounds similar approach)

    My boss would love to use a RTC with an internal crystal but those all seem to cost significantly more than the RTCs with external crystals.

    I'll look at the ones you guys have suggested to see how they compare with the price of the other RTC options.

    Thanks for the suggestions.

    JonnyMac wrote: »
    I built a little DS1337 add-on for an EFX-TEK board. Schematic is attached. Of course, in your app, you'd want to use pins 28 and 29 for the I2C connection. Back-up is with a 3v coin cell battery.

    Wow, thanks for the schematic and the BOM. I feel like I'm getting better at finding my way around Mouse and Digi-Key but it's really nice to have some part numbers to be sure I'm looking at the right part.

    Thank you.
    Peter Jakacki wrote: »
    @Duane: That crystal there is an ancient monster, why not use the small 3.2x1.5mm (or less) crystals that are readily available?

    Thanks for the link to the smaller crystal. I agree, it looks like a better option.

    Thanks.

    Edit: Link to the same part at Mouser in USA.

    Cluso99 wrote: »
    I have been using the expensive Maxim RTCs with inbuilt xtal s. Work fine but they were about $6 last time I bought them.

    My boss likes the lower part count of the internal RCT chips but, as you say, they cost significantly more than the other RTC chips.

    So far the price difference has been too high to justify moving to the internal crystal RTC. I'll take a look at the RTC chips suggested in this thread to see if any of these other chips cost less than the ones I've already found.

    Thanks for all the great input guys.

  • Duane DegnDuane Degn Posts: 10,588
    edited 2016-09-20 14:38
    In case others are interested, here are some links to the RTC chips with internal crystals mentioned above.

    ISL12022MA $4.79 each when purchasing 100 chips.

    PCF2129T $2.52 each when purchasing 100 chips.

    PCF2127T $3.01 each when purchasing 100 chips.
    Peter Jakacki wrote: »
    That PCF2127T looks like a pretty good all-in-one for a good price.

    Agreed. I hadn't looked up the prices when I replied earlier.

    Thanks for the suggestion jmg.

  • MikeDYurMikeDYur Posts: 2,176
    Why has the Dallas DS1302 fallen from grace, maybe the price? It has a very low BOM, and I have gotten more than three months backup on a 3.3v supercap. It does have some drift if your application is time critical.
  • kwinnkwinn Posts: 8,697
    MikeDYur wrote: »
    Why has the Dallas DS1302 fallen from grace, maybe the price? It has a very low BOM, and I have gotten more than three months backup on a 3.3v supercap. It does have some drift if your application is time critical.

    More likely a combination of factors. Newer chips have on chip crystals and additional benefits so may be of lower total cost when all the costs of placing it on a board are taken into account.
  • Duane DegnDuane Degn Posts: 10,588
    MikeDYur wrote: »
    Why has the Dallas DS1302 fallen from grace, maybe the price?

    I don't have anything against the DS1302 but as kwinn and others have pointed out there are new chips with internal crystals.
    MikeDYur wrote: »
    I have gotten more than three months backup on a 3.3v supercap. It does have some drift if your application is time critical.

    I don't think we're worried about a bit a drift. The time will be reset each time the device connects with a PC or smartphone.

    Do you have a supercap you like? There are so many different kinds it's hard for me to figure out which ones are good options for this application.



  • MikeDYurMikeDYur Posts: 2,176
    Duane Degn wrote: »

    Do you have a supercap you like?




    I bought mine awhile back from Mouser, I remember there was a few choices @3.3v, I ordered a couple of each, but the physically larger ones we're easyer to work with, and their higher capacity, so that is what I used. This one is plugged into the PPDB.
    2016-9-20 12-43-9.jpg
    1280 x 720 - 90K
  • tomcrawfordtomcrawford Posts: 1,126
    If you already have a I2C bus in your application, the ds1307 might be a good choice. It uses a battery rather than the supercap. 
    ds1302     spi bus     supercap
ds1307     i2c bus     coin cell
  • MikeDYurMikeDYur Posts: 2,176
    tomcrawford wrote: »
    If you already have a I2C bus in your application, the ds1307 might be a good choice. It uses a battery rather than the supercap. 
    ds1302     spi bus     supercap
ds1307     i2c bus     coin cell



    Space limitations may have you put a coin cell holder off board, if it is a concern, the supercap can get in tight.
    2016-9-20 13-23-2.jpg
    720 x 1280 - 482K
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2016-09-20 18:24
    The DS1307 and DS1302 are 5 volt parts. The DS1337 family run on 1.8V up to 5.5V and has better micropower characteristics. More modern.

    In any of these, whether it be internal or external crystal, be sure to follow the layout guidance, and best (unless proven otherwise) use the crystal that they suggest.

    My favorite is the ISL12020MIRZ, which is the DFN + internal crystal 5ppm version of the '12022 that Sapphire suggested. Both offer 128 bytes of battery backed ram, which can be very handy for sticky variables. I usually use a CR1225 coin cell for backup, either one that solders onto the board or with a holder. At 48mAh the projected/optimistic backup life is around 10 years (0.5µA). When the board has external power the timekeeping current comes from that, rather than from the coin cell. Layout is crucial, particularly if the board is soldered with water-based flux. Has to be squeaky clean or else the 32kHz will stop, because the internal crystal comes out to lands. I add silicone conformal coating to that area of the pcbs.

    Duane, the MCP7940N sounds quite nice for your purposes, and the price is right!



  • D.PD.P Posts: 790
    edited 2016-09-20 19:29
    I like this one, +- 2ppm internal crystal, 234 bytes of SRAM.

    DS3232

    Not inexpensive.
  • MikeDYurMikeDYur Posts: 2,176
    edited 2016-09-20 20:13
    Tracy Allen wrote: »
    The DS1307 and DS1302 are 5 volt parts.



    The DS1302 has a supply range of 2.0 - 5.5v, current < 300nA@2.0v. It does have TTL out with a 5v supply.
  • dMajodMajo Posts: 855
    edited 2016-09-21 06:27
    I am using this one for many years. It is not cheap but the cost is not a problem. With this I replace also the eeprom and I have figured out a sort-of code/tamper protection.
    FM31L278
  • Cluso99Cluso99 Posts: 18,069
    Here is the one I use with internal crystal _ cheaper than I recalled.
    DS1340C-33# SOIC16 $3.76/100 from Mouser.
    Not the cheapest one around.
  • Don MDon M Posts: 1,652
    dMajo wrote: »
    I am using this one for many years. It is not cheap but the cost is not a problem. With this I replace also the eeprom and I have figured out a sort-of code/tamper protection.
    FM31L278

    Thanks for that. Neat chip.

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