Comparison of RTC Chips

Hardware
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This post is a comparison of various RTC chips supported by Zephyr.

We get a list of supported chips from the Zephyr build system:

  • am1805
  • ambiq
  • ds1307
  • emul
  • fake
  • ifx_cat1
  • mc146818
  • pcf8523
  • pcf8563
  • rpi_pico
  • rv3028
  • sam
  • smartbond
  • stm32
  • numaker
  • rv8263
  • xmc4xxx
  • nxp_irtc

Many of these are embedded RTCs inside MCU chips. Initially, we’ll focus on external RTCs. Most of the below, except for RV-3028 require a separate external crystal.

Part # Manufacturer Cost (100 pcs) Interface Accuracy Power Notes
AM1805 Ambiq $0.65 I2C 2 ppm 22nA Watchdog timer with hardware reset, 256 bytes ram, auto-calibrates internal osc from xt to achieve lower power
DS1307 Analog Devices $3.23 I2C ??? 300nA
PCF8523 NXP $0.82 I2C ??? 150mA
RV-3028 Micro Crystal $1.91 I2C 1 ppm 45nA integrated RTC + Crystal in single module
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Supercap calculations

Sometimes it is desirable to run a RTC from a supercap so you don’t have to worry about issues with battery going bad, etc. Below is a comparison of one range of options. The life assumes we will discharge the cap from 3.3V to 1V @ 45nA. The formula is:

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Cap (F) Part # Size (mm) Cost (qty: 100) Run time (days)
0.47 PHVL-3R9V474-R 9 x 17.3 x 14.5 $5.11 277
1.5 PHVL-3R9V155-R 9 x 17.3 x 22 $3.56 594
2.5 PHVL-3R9V255-R 11 x 21.3 x 23 $3.92 1,479
3.0 PHVL-3R9V305-R 9 x 17.3 x 32.5 $4.07 1,775
5.0 PHVL-3R9V505-R 11 x 21.3 x 32.5 $4.14 2964