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-C7
Micro Crystal
$1.35
I2C
±1 ppm @ 25°C
45nA
integrated RTC + Crystal in single module
RV-3032-C7
Micro Crystal
$1.73
I2C
-40°C to +85°C, ±2.5 ppm
160nA
integrated RTC + Crystal in single module. Temperature compensated. Integrated temp sensor readable over I2C.
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:
That RV-3028 RTC is quite impressive! I hadn’t seen that chip before, thanks!
But there’s quite a few supercaps available at normal retail distributors, like Digi-Key, which come in quite a bit less expensive than your examples if you’re able to accept a bit higher ESR (equiv series resistance), which should be fine for an RTC circuit drawing <1uA. For example, CDA’s CE5R5105HF-ZJ is a 1F 5.5V coin shape supercap for $0.84/ea for qty 100, but the internal resistance is around 30ohms.
Am I misunderstanding something about supercap selection such that this CDA part wouldn’t be reasonable to choose for an RTC circuit?
I assume the 1000 hours means it will hold a charge for that long without self-discharging? Self-discharge is probably the dominant factor with a supercap as the RTC draw is so low.
1000 hours is 41 days.
For the PHVL-3R9V155-R, the lifetime is listed as 0.6 to 2 years:
I assume “endurance” and “lifetime” mean how long the supercap will keep a charge. If these are equivalent, then the more expensive caps do have a significant advantage if you have a scenario where you want to set the time during manufacturing, and have it be valid during installation up do a year later. If you don’t need that, then the cheaper ones seem like a good option.
While lifetime endurance is important, self-discharge is what I was really looking for. The leakage current for the 1.5F PHLV cap is only 2uA - which is relatively insignificant compared to the current consumed by most RTCs.
Additionally, with the RV-3032-C7, you can read the built-in temperature sensor over I2C, eliminating the need for a separate temperature sensor on the board.
The RV-3032 draws more current (160nA vs 45nA in the RV3028), but it also has a built-in charge pump so it can charge super-caps up to 3V or 4.5V, allowing you to store more energy in the super-cap. Measurements here indicate the RV3028 only charges a super-cap up to 2.86V.
