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RTC with 1 Hz output as default (no serial communication wanted)
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KMoffett:
You didn't say if this is a one-off or a design for a device to go into production. If a one off, attached is a circuit I've used for a 1 second square wave. It used the guts from a 1.5VCD desk or wall clock for the 1 second pulse, and a 555 monostable to get the 50% square wave.

Ken



Ian.M:

--- Quote from: eV1Te on June 12, 2019, 02:05:23 pm ---
* Hence I will probably fall back to using a schmitt trigger (or a NAND gate to have one input as an enable pin) oscillator. The 555 also works, but has more pins and require more components.

* - The single NAND SN74AHC1G00 seems to be a good choice:
   Symmetric input levels for close to 50 % duty cycle and low-leakage which enables me to use large resistors (instead of a large capacitor)
--- End quote ---
That's very much not a good idea.  The frequency of a single gate Schmitt inverter oscillator is highly dependent on the width of its hysteresis band, which isn't specified for the SN74AHC1G00, and you will certainly violate its max recommended input transition time (100ns @3.3V).  If you use a dual NAND you can implement a two gate oscillator with predictable period, but that needs three passives (see Fairchild AN-118), and you are back up to an 8 pin package. 

You could use a CMOS 555, with Threshold and Trigger tied together (which gives you an inverter with a guaranteed 1/3 Vcc hysterisis band), and to the timing capacitor, and a single feedback resistor from Out to the Threshold + Trigger + TC node.

However that still leaves you needing a fairly large, extremely low leakage timing cap, and a high value feedback resistor, so it will be highly sensitive to board surface contamination and enviromental conditions.

The best option if you want to avoid programmable chips, is an higher frequency oscillator + a divider chain.  74HC4060 if you can afford the board space, or bite the bullet and pay the premium for the LTC6995 timerblox chip if you need something in a smaller package.
voltsandjolts:
Some EPSON parts will output 1Hz just by setting pins.

e.g. RTC-4543SA (built-in crystal, no external parts required..well, decoupling cap I guess)
https://www.digikey.co.uk/product-detail/en/epson/RTC-4543SA-A0-ROHS/SER3245TR-ND/1021877
FOUT freq set by FSEL pin as 1Hz or 32768Hz, also tie FOE enable pin appropriately.

BTW The above IC is spec'd at 3uA current consumption with FOUT disabled but IIRC this increases significantly with FOUT enabled, like 100uA or something.
Yansi:
Why complicating stuff so much? Do you even need RTC at all?

Use 4060 + crystal as your 1Hz source.

//EDIT: Forgot 4060 will give you 2Hz, so use 4521 instead
NivagSwerdna:

--- Quote from: eV1Te on June 10, 2019, 11:00:45 pm ---Now to my question:  How to generate a reasonably accurate 1 Hz square wave?
--- End quote ---
Define 'reasonably accurate' and the solution will follow.
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