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From the schematic provided for DS3234: enter image description here

It can be seen that the RESET pin is active low, and is connected to unit that provides Vcc. However, if power is disconnected and only Vbat is left, this would mean that RESET pin would be left floating? Wouldn't this configuration render the Vbat useless, as it might not even keep the time?

Furthermore, if I do not plan to do hardware reset, would it be wise to connect RESET pin just to Vbat, to ensure that it is always high until battery drains?

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3 Answers 3

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The DS3234 has a reset/brownout detector circuit as needed for µCs without a brownout detect function. It monitors Vcc and checks whether it is higher than roughly 2.5V. If it's below, it holds the RST output low, if it's higher for 250ms, it releases the RST line, which is floating then. In addition, you can connect a push button to the RST line for manual reset, which starts the very same timer of 250ms on release.

You have to use the same Vcc for the µC and the DS3234 to make this work. If Vcc is below 2.5V, RST is not floating but pulled to GND then.

If you don't need this function, simply do not connect the RST pin.

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Here is the description of the /RST pin from the datasheet:

Active-Low Reset. This pin is an open-drain input/output. It indicates the status of VCC relative to the VPF specification. As VCC falls below VPF, the /RST pin is driven low. When VCC exceeds VPF, for tRST, the /RST pin is driven high impedance. The active-low, open-drain output is combined with a debounced pushbutton input function. This pin can be activated by a pushbutton reset request. It has an internal 50k_ nominal value pullup resistor to VCC. No external pullup resistors should be connected. On first power-up, or if the crystal oscillator is disabled, tRST is bypassed and /RST immediately goes high.

Notice “No external pullup resistors should be connected”. You should not connect /RST to Vcc or Vbat, even through a resistor. Instead, it must only be pulled down, either with a switch to ground, or by an open-collector output.

/RST I’d not available while it’s on battery.

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It is not a reset to the RTC chip, it's from the RTC chip to provide a stable reset signal to MCU if it needs it during powerup. If MCU does not need it, don't connect. RTC will keep time.

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