RtcOne - A battery backed up Real Time Clock

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The RtcOne module provides your ESP210 with a battery backed up real time clock. Perfect if you are running a battery operated system and don't want to draw the extra power to ask an NTP server for the time.

The Board


The board is based on a device called MCP79402 from Microchip and the on board battery is a soldered CR1220. The battery will keep the RTC running for at least 10 Years after which the battery must be replaced. The battery that is shipped with the device is a Panasonic CR1220 high quality battery. Please note that for some markets the battery is not included but must be purchased separately. This is purely a customs thing that we are forced to comply with.


The MCP79402 Real-Time Clock/Calendar tracks time using internal counters for hours, minutes, seconds, days, months, years, and day of week. Alarms can be configured on all counters up to and including months. For usage and configuration, the MCP79402 supports I2C communications up to 400 kHz. The open drain, multi-functional output can be configured to assert on an alarm match, to output a selectable frequency square wave, or as a general purpose output.

The MCP79402 is designed to operate using a 32.768 kHz tuning fork crystal with external crystal load capacitors. On-chip digital trimming can be used to adjust for frequency variance caused by crystal tolerance and temperature. SRAM and timekeeping circuitry are powered from the back-up supply when main power is lost, allowing the device to maintain accurate time and the SRAM contents. The times when the device switches over to the backup supply and when primary power returns are both logged by the power-fail time-stamp.

The MCP79402 features 64 bits of EEPROM which is only writable after an unlock sequence, making it ideal for storing a unique ID or other critical information. The MCP79402 is also pre-programmed with a EUI-64 address.


The board have two jumpers that you need to consider before using the board. The first jumper, SJ1, connects a pull up resistor to the SCL signal. The ESP210 does not have a pull up resistor connected to the SCL output so this need to be arranged externally. Most +One modules have a solder jumper that will enable this pull up resistor. You can only have one board with this jumper closed. So if you have a node.IT stack with several I2C boards you need to ensure that only one of these boards have the SCL jumper shortened.

The second jumper is SJ2. this jumper connects the multi function output from the MCP79402 chip to pin GPIO5 of the ESP210. The current Arduino libraries does not rely on having this jumper closed but if your are developing your own software you might consider closing this jumper.

The jumpers are PCB level jumpers that need to be closed using a solder blob. This is very easy to obtain, simply heat your solder iron and melt the already existing solder on the pads of the jumper. Then apply as much solder you need to close the gap between the pads. When you need to remove a solder jumper you simply take your solder braid, heat the braid and the solder and wick the solder away.


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