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oregontrailmike

Low Cost RTC BOB

Status: Proposal
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April 26, 2015
20150426_095906.jpg

This is a Low Cost I2C RTC board I created for a different project recently. This RCT (MCP7940M) has no Battery backup and is meant to be integrated in "permanently" powered devices (like a car). Alternatively a battery backup can be designed into the device where it is integrated in. The goal was a small PCB and overall low cost. Precision of time keeping was not as important but can be achieved if needed.

BOM: 1 x MCP7940MT-I/SN RTC ~ $.58

1 x ABRACON ABS07-32.768KHZ-7-T Crystal 32.768KHz 7pF ~ $.70

2 x AVX 08051A9R0DAT2A 9pF MCC SMD/SMT 0805 100V ~ $.25

1 x Syfer 0805J0500104KJT .1uF MCC SMD/SMT 0805 50V ~ $.05

1 x KOA Speer RK73H2BTTD1002F Thick Film Resistor 10k 1/4W 1206 ~ $.01

1 x 5 pin Header straight ~ $.05

Total cost in parts for one ~ 2$

PCB at OSHPark $3.45 for batch of 3 https://oshpark.com/shared_projects/rZjtmUx4

Total cost for 3: ~ $10 not too shabby.

All parts still hand solderable. No need for reflow oven. The crystal is a bit tricky though. I chose that specific crystal for it's small size to get the board smaller.

 

The main picture shows an earlier version with a different connector (shrouded 3 x2 header) I use for my standard I2C connections between boards.

Link to MCP9740M datasheet

http://www.mouser.com/ds/2/268/22292A-71518.pdf

 

Update 02/15/2016: Fixed Issues with Leap Year bit in test sketch.

Uploaded updated sketch.

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