Saturday, December 31, 2011

MCP79400 RTCC breakout board

Last design for the year! 32.768kHz crystal and capacitors will be onboard as well as the Schottky diode and current limiting resistor for the back-up coin battery (which will be off-board). PCB measures 0.8 x 1.0 inch. All components except for the 0.1" male headers are on the copper side (am using a single-sided presensitized board) because the MCP79400 real time clock calendar chip is available only in surface mount packages. I don't have any chip resistors and capacitors so the discretes are all through-holes. The two headers are spaced 0.7-inch apart so they'll still plug nicely into a breadboard.

I could have this board fabricated tomorrow. Thing is, I have yet to purchase my very first MCP79400! Might do so in mid-January.

Ignore that yellow airwire at pin 6. I think it's a bug in DesignSpark PCB 3.0. Even after routing, some airwires remain. Even after executing a design rule check. And even after closing DesignSpark and opening it again. Go figure.

You'll probably notice the SMD pads are much longer than recommended by chip manufacturers. When the chip is mounted on the board there should be around 2mm of exposed pad. The rationale is that the excess pad area permits the soldering iron tip to simultaneously make contact with the pad and the leads. Without the excess we'd have to position the iron tip on top of the pins to solder them. This translates to higher thermal resistance and longer heating time--not good for the chip.


January 1, 2012 Addendum

Made a couple of revisions. Added a 1uF filter cap just to make sure there's sufficient power on board. Also added a 10K pull resistor to the MFP pin as specified in the MCP79400 datasheet. The I2C's SDA and SCL lines need pull-ups but since their value depends on the I2C frequency I've left them out.

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