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.

 

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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.

Voltmeter woes

Just got duped. The digital panel-mounting voltmeter I bought which supposedly has a range of 0 to 20VDC stops measuring--with fairly good accuracy--at a little over 15V. Feed any voltage higher than that and meter's accuracy starts dropping off fast. By the time the measured voltage is 20V the meter reads less than 17V.

The exasperating part about it is I found this out only after I had redesigned a circuit so as to factor its limited range of 20V. If you haven't guessed it yet, the cheapo meter is made in China. Yes, it came in that blue and white box with Chinese characters. Doesn't come with any instruction leaflet. And there are only Chinese text on all sides of the carton. Frustrating. At least the silkscreen on the board isn't in Chinese! As you can probably make out, the pins for the connectors are marked (left to right) 5VIN, GND, GND, VIN+. The first two pins are for power and the latter two are for the measured voltage input. A check using a DMM shows the two middle pins (GND) are in fact connected to one another on the PCB. The multiturn potentiometer (blue rectangular component at the top) is used to calibrate the meter. Calibration is fine--if you don't need the entire specified range of 20V. Else, it's a joke.

Devil's in the details

And I just got burned. I inadvertently designed a short circuit in a pcb!

I create circuit board artwork using DesignSpark PCB and when printing it out on transparency film I always have the board outline printed out as well. The outline allows for precise alignment of the mask (the film) and the presensitized board prior to exposure. In this particular design of a simple 5-volt power supply I had a wide strip of copper for both the +5V and ground. And I extended those tracks all the way to the edge of the board (see the left side of the artwork image below).

Yep, disaster in the waiting. I printed the artwork, exposed the presensitized board, etched it, drilled the holes, soldered the components, and all the while I was absolutely oblivious to the short I had created. I powered up the board while probing the output with a multimeter. Reading? 23mV. I cut off power immediately. Tried it again. Still the same zero voltage. I was so clueless of the booboo that I even desoldered one of the filter caps suspecting it might be defective. Only while poring through board with a magnifying lens did I finally spot the fine 5-mil trace at the very edge of the board bridging the power rails. Facepalm moment indeed. Actually if I had moved the mask by some half millimeter that outline would've been off the physical board and the short wouldn't have appeared.

It's worth noting that even a 5-mil track is faithfully reproduced--implying there's insufficient undercutting during etching to remove the track. This is good of course--if we want such fine copper traces. Luckily it's easy to cut and gouge out the offending copper track. After making sure the bridge had been completely obliterated I soldered the cap back on and the circuit worked as designed. No electronic component was harmed in the making of this sophomoric design mistake.

Hopefully the stress, anxiety, and panic I was subjected to has burned the lesson in my head--since I'll still continue the practice of printing the board outline, I must remember to leave some 20 mils around the perimeter of the board copper-free. I better burn that minimum gap requirement into DesignSpark's design rules. And more importantly, I better start paying attention to the error messages which the design rule check routine spits out--because I don't usually take a look at the track-to-track, pad-to-track, shape-to-track, shape-to-pad, etc. spacing errors. I have a knack for breaking rules. Unfortunately, this time around I got bitten.

Ambient light sensor on the roof

Installed an ambient light level sensor on the roof. Voltage from the simple voltage divider made up of an LDR and a fixed 5% resistor (750Kohm in this case) is digitized by the analog-to-digital converter of the PIC12F1822 (16 readings are taken successively--takes around 1ms--and averaged) and is sent out through the its serial port at 19.2kbps every 50ms. Communications is simplex--this transmitter merely sends while receivers in remote locations can only receive data. Serial data from the MCU is fed to the SN75176B transceiver which then converts the data stream to an RS-485 compliant signal which is then sent down a pair of unshielded twisted pair cable, for use by any circuit requiring the data. Checksum is sent with every data packet for data integrity verification.

A pair of wires in the UTP cable provides filtered (but unregulated) power to the board. Because the input voltage is somewhere around 8 to 8.5V and may drop below 8 if loads elsewhere (various circuits use the same power supply) increase, I used a 1N5817 Schottky diode instead of a 1N400x for polarity reversal protection. That halves the forward voltage drop across the diode to just 0.3V and thus increases the headroom for the 78L05, ensuring it's able to maintain voltage regulation. There are separate power and ground tracks/traces for the analog and digital sections to minimize digital noise from contaminating the sensor section. The analog section has a 1uF tantalum filter cap and the MCP6232 op amp has the mandatory 0.1uF decoupling cap. You might notice the 1/4-watt resistor adjacent to the LDR. Reason is the dang supplier doesn't have the complete range of values for 1/8-watt resistors.

Image below shows the board in situ with the translucent plastic cover yet to be snapped on. Screws on the board act as standoffs to keep the soldered side from contact with the bottom which might accumulate moisture (hence the drilled holes on the blue plastic to act as drain). The gray 4-pair UTP cable can be seen on the lower left.

Grew up on Casio

Photo of Casio fx-3000 and fx-115MS scientific calculator. An uncle bought me the 3000 back in 1977. Was already gaga over sci calcs back then. It still works but I haven't used it in decades. Though not noticeable in the pic with the 115 (because we're looking head-on through yellow plastic filter) the LCD is already showing its age and deteriorating. Actually it's fared better than my Casio fx-502P whose LCD went kaput years ago. The 3000's power switch had been giving me problems and had to take out its board and clean both the switch and board contacts.

I now use the 6-year old 115 exclusively. It probably has twice the number of functions. The engineering units (nano, micro, kilo, etc) and decimal/binary/hex functions make life easier. And being able to go back and edit the formula/equation and plug in different values for the variables is just indispensable.

As for speed, the two are truly generations apart. A "u" appears as the leftmost character in the 3000 to alert the user that the microcontroller is still running the numbers through its ALU (arithmetic logic unit). Even something as simple as 1+1 takes a quarter of a second. It's that slow. 69! takes 3.3sec. The 115 on the other hand spits out the factorial result in the blink of an eye, literally. Given that both calcs cannot handle or display any number greater than or equal to 10^100, 69 is the highest possible number for factorials for both.

The equation shown on the display of the 115 is (2π*1k*1µ)^-1. This of course is the formula for the cutoff frequency of a simple low pass filter with a 1kohm resistor and 1µF cap. Being able to enter "M" for mega, "k" for kilo, "n" for nano, "p" for pico, etc.comes in very handy. They save me the trouble of keying in values in scientific notation.

Back side of the calculator. The sticker is original. Could be the serial number of the unit. You might be able to make out the printing at the bottom. Part of it says "Made in Japan." All Casios were back then. The 115MS on the other hand is churned out in China. Thumbs down.

Without the front panel you can clearly see the poor state of the LCD.

The microcontroller is a NEC D895G. Googled but can't find any datasheet for it.

This is the side of the board that greets you when you take off the back cover.

Rail-to-rail op amps

Just got bitten. Or rather, just found out after over two years I'd made a booboo. I had used a Microchip MCP618 as a unity gain buffer for an ANLC with the sensor unit located a remote location. I was upgrading the load-control part of the circuit yesterday and while monitoring the output of the op amp at between sunset and dusk I was shocked to see the voltage jump from from V_DD to around 4.80V. (Thereafter it decreased in a smooth continuous gradient). That sent me into a near-panic. Questions raced through my head. Was the long length of cable--some 20 meters--from sensor to control circuit responsible for this behavior? Do I need to add a high value load resistor? Is there some soldering/board defect at the sensor?

Eventually I zeroed in on the fact that the 618 is not a full rail-to-rail op amp. Rechecking the datasheet and refreshing my memory, indeed while its output can, its input does not swing rail-to-rail. So to test whether the 618 is cause of the problem I bench-tested it using a 100K pot as input to the unity gain buffer. Sure enough as I ever so gradually turned the pot down from maximum, the buffer output jumps from 5.18 (V_DD) to around 4.90V.I tried this several times just to make sure its inherent. Well, it doesn't miss a beat. The same discontinuous output occurs every time.

I then popped the 618 off the breadboard and dropped in a pin-for-pin compatible MCP6273. This op amp has true rail-to-rail input and output. As I was hoping, as the pot is wound down the output voltage does not exhibit the sudden drop but instead smoothly and continuously decreases from V_DD.

Whew! I'm just glad it isn't some much more major error or defect.

Moral of the story: Make sure the op amp has rail-to-rail input and output if the application requires the entire range of values.