#pcb work

Box Build Assembly Process in EMS Industry

I am positive I did calculations at some point - but tumblr's tagging system is pretty sucky - so they're difficult to find.  I have some vacation time and wanted to make more headway on my PCB project for the LED Heart.  The following are some of the lessons I've learned since the first schematic (lol - totally Addie style):

Some of the basic information (that may or may not change).

I will be using a Propeller MCU.  The Propeller requires at least 3.3V and can source 3.3v 30mA for a total of 100mA.  Of note, this requires decoupling capacitors between each VDD/VSS pair.

This will need a 3.3V regulated power source so I'll be using the MCP1700 LDO Regulator (MCP1700T-3302E/MB).  Of note, this chip prefers to have 1uf bypass capacitors on both its input and output.

Two LEDs are needed.  I originally selected ones that Jayson uses, but it seems that because their forward voltage is 3.3V, and the Prop's pins sources 3.3v,  that's not enough.  So I'll have to find some LEDs that I can use with lower forward voltage.  (Maybe red?)  This will determine how big the resistors need to be.  

Working out the resistors needed for the LEDs:

Ohm's law says that the resistance of the resistor, R = V/I, where:  V = voltage across the resistor (= VS - VL in this case)  I = the current through the resistor So R = (VS - VL) / I

I will be using the 24LC256 EEPROM.  The SCL pin (6) must connect with 35 on the Propeller (Pin 28) and the SDA pin (5) must connect with 36 on the Propeller (Pin 29). Pins 5 and 6 also need 10k ohm pull up resistors.  And Pin 8 needs a 1uF bypass capacitor.

I have provisioned for a 5.0MHz crystal.  This is connected to 28 and 29 (Pins XI/XO) on the propeller.

To program the propeller - you need to have Ground, RST (7/RESN), TX (38/Pin31), and RX (37/Pin30).  The RST, TX, RX lines need 10k ohm pullup resistors.

While the LEDs are still being...decided upon, so far:

To Do:

Get the vias away from the pads - makes it easier to solder.

Find proper LEDs and calculate the resistor values

Make the ground planes and power planes

Place the header pins for propplug programming (the four vias not connected to anything) a 1/10" spacing.

Decide whether to use a crystal or free up the space and use RCFast - the Prop's internal clock.

Make the traces look prettier!!!

Widen the pads on the switch to make it easier.

MANY thanks to:

The folks at the Parallax Forums: Duane Johnson, Kwinn, tonyp12, doggiedoc, SRLM, RDL2004, Cluso99, Frank Freedman

The folks in the Tymkrs IRC: Whisker, Roy, Tautic, JohnS_AZ, MgBurr, RaivisR, Tom_at, everyone who put up with my LED ramblings :p