README for mu-2b board.

You should have files :

This README txt.
A component list txt.
A schematic pdf.
A layout pdf.



Description.

Mu-2b is a small pcb with a socketed pic32mx170 microcontroller and usb-serial bridge (mcp2200). 
It allows a small interpreter/program to run on the pic and communicate via the USB serial connection. 
The serial bridge connects to the PC usb and also to the pic uart via B4 and A4. The board also has 
connections to the 16 portB pins and 2 of the portA pins (A0, A1), which are arranged physically in 
the arduino pattern. It has a simple power supply that provides 3.3v as the main rail and 5v for 
the mcp2200. 

The copper and soldermask layers were prototyped (version mu-2p) and worked well. The silkscreen 
layers were then configured and applied to mu-2p.  This gave rise to a new verson number mu-2b.
Some very small changes to the copper were also included.

It is not suitable for complete novices but should be ok for people who have some experience, ie
can assemble a simple power supply, do a little intricate soldering and read and use a schematic
or pcb copper trackwork. 

The information is provided "as is",  purely at your own risk. 


Assembly.

Assembly is straightforward, with a few provisos. Before soldering each component, check for 
sensible placement and connections. 
 

The usb socket has the smallest connections and is critical, so it is worth starting with that.
Test continuity between the actual socket inputs, ground, and the two nearby IC pads.
You may as well then add the mcp2200. 
Then add the power connector and diode, 5v regulator, and input capacitor, and test for 5v output.
Add the 3.3v regulator and two capacitors, and indicator led + resistor. The shunt header SH1 feeds 
5v to the 3.3v, it can be "hotwired" with a temporary link if wished at this stage. 
Check voltages and current draw (a few mA quiescent draw from the led and regulators). 
The indicator led should operate.
Then the remaining usb bridge components. Test current draw, voltages, then connect to a PC/terminal 
and check for no USB enumeration gripes. The PC will need a microchip standard 2200 "inf" file 
(on the net) to set up for the first 2200 connected, you may need to override the plug and play. 
There is no serial throughput at this stage, so TeraTerm may connect ok but there will be no 
response to keyboard events other than a glimmering cursor. 
Finally (disconnect from the PC) add the other components and main IC socket, and check 
the voltages on the socket pins.  Don't forget the A/D power supply resistor and capacitors,  
and Vcap capactor which is critical.
If all is OK add the pic32mx170f256b and apply power. Current draw should be about 13 mA 
for an unprogrammed chip, rising to just over 30 mA if it is 
programmed and active, and ca 50 mA with the USB connected. If the board is powered up, 
a pickit3 connected to the ICSP pins should now see the pic32mx ok. 
The second shunt header SH2 and ferrite/diode connect to the USB 5v, and can be added if needed - 
to use it the shunt is moved from  SH1 pins to the SH2. A simple ferrite bead choke works ok, but 
if there is a danger of both connections being used at once, a schottky diode could be used here 
as well - it can be threaded though the ferrite bead. Otherwise do not use both connections at once !!
Lastly add the analog input circuits if needed, check that the diodes are oriented properly to 
clip excess input voltages.



Assembly component notes.
 
It is recommended that smd's be used, however 
the board often provides alternate locations, pads and holes for different format components. 
Most (not all) passive components used have been smd 0805. The alternate locations can use different 
sides of the board. So, check the polarity and placement of a component carefully wrt the copper 
as you may be looking at the reverse-image. 
The capacitor layouts, if not smd, are 0.1" pitch so a bit of wire bendng may be needed for through-hole components. The holes can be solder-filled if needed.  
The power supply uses electrolytic capactors, others are usually smd or ceramic bypass. 
The voltage regulators are smd but older through-hole components could be used vertically (not tried).
The 10uF low esr ceramic cap for the main IC is 1206 format only.
IC1 is sdip 28 pin.
The two larger smd components are IC2 MCP2200 SOIC, and usb socket (min-B).
If through-hole components are used, plating-through is probably not needed albeit a few 
through links mght be required (not tried).


