AVR-Chat

I know this is dangerous, but ... :-)

I'm looking for a critique of the board layout for my first significant project.

This board is designed to control signals for outdoor ride-on railroads (which I
think I've spoken about here before).  The board senses track voltages and drives
multiple 3-LED signal heads accordingly.  I have a previous, much larger setup using
a commercial "development board" with an ATMega16 and an "interface" board I made
for initial feasibility testing.  I'm now adding communications (pseudo-CAN) and
going to an ATMega32a (more program space for communications protocol, etc.).  There
were lots of design decisions that went into it that I won't go into here.  What I'm
looking for is feedback on the circuit design and board layout - be gentle :-)


Images:
	Schematic:
http://www.whitetrout.net/Chuck/temp/Signal_Interface_sch.pdf 
	Board:	http://www.whitetrout.net/Chuck/temp/Signal_Interface_brd.pdf 


Schematic:

- The power, track(rail) inputs, and data bus are protected (up to a point) against
lightning/surge by TVS diodes.  Low current, fast-blow fuses will be mounted on a
"base board" in the enclosure that the board plugs into.  No illusions about
successfully taking a close/direct hit.  My previous prototype survived several hits
with no problems other than blown fuses and one very close hit that took out about
$8 worth of parts across two boards.

- The data bus is pseudo-CAN.  I use a CAN driver but I'll be doing the
beginning-of-message bus arbitration by bit-banging (PD2, PD3) and then switch to
the USART for actual data transfer.  In the future I hope to switch to a AT90CAN128
(built-in CAN support) but I wanted to limit the initial hardware/software learning
curve so I could get the next version out faster.  I realize that this will severely
limit my speed/flexibility for now but it will allow me to test communications on
the long (1,000's of feet) bus, etc. and I have to walk before I run.  Switching to
true CAN later will allow me to take advantage of standard CAN I/O expanders, etc.

- I think I have addressed chip bypass issues - do I need more than one bypass cap
on the ATMega32a?

- Later the 7805 linear regulator will be replaced by a switching regulator to
reduce demand on the power bus (1,000's of feet long - I'm aware that there are lots
of issues here).  Again, I wanted to limit the introduction of too much new
technology in this go-around. 

- I will create a small "Diag" module that plugs onto the "Diag" header.  It will
have LCD screen, "joystick" menu, etc. - possibly based on the AVR Butterfly.  It
will use I2C (new to me) to communicate with the processor, monitor the CAN bus
(hence I brought out CANH/CANL), etc.  "Diag_Present" will be pulled low by the
module to notify the processor that the module is connected and wants to communicate
via the I2C bus (I considered periodic polling of the Diag I2C address but thought
this wasteful of processor time).  "Diag_Int" is there to allow the module to
interrupt the processor but I don't have a specific use at the moment.  Other
signals I should bring out to this header?

- The Railroad Signal Head LEDs (10mm, ~10 ma each) will be connected to the
TLC59116 outputs.  The LED commons will be connected to the unregulated V+ to reduce
demand on the regulator and noise on VCC. 

- I picked a 14.7456 MHz crystal so that I would have 0% USART bit rate error up to
230.4 Kbps.  At the moment, I have no idea what bus rate I'll be able to maintain on
the long cable.  One of the purposes of this board version is to investigate this.
At the moment I'm thinking that I'll be using a lower rate for arbitration (via PD2,
PD3) and then switch to a higher rate for data transfer via the (interrupt driven)
USART.

- Jumpers TB1-TB7 supply bias to the track.  In some cases the bias is supplied at
the distant end of the track section (to allow detection of a "broken rail") by a
board with a similar network.

- Unused processor pins were brought out to the connector to allow for as yet
unknown functionality - hey, I had extra pins :-) ...



Board:

- The board is 2.750" x 2.375" designed to fit into a standard PVC "single-gang"
switch box enclosure available at Lowes/Home Depot, etc.

- The holes in the corners of the board are to facilitate locating pins to
facilitate machining both sides of the prototype boards using "Isolation Milling"
(using pcb-gcode, MACH3 and a Sherline CNC mill).

- The TVS diodes are mounted on the bottom side of the board.  I attempted to keep
the traces short between the TVSs and the connector as well as ground.

- I couldn't seem to find (Mouser/Digikey) a socket for the crystal (FOXSLF/147-20)
so I'm using pin sockets from MillMax for the moment (can anyone point me to an
appropriate socket?).

- JP1 is on the bottom side.  It plugs into a connector on the "base board" within
the enclosure.  The holes at the ends of JP1 are to mount a handle on the top side
to facilitate insertion/removal of the board.

- Signal assignments to connector JP1 were chosen to make board layout/trace routing
easier.

- I know that the 7805 does not have a large heat sink pad but I don't think I'll be
drawing enough for that to be a problem (see also the next item).

- This board was auto-routed (I use Eagle).  The ground needs to be hand-routed but
I have been moving/changing things so much (ergo a lot of "ripup !; auto !;") that I
have stayed with auto-route for the moment.  I realize that this is a crutch and
"real men" don't use the autorouter :-)  This board is mostly to check functionality
and develop the software, I'll hand route GND/VCC/V+ on the next board.  I was also
having problems with the auto-router and GND/VCC planes (the board would auto-route
without them but not with them) so I eliminated them for the moment (I also tried
"FreeRouter" on the web).  I'll be re-addressing this issue when I hand-route the
next board.

Well, that should give you enough to shoot at.  I realize that there are a lot of
design considerations, application considerations, etc. that will cause you to ask
"why did you do it that way" but to describe all that would have required a lot more
reading (and me to remember all the considerations) so I beg your forgiveness on
that aspect.  If something really jumps out at you, feel free to ask ...
 
Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844 http://www.whitetrout.net/Chuck