Building Own Arduino - Couple Quick Questions

Projects General Guidance
1

I'm finally sufficiently happy with my breadboarded setup of parts to build a board to integrate it all. For a variety of reasons, a simple shield is not realistic, so I'm planning to build the Arduino HW (I've been using an Uno) right into the system. I have a few hopefully simple questions.

I believe the attached is a fully-functioning Uno-Equivalent aside from the lack of USB? I used both the Uno and Sparkfun Pro Mini Schematics to piece it all together (hopefully correctly).

  1. I'm planning to program this with an FTDI interface board, so all of the USB circuitry is thankfully un-necessary. The Pro Mini schematic shows how the FTDI board hooks up, looks very simple to me, just TX/RX/VCC/GND/DTR, with DTR on the opposite side of a 0.1uF cap from the reset pin for auto-reset.

  2. I am not going to use a switch for resetting, as my project has too much potential for the button to get jostled too easily and prompt a false reset. Instead I'm just breaking out a pair of pins that I can short with a jumper if needed. Hence the jumper by the reset where there is normally a switch.

  3. The datasheet for the crystal I was looking at using (CTS ATS16B) mentions a load capacitance of 20pF. I assume this is the recommended capacitor value I should be using (most general instruction sources I've seen mention 18-22pF, so this is at least in the range)?

  4. The schematic for the Uno has a 1M resistor across the pins of the crystal. Why? Is this necessary?

  5. Aside from the fact that the DIP 328P-PU can be found with the Arduino bootloader, is there any difference if I were to use an SMD 328P? I am considering this to save space, I've used the Sparkfun ProMini a fair amount too and it seems to be identical to the Uno in capabilities (well, with 2 extra analog pins that I don't need, but same speed, flash, ram, etc). All my peripherals are running on SPI if that matters.

  6. I have 5-6 SPI peripherals (several sensors, a flash mem chip, and a uSD card) and a level shifter (I need the speed of the 16MHz clock otherwise I'd run the whole darn thing at 3.3V and be done with it) that are going to be attached to this system. I know standard practice is to put filtering/bypass caps on the Vin on all of these. Is there any issue with the extra capacitance of having so many caps across the voltage lines? I'm thinking not, but wondering, especially since the board isn't going to be all that big (roughly Uno-sized, maybe 1.5x on the upper end). Power will be supplied through a 5V reg (to the 328p) and a 3V3 reg (to all the SPI devices) from a set of batteries.

Thanks in advance!

UnoEquiv.PNG
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2

Yes, that looks like a typical "Bare Bones" or "Really Bare Bones" Arduino board. It should work but you should probably add a power decoupling cap or two.

3

Yes, add two 0.1uF caps, one each on VCC and AVCC.
Also, a diode across R1 - anode to Reset pin, cathode to +5.

4

I believe that is correct. I don't see why Arduino wouldn't work in that case.

5

I did indeed forget a cap on the power input, woops. I was figuring on using one cap for the two lines, but I guess that will depend on how the board layout falls, I may need to drop the second in if the lines are any significant distance apart.

CrossRoads: why the diode across R1?

Anyone know what the 1M resistor across the crystal is for in the Arduino Uno schematic? More curiosity than anything at this point as it would appear un-necessary.

Any insight on questions 5 or 6 that I posted in the OP? I'm probably over-thinking it and both are fine, but...

6

You need 0.1uF on each power pin of each device for power supply decoupling. Helps provide local current right at the chip while things switch internally.
Have a 10uF electrolytic where the battery cable comes into the board.
The diode prevents any current spikes from a reset switch from letting the chip think high voltage programming mode is starting - and then acting like it's hung until a good reset comes thru.
SMD, DIP, the IDE does not care what form factor you are using. SCK, MISO, MOSI still come out of the same Port on a '328P.
The extra pins a SMD part provides, A6 and A7, are analog input ONLY and cannot be used as extra chip select pins for your SPI devices.