LPC Based Design

Development Boards MicroMod
1

I’m not sure if this is the right forum, but the “Everything ARM and LPC” seems to hint as much.

I am working on an LPC2148 based design. I’m an embedded systems guy by trade, but I have practically no experience with PCB design. I’m trying to create my own first design, and have drawn a schematic using Eagle.

So, I’m wondering if someone would be willing to review it and let me know of any potential issues. Specifically I have the following questions:

  1. Are the sizes of the bypass caps sufficient?

  2. Is the LM1117 sufficient for deriving the 3V3 source from VBUS?

  3. Do I need a protection diode on the USB header?

  4. Is the RC circuit on RESET sufficient (47kohm/100nF RC, which I think is a rise time to Vih of about 2ms)?

  5. Do I need series terminations on the JTAG header?

  6. I’m using a 74LCX07 to interface with a 5V open collector device. The device has its own 5V pullups. I don’t want to have my own 5V supply so I’m relying on the external pullups. Is this safe?

I have some general PCB questions, but I’ll save them for the PCB board.

You can view the schematic here:

http://docs.google.com/fileview?id=F.33 … eb23c431da

Thanks!

2

Sorry, the page (or document) you have requested is not available.

Also, feel free to post Eagle schematics if you can. Lots of us have Eagle and will look at stuff, if you don’t mind sharing.

Also, look at the Olimex schematics for the LPC2148, or SparkFun’s PackageTracker. They’re both good reference designs (minus a few changes I would make which are more personal taste).

  1. LM1117 should be good.

  2. No, not really. Up to you.

  3. Risky, depending on the device in question, how much power it provides, and if the pullups are always present.

3

Ok, try this link instead.

http://www.4shared.com/file/73237133/4e … F1541.html

4

Thanks for the response! I’ve been trying to reply, but it keeps telling me I am using a forbidden word. But it doesn’t tell me what word it is…

theatrus:
Also, feel free to post Eagle schematics if you can. Lots of us have Eagle and will look at stuff, if you don’t mind sharing.

I would, if I knew how to attach files. The best I could figure out was to post the PDF.

theatrus:
Also, look at the Olimex schematics for the LPC2148, or SparkFun’s PackageTracker. They’re both good reference designs (minus a few changes I would make which are more personal taste).

Thanks for the mention of Olimex. They will come in handy.

They don’t show any terminations on the JTAG, so I’ll probably be fine.

And the small, 3-pin MCP130T looks better than the RC circuit I was using for the reset line. The 350ms hold time on RESET is significantly better than any RC circuit. Plus it does power supply monitoring.

theatrus:
6. Risky, depending on the device in question, how much power it provides, and if the pullups are always present.

The remote device always has pullups. As for the power....I dunno. If the pullups are present, does it matter?

I didn’t want to have pullups to 3V3 because I didn’t want reverse current to flow 5V–>3V3 from the remote device (with 5V pullups) into my board.