Need PCB advice

PVJohn

I would like to make my first PCB with SMD components, but before I do that, wanted to ask an expert(s) if this is going to work or not. I'm not sure which FT232 chip to use since I'll solder it by hand. Is there any example how to read/write to additional EEPROMs?

Thanks,
PVJohn

Paul Sr.

PVJohn said...
I would like to make my first PCB with SMD components, but before I do that, wanted to ask an expert(s) if this is going to work or not. I'm not sure which FT232 chip to use since I'll solder it by hand. Is there any example how to read/write to additional EEPROMs?

Thanks,

PVJohn

First thing that came to me was that I would parallel the regulators off the source rather than have the 5V regulator bear the loads of 5V AND 3.3V...

Peter Jakacki

This is a real bare bones implementation so not much should go wrong but the first thing I noticed was the I2C EEPROMs. They have hardwired address lines for a reason so they can be differentiated from one another. The primary boot EEPROM address is always wired as A2=0, A1=0, A0=0 so the device write address becomes $A2. The other EEPROMs need to have the A2,A1,A0 wired differently from one another and obviously it would make sense to do so in a sequential order so that the next device addresses are $A4 and $A6. BTW, I think that WP needs to be pulled low, not high.

As to the +5V regulator cascading into the 3.3V one there is nothing wrong with doing this in this application. The +5V rail is only ever really powering the keyboard and maybe some light external loads. There is no real load on·the +5V·plus it helps to spread the power/heat dissipation across two regulators.

As the I2C bus is only ever a single master you can omit the pullup for the clock line. You have a resistor going from SDA to reset, a drafting error perhaps? The reset line does not need a pullup as it the BOE line is pulled low and this makes the reset line a weak output/input with internal 5K pullup.

Observe good PCB layout especially for crystal and ground and power-supply decoupling.

As an option it can be a good thing to have a status led and you could make good use of the SCL line if you did not have any others available.

Remember too that if you want good·A/D conversion using sigma-delta that you should have components mounted as·close as possible to the chip, inside if that were possible.

*Peter*



·

Mark Pepper

I have published a simple design for a USB to TTL serial circuit on the Megabitty Yahoo Forum and you can download the Eagle schematics and PCB layout here: http://robosavvy.com/Builders/pepperm/USB Serial V1.zip You should be able to modify the design for your needs. I managed to solder the SMD version of the FT232 by soldering all the pins then removing the excess with solder wick..

Hope this helps

Mark

PVJohn

Thank you guys for your replays. My schematic was "copy/paste design" from PropStick Kit and Demo Board rev. C. I knew that something isn't right since Demo Board rev. C has 10K resistor between SDA and RESn pins, and it's not shown in schematic for PropStick Kit. That resistor is removed in Demo Board rev. D. Also, resistor between SCL pin and VDD has been removed in Demo Board rev. D, and it was shown in PropStick Kit. Demo board rev. C·doesn't have any resistor·on SCL and SDA lines.·Very confusing! I'll leave them in schematic and I'll se if I have to install them on my board. Wish me luck with hand soldering SMD components since I can hardly see them [noparse]:)[/noparse] Addressing 24LC256 ICs is fixed. Now I understand how it works.

Mark, thanks for your files.

Regards,

PVJohn

Javalin

The Parallax object library contains an i2cObject and examples for eeproms, and popular devantech robotics bits (www.robot-electronics.co.uk)
Oh and you shouldn't pull the i2c lines using the reset line, as this prevents the chip doing a controlled reboot.· Best practice is to pull both SDA and SCL lines to +3.3v via a 4.7K resistor - this is documented in the i2cObject referenced above.

James