Designing a motherboard

I was very impressed at how easy it had been to make a simple sig-gen with this card but now I wanted something a bit more permanent. So over the next few weeks I decided to design a "motherboard" for the card. This would hold the PIC uController and a few extra chips to provide extra functions.

I decided on the following:

• Keypad to allow direct entry of frequencies
• Rotary encoder (mouse wheel) input
• 1x16 large format backlit LCD display (HD44780 compatible)
• Analogue input with 9kHz low-pass filter for modulation input
• 1 TTL input for FSK, QSK, serial comms
• 1 TTL output for scope synchronisation with "wobbulator" 
• PIC16F876A uController
• ICSP - in circuit serial programming - to allow programming of the PIC device whilst on the board - saves on bent and broken pins

Over the next few weeks I spent a good while "designing" this...well trying bits out on breadboards then adding them in to the schemas...Here's the schemas I came up with: (click on pictures for full size)

Keypad, LCD and rotary encoder connectors

 

Modulation input and filter

Main Processor board and ICSP

After testing there are a few problems with this design - none major! 

1) The rotary encoder doesn't work when the ICSP programmer is connected - this was deliberate as I wanted to re-use the I/O pins. If you would like to build a similar board a 40-pin uController might be a better idea!

2) The mod-in filtering is pretty rubbish. This is supposed to be an anti-aliasing filter for the A-D converter, it should have a sharp cut-off at 9kHz, whereas this gives a pretty gentle cut off. However for my purposes it is fine!

3) This uses the DDS-60's internal 5V regulator to supply the power for the uController, LCD display (not backlight) and all the other bits and bobs. Whilst this shouldn't over tax the regulator a nicer (less mean) designer might add a separate 5V regulator!