Friday, April 22, 2011


The niblet is a small card designed for the tanukifu alpha backplane (DIN 41612 based with non-standard pinout).  The primary purpose is for adding terminations for the differential pairs and single ended lines (which could be very different depending on the actual board production quality and the other cards attached to the system - not a problem at lower frequencies, but very important at high ones).

The differential pairs are placed on the top copper layer (red lines here) and the single ended lins are placed on the lower copper layer (green lines here).  I know it would be much better to use more than 2 layers, but part of this project is trying to see how things actually work in reality (not just on paper) and 2 layers are much cheaper to have produced (and I really want to see what kind of performance I can get out of two layers).  The copper fills are not shown here.

If we look more closely are the differential pairs here you can see that there are two sets of pads for each pair... This will make it significantly easier to look at fine grain adjustments to the resistance and will project access points for testing.

Looking at the single ended lines you can see that I put two pads (one for binding to ground and one for binding to 3v3).  These may be used with resistors to drain current (limiting the magnitude of reflections), or to pull up/pull down the line, or to set the nominal voltage level.  They may also be used to adjust the line capacitance (allowing for slew rate adjustment).  There is also an access point that can be used for prototyping or signal access.

The msp430 controller section is just like the template stub and serves as a means to control the activation state of the card over I2C/TWI from the backplane.  Here the 6 available lines are brought out to access points for prototyping.  If the card is only being used for termination these parts can be left empty on the board.  To the left is the 3v3 LDO section, which can also be left unfilled if unneeded.

Since the board space was available I added access ports for the bus voltage supply lines (1,2 and 3), for 3v3 with ample ground points.  I also added access to the board I2C/TWI lines and the bus SPI lines.  Again, if the board only needs to be used for termination then these components do not need to be placed.

The schematics and board files are here along with other files associated with the tanukifu project.

The goal of the Tanukifu project is to build a flexible and open set of tools to explore system development for purely educational/academic reasons.