The Third Revision with A/D:

The third, and final, board in this series (see schematic 3 and Fig. 3) adds the ability to use the A/D convertor which is present on several of the Rabbit 4xxx Core Modules.  This board needed some more room for the analog input circuit and connector so it is a little bit wider than the RCM41xx.  However, it can still be implemented with a two sided board.

Much more going on here.

The A/D on the RCM4xxx devices is the ADS7870 from TI.  It has 11/12 bits of resolution, eight inputs and a Programmable Gain Amplifier.  The inputs can be used in a single ended configuration or they can be paired to measure differential voltages.  All the voltages applied to the input of the ADS7870 must be positive so if you need to measure negative voltages you will need to implement some external circuitry.

The input circuit is a voltage divider that allows for an input range of 0 to +10 volts.  This input range can very easily be changed by modifying either or both of the input divider resistors as required.  The ADS7870 allows you the choice of any of these three voltage reference values: 1.15, 2.048 or 2.5V.  The schematic shows the divider with 100K and 33.3K resistors.  This provides a voltage division of four, so with a reference of 2.5V you will get a 0 to 10V range.  These same resistors will also allow a full scale input of 4.60V if the 1.15V reference is selected.  You can customize these resistor values on a per input basis if your application has different ranges to measure.

Notice that the sequence of the inputs on the connector is not channel 0, 1, 2…  I did this in order to make routing the traces easier.  If having the connector pins and the channels sequence together is important to you then you will need to do some rerouting.  I basically made an engineering decision that it was not important enough to be concerned about.

Another “feature” of this particular implementation is that it can easily be built to make use of eight additional parallel I/O lines on those RCMs that do not have the A/D.  For those RCMs Parallel Port D is available on the pins that would otherwise have the A/D inputs on them.  You will have to exercise some additional caution if you decide to make use of this feature.

When using a pin as an input you have several options and one limitation.  The options deal with how you use the voltage divider circuit on each of the eight pins.  If you leave the resistors as they are defined on the schematic then you will be able to sense digital inputs of 13.2V (4 x 3.3V).  You can adjust the ratio of the resistors to meet whatever input voltage range you require – just remember that the trip point of the typical CMOS circuit is typically VDD/2.   Whatever you choose you should always have a resistor from the input pin of the Rabbit to ground (or power) so that the input is not left floating.

The limitation is, since there are no clamp diodes, you need to insure that the voltage applied to the Rabbit I/O pin does not exceed 3.3 volts.

To use a pin as an output you will probably want to use a zero ohm jumper in place of the series resistor if you are going to use the CMOS signal directly.  Or, if you want to drive a transistor, you may want to put in a current limiting resistor whose value is determined by the circuit it is driving.  You should be aware that the maximum current per output pin is 8ma sinking or sourcing.

Something to keep in mind if you decide to use this feature is that the Port D pins can be individually configured as either input or output.  This allows you to easily extend the functionality of the board without redesigning the PCB.

A Few Conclusions

• It was fun finding out that it is not very difficult to develop a minimal function SBC
• It did not take very long – the initial design and layout took about 4 hours
• The result is a small, useful and inexpensive (two board) SBC.

Third Schematic (Click for high resolution)