you see in Figure 2. Instead of
columns of odd numbered and
even numbered pins, the
Digilent Pmod standard uses the
pattern shown in Figure 2. We
must keep this Digilent layout in
mind as we assemble our SPI
portal interface cable.
Digilent supplies a six inch
cable (Part# 250-030) that splits
out the 12-pin female header
layout you see in Figure 2 to a
pair of six-pin female headers.
The female headers on both
sides of the cable are tightly
wrapped with heat shrink tubing.
There is no reason to
remove the heat shrink on the
12-pin side as we will use it as a
reference. Removing the heat
shrink tubing on the six-pin side
reveals plastic housings that can
be manipulated to allow the
nondestructive removal of the
six female header pins.
All we have to do is remove and replace the
pins on the six-pin side of the cable to match the
pinout demanded by the EVE 10-pin male
header. Digilent also provides elongated male
header pins (part# 240-004) that act as a gender
changer, so the 12-pin side of the cable can be
connected to the chipKIT MX3 12-pin female
Note that there are no 5V0 connections at
connector JE on the chipKIT MX3 side. In reality,
there are no 5V0 connections on the EVE side
either. We are, in reality, connecting the second
3V3 pin on the JE connector to the 5V0 pin on
the EVE header. This bypasses the 3. 3 volt
voltage regulator that is part of the EVE
Alternately, we could jumper JPE to output
five volts at JE pins 6 and 12. This would allow us
to feed five volts to the EVE 5V0 pin which is the
input of the onboard 3. 3 volt regulator. I try not
to mix 3. 3 and 5.0 volt rails without reason. The
elimination of the five volt power signal keeps
things simple. The customized cable is shown
doing its thing in Photo 4.
The X Factor
Although the chipKIT MX3 was intended as an MPIDE
hardware component, the chipKIT MX3 can also be
programmed in the traditional way. So, we will build the
FT800 driver using MPLAB X, XC32, and a PICkit 3.
This is validated in Screenshot 1. Now that our
MPLAB X project is set up, let’s walk through putting the
firmware framework together.
It is of utmost importance to establish MCU clock
speeds. The chipKIT MX3’s PIC32MX320F128H is capable
of running at 80 MHz. Let’s make that happen:
June 2014 71
■ Photo 4. A neat and easy way to make a custom cable based on
off-the-shelf Digilent components.
■ Screenshot 1. The more you use MPLAB X, the more you will
grow to like it. As you can see here, I've created a configuration
called FT800_Driver that uses the latest version of XC32 to
build code for a PIC32MX320F128H. The programmer/debugger
of choice is a PICkit 3.