of our user-controllable LEDs through a NUD3105
MOSFET relay driver. The NUD3105 includes all of the
necessary bias resistors and steering diodes on the chip.
So, we don’t have to mount the gate and drain resistors
normally associated with standard MOSFET switch circuits.
The power supply design will be simple and rugged. A
Micrel MIC29150 will provide a regulated + 3. 3 VDC
power supply rail. The MIC29150 is capable of supplying
up to 1.5A of current from input voltages as high as 26
VDC. This will allow us to power the circuitry from
common wall wart power bricks.
The PIC32 was not only chosen for its large number
of UARTs. This particular microcontroller also contains an
on-chip Ethernet MAC and is capable of driving an
external PHY device. In this case, our choice of external
PHY devices is provided by Microchip in the form of the
SMC LAN8720A. The addition of the LAN8720A gives
our device the capability of web serving, acting as a
TCP/IP client or server, and providing email (SMTP), time
(SNTP), UDP, and FTP services.
Timing is always important when it comes to
microcontroller circuitry. Time is also important to
humans. To appease both human and silicon, the
PIC32MX795F512L’s internal RTCC (Real Time Clock and
Calendar) will be attached to an external 32.768 kHz
crystal. This will provide a real time clock that can be used
for various timing requirements. For instance, the one
second tick from the RTCC can be used to blink an “I’m
alive” LED. The finished product is shown in Photo 1. To
make the assembly you see in Photo 1 possible, we had
to combine and interconnect a number of electronic
The Heart of the System
Schematic 1 is a graphical depiction of the
PIC32MX795F512L and its supporting resistors,
capacitors, crystals, switches, LEDs, and LED drivers. The 8
MHz CPU clock crystal is the seed for the PIC32’s PLL,
which ramps up the CPU clock frequency to 80 MHz. The
PIC32MX795F512L’s internal peripherals are also clocked
at 80 MHz. Coupled with the PIC32’s immediate I/O
commands (LATxSET, LATxCLR, LATxINV), the 80 MHz
clock makes for some very fast I/O switching.
You can also see how clean the LED NUD3105
MOSFET driver circuits are. All we have to do is connect
an NUD3105 to its respective PIC32MX795F512L I/O line
and write the code to drive it.
The NUD3105 also allows us to directly drive small
relays in the same way we are driving the LEDs.
ADVANCED TECHNIQUES FOR DESIGN ENGINEERS
August 2015 61
■ Schematic 1.
There are 100
pins on the
package, and we
can access almost
every one of them
in one way or
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