detect logic level. In that
controlled microSD card is
seen as an SPI portal to
the PIC32, there isn’t
much hardware work to
be done in this area. The
minimal “other” microSD
card hardware setup can
be seen in the contents of
What you don’t see in
the schematic is the SPST
switch between pins 9 and
10 of MSD95. The switch
is an integral part of the
microSD socket. When a
card is present, the switch
The switch feeds the
gate of the MOSFET.
Depending on the
JumperX settings, the MOSFET is either turned ON or
OFF by the state of the card detect switch. Normally,
the gate of the MOSFET is tied high and the SPST
switch within the microSD card socket is jumpered to
go logically low when closed.
The 25LC1024 EEPROM signals are shared with the
PIC32MX795F512L’s microSD card. The EEPROM is
selected using the PIC32’s EE-CS I/O pin.
All Powerful Ethernet
Despite all of the SPI portals, UARTs, clocks, and
timers that the PIC32MX795F512L possesses, its most
powerful asset is its Ethernet interface. As you can see
in Schematic 6, the Ethernet circuitry isn’t particularly
complex. However, by including this circuitry, we
instantly have the ability to contact other network-capable nodes on an LAN or over the Internet. Our
board can suddenly become a TCP/IP server or client.
We can use the Ethernet portal to serve web pages.
We can send email messages. We can synchronize with
Internet time servers. If necessary, we can even
bootload the PIC32MX795F512L over an Ethernet
connection. The inclusion of the LAN8720A interface is
well worth the loss of a few of the PIC32’s I/O pins.
No data will flow and no LEDs will blink if the power
supply is inadequate. So, we must design in a robust and
noise free power supply circuit. Fortunately, that spectrally
clean power system consists of only three components.
If you include the optional power indicator LED and
its current-limiting resistor, the parts count rises to five
Wired Ethernet portals are typically power hungry.
This design will draw around 250 mA while running the
Microchip TCP/IP stack with all LEDs illuminated. The
Micrel MIC29150 won’t even break a sweat with that
load. All five components plus the power jack make up
August 2015 63
■ Schematic 4. This is an implementation of the OpenLog, which can be purchased as a
module from SparkFun. Any serial input is directed to the microSD card by the firmware
contained within the ATmega328P microcontroller.
■ Schematic 5.
blocks allow us to
set the card
detect logic level.
Jumper2 is tied
logically low, and
Jumper1 is tied
This results in an
active low card
The "board" printed circuit board