MANAGING YOUR MOBILE MONKEY
movement of data between it and your PIC.
The uDrive requires a power source between + 3. 6 VDC
and + 6.0 VDC but only a paltry 23 mA of current. I actually
used a + 6.0 VDC AA battery pack to power my entire
design, which includes a PIC18F2620 microcontroller.
Take another look at Photo 1. The component just
above pin 1 of the five-pin header is a diode, which is in
series with pin 5 — the uDrive’s incoming power source
portal. The diode is there to prevent damage if the power
is applied with the wrong polarity. If power is attached
correctly, the blocking diode feeds the input of a + 3.3-volt
linear regulator. How do I know this? The letters “LORA”
are embossed on the regulator. A datasheet search exposed
the voltage regulator as a National Semiconductor LP2985.
I revealed the identity of the LP2985 voltage regulator with
a clue provided by the uDrive’s I/O specifications.
The uDrive communicates serially with a host processor
using its TX and RX pins. The RS-232 voltage levels for the
serial I/O are at 3. 3 volts according to the user manual. The
3. 3 volt RS-232 levels set me on the path of a + 3. 3 volt, five-pin SOT-packaged linear voltage regulator. I found a package
description of “LORA” in the LP2985 datasheet which, according to the datasheet, can be found only on the National
Semiconductor LP2985AIM5-3.3. With this information, I
was able to match the uDrive’s voltage regulator input and
ground pins to the LP2985 datasheet connection diagram.
The LP2985 is a low-dropout linear regulator, which
explains the minimum input voltage of + 3. 6 volts. The inclusion
of the + 3.3-volt linear voltage regulator in the uDrive circuitry
allows it to be used with + 5.0 volt microcontrollers and their
+ 5.0 volt peripherals. The only external component that is
recommended for + 5.0 volt systems is to add a 100Ω to 220Ω
resistor in series with the uDrive RX I/O pin. The uDrive can be
part of any + 3. 3 volt microcontroller system, as well. If you
absolutely have the need for a + 3. 3 volt system, you can choose
to bypass the uDrive’s onboard voltage regulator. I don’t
+6VDC to +9VDC
LM2940
VR1
5.0V LM3940
VR2
5.0V
3.3V
J1
C1 +
.1uF C2
220uF
IN OUT
GND
C3
220uF
+ C4
.1uF
IN OUT
GND
PIC VOLTAGE SELEC T JUMPER
R1 + 3.3V
390
C5
220uF
+ C6
.1uF
JP2
VCC
LED1
+ 5.0V
VCC
ICSP CONNECTOR
+ 5.0V
63
56 32
45 21
41
R3
10K
C8
.1uF
uDRIVE-uSD-G1
C7
.1uF
VCC
R1
100
OPTIONAL
390
RESET
GND
RX
TX
VCC
R2 1K
U2
2
5
4
3
1
1
MCLR
2
RA0 3
RA1 4
RA2 5
RA3 6
RA4/T0CKI 7
RA5 10
OSC2/RA6
OPTIONAL
ACTIVITY LED
C10
.1uF
C11 20pF
Y1 20MHz
28
RB7/PGD
27 9
RB6/PGC OSC1/RA7 21
RB0 22
RB1 23
RB2 24
RB3 25
RB4 26
RB5 11
VCC RC0 12
RC1/CCP2 13
20 RC2/CCP1 14
C9 VDD RC3 15
RC4 16
.1uF 8 RC5 17
19 RC6/TX 18
RC7/RX
C12 20pF
VCC
R5
10K
RESET
TX
RX
R4 200
GND
GND
PIC18F2620/PIC18LF2620
JP1
SCHEMATIC 1. Even with a
uDrive-uSD-G1 in tow, nearly
all of the PIC18F2620’s I/O is
available to you for your
application. If you need to
make the design more “green,”
you can eliminate the indicator
LEDs. Using the “LF” variant of
the PIC18F2620 will also allow
you to save some power as
the PIC18LF2620 can run the
20 MHz clock at + 3. 6 volts.
The 20 MHz crystal can also
be eliminated as both the
PIC18F2620 and the
PIC18LF2620 contain
internal CPU clocks.
SERVO 05.2008 51
