and copper planes. Another look at Photo 3 shows that we
have complied it with the antenna rule. The majority of the
unused pins are connected to ground. The remaining
unconnected ETRX2 pins are intended for use as custom
firmware I2C and SPI interfaces. A look-through graphical
view of the ETRX2’s SMT pad layout is presented in Figure 1.
As you can see in Schematic 1, I have attached my
ETRX2 to a PIC18LF2620. You can use any PIC that you
desire. Just make sure it will run at 3. 3 volts. I’m using the
PIC18LF2620 here because it is a plain Jane PIC with plenty
of punch. Another reason for choosing this particular PIC is
that the HI-TECH PICC- 18 PRO code we use to drive it can
be easily ported to any other PIC microcontroller.
The hookup between the ETRX2 and the PIC consists of
only two wires. All of the data and command transfers
between the ETRX2 and the PIC take place over a simple
19200 baud RS-232 connection. The baud rate is based on
the PIC’s 4 MHz internal system clock. If you need to run
faster, use the PIC18LF2620’s PLL to double the internal
oscillator’s frequency to 8 MHz. If you still feel the need for
speed, load up the 20 MHz crystal and its supporting
capacitors. After installing the crystal and capacitors, be
sure to change the PIC’s oscillator fuse setting in the code
to reflect the use of the crystal instead of the internal clock.
There are only two ETRX2 pins you may think you need
to connect that should not be electrically attached to other
components in the system. The ETRX2’s VREG pin is an
outlet for the internal regulated voltage and should not be
presented with a load. A pullup resistor can be attached to
the ETRX2’s RESET pin if you have excessive noise in your
system. Otherwise, the RESET pin’s internal pullup will
suffice. If things are noisy in your circuitry, my advice is to
fix it at the source. If the noise is affecting the ETRX2, it will
adversely affect other components in the system, as well.
There are no special communications firmware drivers for
I2C and SPI. So, it’s obvious that we also need to leave pins
16 through 19 flying in the wind. If you want to take full
advantage of the ETRX2’s sleep mode, you may want to
pull down the MOSI pin (pin 18) with a 10K resistor. The
pulldown resistor prevents the MOSI pin from floating and
+6VDC to +9VDC
LM2940
VR1
5.0V LM3940
VR2
3.3V
J1
IN OUT
GND
IN OUT
GND
NOTES:
1. Y1, C10 , C11 ARE OPTIONAL
C1 +
.1uF C2
220uF
C3
220uF
+ C4
.1uF
C5
220uF
+ C6
.1uF
3.3V
ICSP CONNECTOR
66 33
55 22
44 11
R3
10K
C8
.1uF
SCHEMATIC 1. The ETRX2 can be used as an
I/O extender as it includes a pair of on-board
analog-to-digital converter inputs and a dozen
general-purpose I/O lines.
U1
1 MCLR
R1
100
R2 1K
RA0 2
RA1 3
RA2 4
RA3 5
RA4/T0CKI 6
RA5 7
OSC2/RA6 10
C7
.1uF
C11 20pF
U2
4 I/O9
5 VREG
6 GND
7 VCC
8 GND
9 A/D1
10 A/D2
11 I/O7
12 I/O6
13 I/O5
14 I/O4
ETRX
I/O11 31
I/O10 30
GND 29
RXD 28
TXD 27
I/O0 26
I/O1 25
RESET 24
I/O3 23
I/O2 22
I/O8 21
Y1 20MHz
28 RB7/PGD
27 RB6/PGCOSC1/RA7 9
RB0 21
RB1 22
RB2 23
RB3 24
RB4 25
RB5 26
RC0 11
3.3V RC1/CCP2 12
RC2/CCP1 13
20 RC3 14 C9 VDD RC4 15
RC5 16.1uF RC6/TX 17
RC7/RX 18
C10 20pF
8 GND
19 GND
PIC18LF2620
52 SERVO 09.2008
