The Design Cycle
connector is wired as data communications equipment (DCE) to eliminate the need for a crossover serial
cable or null modem when connecting to data terminal equipment
(DTE) devices like your personal
computer’s serial ports.
You may wonder why a serial
port would be something necessary
to have on an 802.11b wireless
device. Well, it’s not really necessary,
but it was great to have when I was
writing and debugging the 802.11b
driver code. It’s a nice luxury to have
when you want to mark places in the
code that “talk back” using the serial
port during normal 802.11b wireless
operation. It’s also good to have serial ports when writing columns like
this, as you can dump data to a termi-
nal emulator and physically see what
is going into and coming out of the
AirDrop-P.
Let’s move on to the microcontroller. The PIC18LF8621 is the low-voltage, lower power cousin of the
PIC18F8621. Both the PIC18LF8621
and the PIC18F8621 are capable of
operating with a maximum clock frequency of 40 MHz with a Vdd of + 5. 5
VDC. However, the AirDrop-P is powered with a 3.3-VDC power supply
and the PIC18LF8621’s maximum
clock frequency is reduced to a bit
over 25 MHz. Here’s the
PIC18LF8621 maximum clock frequency versus maximum applied Vdd
equation:
Fmax = ( 16. 36 MHz/V) (Vddappmin
– 2.0 V) + 4 MHz
where:
Fmax = Maximum operating
clock frequency
Vddappmin = Minimum Vdd
power supply voltage
Fmax = ( 16. 36 MHz/V) ( 3. 3 V –
2.0 V) + 4 MHz = 25.268 MHz
To guarantee stability, I decided
to run the AirDrop-P clock at 20 MHz.
If you can handle changing the
CompactFlash card’s mandatory I/O
delays, you can run your AirDrop-P at
the clock rate ceiling if you desire.
The PIC18LF8621 ICSP programming interface is standard Microchip
issue. The AirDrop-P is designed to be
programmed and debugged easily
Figure 2. The AirDrop-P hardware design.
+ 3.3VDC
ICSP CONNECTOR
J1
63
PGC 5 2
PGD 4 1
63
52
41
R1
RESET
10K
D1
MCLR
C1
.1
1N5819
U2
RXIN 13
RTSIN 8
CTS 10
TX 11
1
C4
.1uF
3
2
C6
.1uF
R1IN R1OUT
R2IN R2OUT
T2IN T1OUT
T1IN T2OUT
C1+ C2-
C1-
V+
C2+
V-
12 RX
9 RTS
14 TXOUT
7 CTSOUT
5
4
6
C5
.1uF
C7
.1uF
SP3232
SP3232 PIN 16 = Vcc
SP3232 PIN 15 = GND
SP3232 BYPASS CAP
+ 3.3VDC
C8
.1uF
RS-232 CONNECTOR
9
RTSIN 7
5
RXIN 3
1
9
8
7
6
5
4
3
2
1
8
6
4
2
CTSOUT
U1
30
29
28
27
34
33
58
57
56
55
54
53
PGC 52
PGD 47
36
35
43
44
45
46
TX 37
RX 38
D00 72
D01 69
D02 68
D03 67
D04 66
D05 65
D06 64
D07 63
4
3
78
77
76
75
74
73
TXOUT
J2/DB9 FEMALE
+6-9VDC
J3
D2
1
1N5819
2
C2 +
10uF
VR1
LM1086CS- 3. 3
13
+ 3.3VDC
R2
IN
OUT
GND
332
C3 +
10uF
RA0 OSC1
RA1
RA2
RA3
RA4 OSC2
RA5
RB0 RF0
RB1 RF1
RB2 RF2
RB3 RF3
RB4 RF4
RB5 RF5
RB6 RF6
RB7 RF7
RC0 RG0
RC1 RG1
RC2 RG2
RC3 RG3
RC4 RG4
RC5 MCLR/RBG5
RC6
RC7 RH0
RH1
RD0 RH2
RD1 RH3
RD2 RH4
RD3 RH5
RD4 RH6
RD5 RH7
RD6
RD7 RJ0
RJ1
RE0 RJ2
RE1 RJ3
RE2 RJ4
RE3 RJ5
RE4 RJ6
RE5 RJ7
RE6
RE7 VDD
AVDD
VDD
VDD
VDD
VSS
AVSS
VSS
VSS
VSS
PIC18LF8621
LED1
2
C16 20pF
49
Y1
20MHz
C17 20pF
50
24
23
18
17
16
15
14
13
5
6
7
8
10
9
79
80
1
2
22
21
20
19
62
61
60
59
39
40
41
42
12
25
32
48
71
OE
IORD
REG
RESET
IREQ
WE
IOWR
A08
A09
A10
J4
1
D03 2
D04 3
D05 4
D06 5
D07 6
CE1 7
A10 8
OE 9
MCLR A09 10
A08 11
A00 + 3.3VDC A07 12
A01 13
A02 C14 A06 14
A03 A05 15
A04 .1uF A04 16
A05 A03 17
A06 A02 18
A07 A01 19
A00 20
D00 21
D01 22
D02 23
24
CTS 25
RTS
GND CD1
D03 D11
D04 D12
D05 D13
D06 D14
D07 D15
CE1 CE2
A10 VS1
OE IORD
A09 IOWR
A08 WE
A07 IREQ
VCC VCC
A06 CSEL
A05 VS2
A04 RESET
A03 WAIT
A02 INPACK
A01 REG
A00 BVD2
D00 BVD1
D01 D08
D02 D09
WP D10
CD2 GND
26
27
28
29
30 + 3.3VDC
31
32 CE2
33
34 IORD
35 IOWR
36 WE
37 IREQ
38
39
40
41 RESET
42
43
44 REG
45
46
47
48
49
50
+ 3.3VDC
C15
.1uF
CF CARD CONNECTOR
+ 3.3VDC
C9 C10 C11 C12 C13
11.1uF .1uF .1uF .1uF .1uF
26
31
51
70
AirDrop-P
APRIL 2005
31