Nuts & Volts - April 2005

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 ₅₂

PGD ₄₁

63

52

41

R1

RESET 10K

D1

MCLR

C1

.1

1N5819

U2 RXIN ₁₃RTSIN 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-

₁₂RX ₉RTS ₁₄TXOUT ₇CTSOUT

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 ₅₂PGD ₄₇

36

35

43

44

45

46 TX ₃₇RX ₃₈D00 ₇₂D01 ₆₉D02 ₆₈D03 ₆₇D04 ₆₆D05 ₆₅D06 ₆₄D07 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

^C1720pF

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 ₂D04 ₃D05 ₄D06 ₅D07 ₆CE1 ₇A10 8

OE ₉

MCLR A09 ₁₀

A08 11

A00 + 3.3VDC A07 12 A01 ₁₃A02 C14 A06 14 A03 A05 ₁₅A04 .1uF A04 16 A05 A03 ₁₇A06 A02 18 A07 A01 ₁₉

A00 20

D00 ₂₁

D01 ₂₂

D02 ₂₃

24

CTS ₂₅RTS

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