e. How do I use PIC18FxxJ50 A/D converter with USB memory read (DateMemRead) and write (DataMemWrite) commands?

ADFM = A/D result format select bit

“0” – left justified result (default)

“1” – right justified result

ADCAL = A/D calibration bit

“0” – A/D conversion

“1” – Will be performed on next A/D conversion (bit must be cleared by SW after the calibration)

<ACQT2 ACQT1 ACQT0> = A/D acquisition time

111 = 20 TAD

110 = 16 TAD

101 = 12 TAD

100 = 8 TAD

011 = 6 TAD

010 = 4 TAD

001 = 2 TAD

000 = 0 TAD

T_AD = A/D conversion time per bit. A/D acquisition time is defined on basis of conversion time per bit. This is the minimum time to wait before the next acquisition starts.

< ADCS2 ADCS1 ADCS0> = A/D Conversion Clock Select bits

110 = FOSC/64

101 = FOSC/16

100 = FOSC/4

011 = FRC (clock derived from A/D RC oscillator)

010 = FOSC/32

001 = FOSC/8

000 = FOSC/2

FOSC is microcontroller CPU oscillator frequency. A/D Conversion Clock Select bits combination defines A/D clock frequency divider. Higher crystal resonator frequency requires a lower conversion clock frequency (higher divider).

Here is a sample program to execute A/D conversion and acquire the 10-bit result using DataMemWrite, DataMemRead, SetDataMemBit and ClearDataMemBit commands:

Dim ADchannel As Byte = 0 ' Select an analog input channel

Dim result As Int16

Const ADCON0 as int16 = 0xFC2

Const ADCON1 as int16 = 0xFC1

Const ANCON0 as int16 = 0xF48

Const ANCON1 as int16 = 0xF49

Const ADRESH as int16 = 0xFC4

Const ADRESL as int16 = 0xFC3

If PIC.IsConnected(&H4D8, &HE001) Then

If Not PIC.IsConnected() Then ' Connect microcontroller if not connected

If PIC.Open(&H4D8, &HE001) < 0 Then

goto Error

End If

End If

Dim PIC As SVLib_PIC18F24J50.SVPICAPI = New SVLib_PIC18F24J50.SVPICAPI

PIC.DataMemWrite(ADCON0, 1 Or (ADchannel << 2))

PIC.DataMemWrite(ADCON0, 0x9E) ‘ Set ADC operation mode

PIC.SetDataMemBit(ADCON0, 1) ' Start a conversion

While (PIC.DataMemRead(ADCON0) And 2) = 2 ' Wait until the conversion is finished

End While

result = (PIC.DataMemRead(ADRESH) << 8) Or PIC.DataMemRead(ADRESL)

End If

Exit ....

Error:

Result is a 10-bit value. The example above is without error control and will malfunction is an error occurs. Please, include “if” sentences to verify each of the 4 data memory access functions for negative values upon execution. A negative value is an error code. Here is an example on how to include error code verification:

If PIC.DataMemWrite(ADCON0, 0x9E)<0 then

goto Error

End If

Dim x As Integer

x = PIC.DataMemRead(ADCON0)

If x < 0 Then

goto Error

End If

Label1.Text = CStr(x) ' print out a decimal value of a memory location