PIC Analog to digital conversion registers

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There are totally four registers for A2d conversion in PIC16f877 and PIC16f877A .They are the following .
(NOTE : Almost all the PIC series of IC's have this same register configuration .So knowing this is enough to program all the PIC series IC's )

registers for analogue inputs.

Name	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
ADRESH	A2D Result Register - High Byte
ADRESL	A2D Result Register - Low Byte
ADCON0	ADCS1	ADCS0	CHS2	CHS1	CHS0	GO/DONE	-	ADON
ADCON1	ADFM	-	-	-	PCFG3	PCFG2	PCFG1	PCFG0

ADCON0 Configuration

Since PIC has 10 bit A2D and its a 8 bit microcontroller it needs an extra register to store the extra 2 bits resulting in the A2D conversion .Thats the job the first two registers ADRESH and ADRESL name itself is self explanatory .
ADRESH:Stores the higher 8 bits .
ADRESL:Stores the lower 8 bits .
So now we are done with the first two registers .

The next register that we are gonna see in PIC16F877A is ADCONO .
The first two bits ADCS1 and ADCS0 is required to select the A2D clock .The table for clock selection is given below .If I am using 5 MHZ I will select 01 thats fosc/8 .

ADCS1	ADCS0	A/D Conversion Clock Select bits.	Max. Clock Freq.
0	0	Fosc/2	1.25MHz
0	1	Fosc/8	5MHz
1	0	FOsc/32	20MHz
1	1	Frc (Internal A2D RC Osc.)	Typ. 4uS

The next set of bits are channel select .YOu can select the appropriate channels by selecting the appropriate bits .So now I am using 5 MHZ and selecting RA2 for example , now bit selection will be 01010xxx .The bits that are named as X are still to be selected .

The next bit is GO/DONE .Setting this bit high starts A2D conversion and it is automatically cleared when it is complete .So monitoring the status of this bit can tell us about the status of Analog to digital conversion .So we have to set it high to start conversion .So now our bit configuration will be 010101xx .

If you look at the first table you will see bit1 is useless .So put a zero there .So now our bit configuration becomes 0101010x .
The last bit is ADON .When the bit is set it turns on the A2D when cleared it turns off .So we are setting 1 there .Now it becomes
01010101 .This is the bit configuration to select 5 MHZ clock channel AN2 .The next three bits are useless .

CHS2	CHS1	CHS0	Channel	Pin
0	0	0	Channel0	RA0/AN0
0	0	1	Channel1	RA1/AN1
0	1	0	Channel2	RA2/AN2
0	1	1	Channel3	RA3/AN3
1	0	0	Channel4	RA5/AN4
1	0	1	Channel5	RE0/AN5
1	1	0	Channel6	RE1/AN6
1	1	1	Channel7	RE2/AN7

ADCON1 Configuration

The next channel is ADCON1 . The bit7 ADFM(Analog to digital conversion format ) is used to select the format .Setting this bit high makes it 10 bit converter .And clearing it makes it 8 bit ie the result only appears in ADRESH and ADRESL is ignored .
Now we are entering thre most complicated part of the register selection .Please look at the following table to select the configuration you need .

Visit WINPIC PROG to learn more in detail about the register selection .

PCFG3: PCFG0	AN7 RE2	AN6 RE1	AN5 RE0	AN4 RA5	AN3 RA3	AN2 RA2	AN1 RA1	AN0 RA0	Vref+	Vref-
0000	A	A	A	A	A	A	A	A	Vdd	Vss
0001	A	A	A	A	Vref+	A	A	A	RA3	Vss
0010	D	D	D	A	A	A	A	A	Vdd	Vss
0011	D	D	D	A	Vref+	A	A	A	RA3	Vss
0100	D	D	D	D	A	D	A	A	Vdd	Vss
0101	D	D	D	D	Vref+	D	A	A	RA3	Vss
0110	D	D	D	D	D	D	D	D	Vdd	Vss
0111	D	D	D	D	D	D	D	D	Vdd	Vss
1000	A	A	A	A	Vref+	Vref-	A	A	RA3	RA2
1001	D	D	A	A	A	A	A	A	Vdd	Vss
1010	D	D	A	A	Vref+	A	A	A	RA3	Vss
1011	D	D	A	A	Vref+	Vref-	A	A	RA3	RA2
1100	D	D	D	A	Vref+	Vref-	A	A	RA3	RA2
1101	D	D	D	D	Vref+	Vref-	A	A	RA3	RA2
1110	D	D	D	D	D	D	D	A	Vdd	Vss
1111	D	D	D	D	Vref+	Vref-	D	A	RA3	RA2

in the next tutorial we are going to kick start with PIC16F877A programming in C .I am gonna use mikroc compiler .

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