AN200

PicStic 5

Accessing the 10-bit and 12-bit ADC

December 4, 2001

Introduction:  This application note illustrates how to access the internal 10-bit and external 12-bit ADC on the PicStic 5.

Background:  The PicStic 5 is equipped with a total of 6 channels for analog-to-digital conversion. There are four 10-bit channels and two 12-bit channels. The four 10-bit channels are supplied by the PIC16F876 processor. While the PIC processor is equipped with five 10-bit channels, only four of these channels are available on the PicStic 5. The two 12-bit channels are available through a separate IC internal to the PicStic 5 but external to the processor. Generally, the two main characteristics that are looked at when choosing an ADC is speed and resolution. With the PicStic 5 both are available. The two 12-bit channels give the user the great resolution at a fast rate ( up to ~10K samples/sec). The four 10-bit channels give the user good resolution at much higher data rate ( up to ~21K samples/sec). The user has the choice of which ADC best suits the given application.

How it works:   The four 10-bit channels can be easily accessed through the PicBasicPro command ADCIN.  There are only two steps that need to be followed before a 10-bit channel can be sampled:

1.        The appropriate ADC pins need to be set as inputs. This is done through the TRISA register.

2.        The pin function needs to be selected by setting the ADCON1 register.

Once these two steps are completed, the 10-bit channels can be sampled. The available settings for the ADCON1 register are located on page 11 of the PicStic 5 data sheet. For a more detailed description of the ADCIN command please refer to the PicBasic Pro Programmers Reference Manual.

     The 12-bit channels are accessed using a call routine that is included when a PicStic 5 program is compiled using PicBasic Pro. These routines were written and incorporated into PicBasic Pro to simplify the use of the external ADC.

     When a channel is ready to be sampled all that needs to be done is to execute the call for that channel. The result will be returned in the predefined variable PS5ADC. There are four different routines that can be called to access the two 12-bit channels. These calls are listed and described on page 14 of the PicStic 5 data sheet.

Program Listing: 1 of 2

‘ This program will illustrate how to use the two 12-bit channels of the PicStic 5.

‘ All four of the callable routines will be executed to show their uses.

‘ The hardware serial port will be used to display the results of the conversions on a PC.

‘ The hardware serial port defaults to 8N1 @ 2400Bps.

ANY_KEY    VAR  BYTE                                    ‘ program to read the A/D

START:  

HIGH PORTC.1                                                                     ‘ Enable the transmitter

PAUSE 100                                                                            ‘ Tenth sec pause to enable trans.  

HSEROUT ["Verifying that the ADC call works.",13,10]

HSEROUT ["Press any key to continue.",13,10]

HSERIN  [ANY_KEY]                                                         ‘ Wait for user to confirmation

PAUSE 1000                                                                          ‘ One second pause

CALL AIN5                                                                           ‘ Sample channel 0

HSEROUT ["Channel 0 of the 1298 reads ", dec PS5ADC, 13,10]

PAUSE 1000                                                                          ‘ Print results serially

CALL AIN6                                                                           ‘ Sample channel 1 of LTC1298

HSEROUT ["Channel 1 of the 1298 reads ", dec PS5ADC, 13,10]

PAUSE 1000                                                                          ‘ Print results serially

CALL AD01                                                                          ‘ DIFF read of CH0 to CH1

HSEROUT ["Channel DIF of the 1298 reads ", dec PS5ADC, 13,10]

PAUSE 1000                                                                          ‘ Print results serially

CALL AD10                                                                          ‘ DIFF read of CH1 to CH0

HSEROUT ["Channel DIF of the 1298 reads ", dec PS5ADC, 13,10]

PAUSE 2000                                                                          ‘ Print results serially

END

Program listing: 2 of 2

‘ This program will show how to sample the four 10-bit channels using PicBasic Pro.

‘ The hardware serial port will be used to display the results of the conversions on a PC.

‘ The hardware serial port defaults to 8N1 @ 2400bps.

Define     ADC_BITS                             10            ' Sets the number of bits in the result

Define     ADC_CLOCK                        02            ' Selects clock source

Define     ADC_SAMPLEUS                50            ' Sample time in µSec

TRISA = 255                                                          ' Set PORTA to outputs

ADCON1 = %10000000                                        ' Select right justify for the results

RSLT      VAR       WORD                                    ' Variable to store the results

SAMPLE VAR       BYTE                                      ' Variable to select the channel to be sampled

ANYKEY VAR      BYTE     

HIGH PORTC.1                                                    ' Enables the serial transmitter

PAUSE 100

START:

                FOR SAMPLE = 0 TO 3                       ' Selects the channel to be sampled

                ADCIN   SAMPLE,RSLT                     ' Go sample the selected channel

                HSEROUT [HEX RSLT,10,13]             ' Display the result followed by CR,LF

                NEXT SAMPLE                                    ' Go do the next channel

                HSERIN [ANYKEY]                             ' Press any key to continue

                GOTO START                                      ' Start again

END