How to increase the 8-bit A/D in PIC16C711 to 11-bit

At present, the A/D embedded in the microcontroller is generally 8-bit to 10-bit, which is difficult to meet the high-resolution requirements in signal processing applications; and the external high-resolution A/D will significantly increase the cost, because the price of the A/D converter will increase exponentially with the increase of its bit number. This article introduces a method to improve the resolution of the A/D on the PIC16C711 microcontroller chip, increasing the 8-bit A/D on the PIC16C711 chip to 11 bits. This method is also applicable to other microcontrollers in the PIC series.

The 8-bit MCU PIC16C711 launched by Microchip of the United States is a MCU with a high performance-price ratio. It is low-priced, small in package, uses CMOS technology, has OTP type, and is very convenient to develop. It contains 4 8-bit high-speed A/Ds. After expanding it to 11 bits, the performance-price ratio of the MCU application system can be greatly improved.

1. Hardware Circuit

The schematic diagram of the expansion is shown in Figure 1. Eight precision resistors R of equal value divide the 0-5V reference voltage into eight levels, each with a range of 0.625V. The eight taps are connected to the eight input terminals of the CD4051 eight-to-one analog switch, and the channel selection control terminals C, B, and A are controlled by RB2, RB1, and RB0 of the PIC16C711 microcontroller, respectively. IC2 and IC3 are high input impedance op amps, IC2 forms a follower, and IC3 forms a precision differential amplifier.

2. Working Principle

The 11-bit A/D conversion is achieved by PIC16C711 doing two 8-bit A/D conversions. PIC16C711 has four analog input channels RA0~RA3, which multiplex a sample-and-hold device into the A/D converter. The reference voltage Vref can come from the outside or from the internal VDD. The A/D converter is a successive approximation type, and the conversion result (8 bits) is stored in the ADRES register. Before the A/D conversion, it is necessary to select the appropriate channel and set enough sampling time. The user can control the conversion process by setting the A/D control registers ADCON0 and ADCON1, and the status of the A/D conversion will also be reflected in ADCON0.

First, send the voltage Vi to be converted to the RA0 channel of PIC16C711 for an A/D conversion. According to the digital quantity obtained by the conversion, the software calculates the position of Vi in the 8 gears, and subtracts the starting voltage of the gear where Vi is located from Vi. The difference is amplified by 8 bits to convert it into a 0-5V voltage signal, and then sent to the RA1 channel of PIC16C711 for an A/D conversion. The digital quantity obtained is the lower 8 bits of the 11-bit AD conversion, and the gear position CBA is the upper 3 bits, thus realizing 11-bit A/D conversion.

An example is as follows: Assuming the input voltage Vi is 3V, the program controls the RA0 channel of PIC16C711 to perform the first A/D conversion, and the result is 153, that is, 3/5×255=153, corresponding to the 8-bit digital quantity 10011001. Mask the lower 5 bits to get 10000000, and shift right 5 times to get 00000100. At this time, the lower 3 bits correspond to the gear position when Vi=3V voltage, that is, the 4th gear, CBA=100. The program outputs this gear position from RB2, RB1, and RB0 as the channel selection of the 8-to-one analog switch, so that CD4051 outputs 2.5V, that is, 5/8×4=2.5V. This voltage is followed by the IC2 op amp and then differentially amplified by IC3. When designing, adjust the resistance values ​​of R11, R12, R13 and R14 to make the amplification factor 8 bits, and the amplifier IC3 outputs Vo = 8 (Vi-2.5) = 8 (3-2.5) = 4.0 V. Select RA1 channel for the second A/D conversion, and the result is 11001100, which is the lower 8 bits of the 11-bit A/D conversion result. The gear position 100 is the upper 3 bits of the 11-bit A/D conversion, and the result of the 11-bit A/D conversion is 10011001100.

III. Procedure

The following is a program segment that uses the PIC16C711 instruction system to complete an 11-bit A/D conversion.

BSF STATUS, RP0; Select page 1

MOVLW 00000010B; RA0 and RA1 are analog channels

MOVWF ADCON1 ; internal reference voltage

BCF STATUS, RP0; Select page 0

MOVLW 11000001B ; Select RA0 channel and internal clock

MOVWF ADCON0

MOVLW .125 ; Delay 125μs, wait for input

CALL WAIT ; stable

BSF ADCON0, GO; Start A/D

LOOP BTFSC ADCON0, GO; Determine whether A/D is completed

GOTO LOOP

MOVF ADRES, W; get the conversion result

ANDLW 0E0 ; shield the lower 5 bits

MOVWF D1 ; store in D1

BCF STATUS，C

RRF D1,1; shift right 5 times

RRF D1，1

RRF D1，1

RRF D1，1

RRF D1，1

MOVF D1，W

MOVWF PORTB ; gear output

MOVLW 11001001B ;Select RA1 channel

MOVWF ADCON0

MOVLW .60 ; Delay 60μs

CALL WAIT

BSF ADCON0, GO; Start A/D

LOOP1 BTFSC ADCON0, GO; Determine whether A/D is completed

GOTO LOOP1

MOVF ADRES, W; A/D conversion result is stored in D0

MOVWF D0

WAIT MOVWF TEMP ; Delay subroutine

NEXT DECFSZ TEMP，1

GOTO NEXT

RETURN

The 11-bit A/D conversion speed realized by this method is fast. PIC16C711 is basically single-cycle instruction. At 4MHz oscillation frequency, the instruction cycle is 1μs, the shortest time for one A/D conversion is 20μs; two A/D conversions are 40μs; two op amp conversion time is 20μs×2=40μs. The total conversion time is hundreds of μs.

The method introduced in this paper has a simple circuit, fast speed and convenient debugging. It has been used in the single-chip microcomputer resistance furnace temperature automatic control system developed by us and achieved the expected effect. It can be used for other single-chip microcomputers with 8-bit and 10-bit A/D with slight modifications, and has promotion value.