Final Project
Abstract
A
data acquisition system was developed as a final project for the
microcontroller design course at UW-Platteville. This system was developed to
monitor power supplies and aid as a development tool for the design of a 200 W
switch-mode power supply. A PIC microcontroller from Microchip was used as the
heart of the data collection system. Internal analog to digital converters acquired
data from an analog interface. The analog subsystem gathered data from
temperature, voltage, and current sensors. Data was recorded through
HyperTerminal in Windows. Once the data was gathered MS Excel was used to
convert, scale, and plot the data.
Introduction
Measurement
of the three basic quantities: voltage, temperature, and current can provide
enough information to allow for debugging of almost any electrical circuit.
During the development of a switch-mode power supply it was determined that
some sort of data logging was necessary to protect the supply and determine if
the supply was operating properly. Out of this need a data acquisition system
was developed. The data acquisition system measures one channel of voltage from
0 to 20 volts, one channel of current from –50 to +50 amps, and two channels of
temperature (one ambient and one load). This amount of data is enough to
determine supply efficiency and temperature rise. With additional channels or
faster sampling rates it would be possible to measure and calculate inrush
current, supply stability, and transient response.
Implementation
options
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Two options were initially considered to implement the data logger. First, adding an outboard analog to digital converter (ADC) to the 8052 board used for development in class was considered. This approach had the advantage of allowing me to use a known good development system, software toolset, and the ability to get support from teachers and other students. Unfortunately, I had a difficult time finding multi-channel ADC converters featuring parallel interfaces to 8-bit busses that could resolve more than 8 bits. Most modern ADCs found used a serial interface. This was a problem on the 8052 board as the serial port was tied up for communication to the PC. Also fully developing a software-based serial protocol on a relatively slow processor like the 8052 could be difficult. Figure 1 shows the proposed block diagram for the 8052 based data acquisition system.
Figure 1, data acquisition system using 8052 board
The
second option considered the use of an altogether different processor, the
Microchip PIC controller. PICs are self-contained microcontrollers often
including clock, I/O, and a host of peripherals on-chip. The great advantage
seen by adopting the PIC was a chip with onboard analog to digital converters
was available in a small 14-pin DIP package. In addition a serial port, and
multiple timer/counters were available. A low-cost ($35) development kit is
available from Microchip to try out any of the 14-pin series of
micrcontrollers. Additionally, for the intended application a low-power small
form factor device was a plus. Essentially all the PIC needs to create the
system is the analog interface and a voltage regulator. Just the 8052 board requires
2 to 3 times the space of the board designed for the PIC controller and analog
board. The downside of this approach hinged around learning a new assembly
language for the PIC microcontroller and learning a new development environment
and device programmer. Figure 2 shows the block diagram for the PIC data
acquisition system.
Figure 2, PIC processor based data acquisition system
PIC
selection and setup
There
are literally hundreds of PIC microcontrollers to pick from. The programmer I
had already purchased narrowed this selection down to 8 or 14 pin devices. At
minimum 4 ADC channels, a UART, and one counter timer were needed. The first
device found to meet these specifications was the PIC16F688. The ‘688 contains
8 channels of 10-bit AD converters, an enhanced UART, two timer counters,
analog input comparison modules, an internal 32kHz to 8MHz clock, and flash
program memory.
In
order to use the PIC, settings for all the internal registers needed to be
determined or calculated via the datasheet. The internal oscillator was used
and set to 4MHz. Next, the serial port was configured. The enhanced UART
(EUSART) has an internal baud rate generator (no external timer is needed). The
EUSART was setup to communicate at 9600 bps, 1 stop bit, 8 data bits, and no
parity. This seemed to be a common serial data rate that was easily achieved
with little error in bit-rate on the PIC (about 0.16%). The commonly used baud
rates were all available in tables in the ‘688 datasheet. After the serial port
was configured the analog to digital converter was set-up. A conversion time of
4.0us as dictated by the datasheet was selected. Then, registers were set up to
use the positive supply as the ADC reference voltage along with selecting the
location of the most significant bit of the result. Finally, a timer was set up
to control the rate of data sampling. The timer values were set to allow for
maximum delay that turned out to be around ¾ of a second. The timer overflow
bit was checked via polling. This approach was used because exact time
intervals were not needed and quick response was not necessary.
Microchip
provides an integrated development environment (IDE) called MPLAB for coding,
compiling, setting up, and controlling programmers for the PIC series of
microcontrollers. Included in the IDE is a debugger and compiler. The debugger
worked well until additional peripherals were initialized and used which then
caused the debugger to crash the IDE. Thus, all further testing needed to be
conducted on actual hardware. A simple USB powered programmer interfaced to the
IDE and reprogrammed the flash program memory in PIC controllers.
Microcontroller
Firmware
After
configuration of peripherals the microcontroller firmware consisted of a simple
loop that acquired samples, converted them to a format acceptable to
HyperTerminal, and echoed them to the serial port. Code is attached to the end
of this document in listing 1. Peripherals were first initialized as described
in the previous section. Following this a timer set to approximately one second
would overflow triggering a capture event. The capture event consists of
setting ADC registers, then waiting for the conversion to be completed. These
events are repeated four times to cover all the input channels. The results are
then converted to a three digit octal number via shifts and bit masks. Only 8
bits of the 10-bit result are converted to octal, as it appeared the lower
value bits only added noise to the acquired signal. Finally, the converted
values were output on the serial port to HyperTerminal in ASCII format
delimited with commas. Figure 3 shows the block diagram for the microcontroller
firmware.
Figure 3, A block diagram of the PIC’s firmware
Hardware
The high
integration of the PIC controller leads to a very simple hardware solution. On
the digital side the PIC controller is connected to a MAX232, RS-232 to
logic-level converter. A 5 V power supply and some supply decoupling capacitors
round out the digital section of the hardware. Figure 4 shows the
implementation of the digital board that was constructed on the PICKit-1
development board.
Figure 4, the digital section of the data acquisition system
Highly integrated sensors reduced the difficulty of implementing an
analog interface board. The LM35 temperature sensor features a conditioned
output with a 10mV/C output slope. It was decided to use the LM35’s output
directly, without amplification, with slightly reduced resolution. A
hall-effect current sensor the ASC750SCA-050 made by Allegro was chosen as an
easy integrated solution for current sensing. The current sensor is capable of
resolving –50 to +50 A and outputs a 0 to 5 V signal corresponding to the
current through the device. In the original application it was expected to see
load currents up to 30 A. However, when demonstrating the device it was not
possible to find power supplies capable of supplying more than 2.5A, thus the
captured waveforms appear very noisy due to the small currents measured. Figure
5 shows the schematic of the analog board.
Figure 5, analog interface section of the data acquisition system
The
physical hardware is shown in photo 1. Two LM35 temperature sensors are
attached via twisted cable, enabling them to be clamped onto heatsinks to
measure power supply temperatures. A set of binding posts is provided for
voltage measurement and current measurement input. The 9 pin serial port hangs
off the left side of the development board (black PCB on top). Unregulated (9
to 37 VDC) DC input is supplied via two wires exiting the back of the board.
Photo 1, physical realization of the data acquisition
system
Results
To
test the data acquisition system an unregulated power supply was used. Real
power supplies have internal resistance that can be demonstrated if the load of
the supply is varied. The circuit in figure 6 was built to show the effect of
loading the supply under various conditions.
Figure 6, an unregulated power supply loaded with various power resistors
The output voltage, current, ambient temperature, and
temperature of the load resistor were all monitored via the data acquisition
system. After logging the octal numbers to a text file via HyperTerminal, the
data was scaled and plotted in MS Excel.
Plot 1 shows the results of loading the
supply with three different values of load resistors. As expected the supply
voltage sags with decreasing resistance. The output current changes from 400mA
to about 1A over the three different resistors. Finally, the temperature of the
load resistor peaks at about 55 degrees Celsius. This simple application
example shows the utility and some of the capabilities of the data acquisition
system.
Conclusions
and Future Work
The data
acquisition system proved to be a successful, yet challenging final project.
Choosing a different microcontroller platform on which to base the project made
hardware design much simpler. However, using the PIC controller greatly
increased the difficulty of software design. A new assembly language was
learned to take advantage of the PIC. Additionally, it was necessary to learn
the development environment, compiler, and device programmer. Hardware design
went relatively smoothly. Highly integrated sensors made the analog interface a
snap. For some sensors such as the current sensor and to a lesser degree the
temperature sensors the data acquired was of low resolution. Ideally, a higher
resolution ADC converter would be needed to take better data, or a variable
gain amplifier could be used. The primary problem with the current sensor was
because it was intended for a 0 to 50 A range. In the original application this
high current sensor was fine. Under testing, low-current supplies were used
that showed the low resolution of the ADC. This combined with using a very
small region of the current sensors range resulted in noisy captured data.
This
data acquisition system, while functional, could be improved. Primarily, the
improvements are centered on the lack of resolution of the ADC in the PIC
controller and the need for better PC interface software. Other
microcontrollers such as the MSC series from Texas Instruments have on-board
ADC converters with up to 24 bits of resolution. These microcontrollers also
feature programmable gain amplifiers, which effectively increase the dynamic
range of the ADC and thus further increase resolution. An improved voltage
measurement input is also desired. Currently, the system can only measure
positive ground referenced voltages. With a true differential amplifier input,
positive and negative voltages could be read without the fear of shorting a
measurement channel to ground. On the software side two major improvements are
warranted. First, the capability to display real-time values of the captured
data would greatly increase the utility of the data acquisition system. Next,
an integrated capture and display program would greatly simplify data capture.
The current system requires capturing data via HyperTerminal to a text file. This
text file then is updated in an excel spreadsheet. Excel’s update feature works
most of the time, however, trouble was occasionally experienced resulting in
Excel crashing. The current solution is far from a one-click data capture. With
dedicated software it is possible that a single click would enable a user to
view live data and concurrently save the trend data to a file.
Listing 1, PIC16F688 assembly code for the data acquisition system
Listing 1, PIC16F688 assembly code for the data acquisition system
list p=16f688 ; list directive to define processor
#include <P16F688.inc> ; processor specific variable definitions
__CONFIG _CP_OFF & _CPD_OFF & _BOD_OFF &
_PWRTE_ON & _WDT_OFF & _INTRC_OSC_NOCLKOUT & _MCLRE_OFF &
_FCMEN_OFF & _IESO_OFF
; '__CONFIG' directive is used to embed
configuration data within .asm file.
; The labels following the directive are located in
the respective .inc file.
; See respective data sheet for additional
information on configuration word.
;***** VARIABLE DEFINITIONS
w_temp EQU 0x71 ;
variable used for context saving
status_temp EQU 0x72 ;
variable used for context saving
pclath_temp EQU 0x73 ;
variable used for context saving
ADCH0 EQU
B'00000001' ;setup ch 0, Vdd for
Vref, and left justified result
ADCH1 EQU
B'00000101' ;set to Ch1 same settings
as above
ADCH2 EQU
B'00001001' ;set to Ch2 same settings
as above
ADCH3 EQU
B'00001101' ;set to Ch3 same settings
as above
COUNTER EQU
0x60 ;counter variable
TXPREBUF EQU
0x61 ;A buffer for the serial
transmit buffer (a prebuf)
OCTTOVERT EQU
0x20 ;stores the a/d value when
it's being converted to octal
;**********************************************************************
ORG 0x0 ;
processor reset vector
goto main ; go to beginning of program
main
;***********************************************************
;Setup For 9600 bps tx serial interface
;***********************************************************
bsf STATUS, RP0 ;bank 1
movlw
B'00111111' ;setup RC0-5 as
inputs for serial i/o and A/D
movwf
TRISC
bcf STATUS, RP0 ;bank 0
bsf RCSTA, SPEN ;serial port enable
bsf BAUDCTL, BRG16 ;use 16 bit baudrate generator
bcf TXSTA, BRGH ;use low baudrate speed
movlw
d'25'
movwf
SPBRG ;setup baudrate
generator
bcf TXSTA, SYNC ;asynchronous serial i/o
bsf TXSTA, TXEN ;enable transmission
;***********************************************************
;Setup A/D converter
;***********************************************************
bsf
STATUS, RP0 ;bank 1
movlw
B'00011111' ;Setup all of port A
as inputs for A/D
movwf
TRISA
movlw
0xFF
movwf
ANSEL ;Enable all available Analog channels
movlw
B'01010000'
movwf
ADCON1 ;Set 4uS conversion
time w/ 4MHz internal clk
bcf STATUS, RP0 ;bank
0
movlw
ADCH0
movwf
ADCON0 ;AD: right justified
result, ch 0, VDD ref. voltage
call
WaitForAD
;***********************************************************
;Setup Timer1 for 1/10 second wait period
;***********************************************************
movlw
B'00110001'
movwf
T1CON ;load timer1 config
register
GoAgain:
movlw
0x01 ;set to max for
approx 1 sec delay
movwf
TMR1L
movlw
0x00
movwf
TMR1H
;bsf
T1CON, TMR1ON ;start timer
bcf
PIR1, TMR1IF ;clear overflow
flag
;This is the delay between sample periods from the
analog inputs
Wait:
btfss
PIR1, TMR1IF ;check if timer has
overflowed
goto
Wait
call
GetADValues
; loop through 4 values here and print them out
movlw
0x4 ;loop 4 times
movwf
COUNTER
movlw
0x21 ;point to first a/d
value
movwf
FSR ;data pointer
OutLoop:
movf
INDF, W ;getting indirect data
movwf
OCTTOVERT ;store a/d value we need to
convert
call
TO_OCTAL ;convert currently
selected A/D Channel to octal
movf
0x30, W ;output all bytes
movwf
TXPREBUF
call
SENDBYTE
movf
0x31, W
movwf
TXPREBUF
call
SENDBYTE
movf
0x32, W
movwf
TXPREBUF
call
SENDBYTE
movlw
',' ;do checking on this one to see if
we're at the last byte
movwf
TXPREBUF
call
SENDBYTE
incf
FSR ;goto next a/d value
decfsz
COUNTER ;is counter zero yet?
if so skip goto
goto
OutLoop
; end of above loop
movlw 0x0A ;Line Feed (newline, hopefully)
movwf
TXPREBUF
call
SENDBYTE
;movlw
0x0D ;carridge return
;movwf
TXPREBUF
;call
SENDBYTE
goto
GoAgain ;Reset the timer for
another go around
;***********************************************************
;Subroutines
;***********************************************************
WaitForAD: ;wait
44us to init, 176 instruction cycles
movlw
D'176' ;really only need 1/2 this
many cycles due to test and set
movwf
COUNTER
FourFourWait:
decf
COUNTER
bnz FourFourWait ;keep decrementing until we reach 0
return
;get a/d values for 4 ch
GetADValues:
movlw
0x21 ;start of free ram
movwf
FSR ;setup indirect
addressing register
movlw
ADCH0 ;goto channel 0
movwf
ADCON0
bsf
ADCON0, GO ;start
conversion
btfsc
ADCON0, GO ;conversion done?
goto $-1 ;keep looping
movf
ADRESH, W ;get high
byte into acc W
movwf
INDF ;store adresh to 21h
incf FSR ;move ptr to 22h
call
WaitForAD
movlw
ADCH1 ;goto channel 1
movwf
ADCON0
bsf
ADCON0, GO ;start
conversion
btfsc
ADCON0, GO ;conversion done?
goto $-1 ;keep looping
movf
ADRESH, W ;get high
byte into acc W
movwf
INDF ;store adresh to 22h
incf FSR ;move ptr to 23h
call
WaitForAD
movlw
ADCH2 ;goto channel 2
movwf
ADCON0
bsf
ADCON0, GO ;start
conversion
btfsc
ADCON0, GO ;conversion done?
goto $-1 ;keep looping
movf
ADRESH, W ;get high
byte into acc W
movwf
INDF ;store adresh to 23h
incf FSR ;move ptr to 24h
call
WaitForAD
movlw
ADCH3 ;goto channel 3
movwf
ADCON0
bsf
ADCON0, GO ;start
conversion
btfsc
ADCON0, GO ;conversion done?
goto $-1 ;keep looping
movf
ADRESH, W ;get high
byte into acc W
movwf
INDF ;store adresh to 24h
return
;convert to 3 octal bytes in ASCII
;stores bytes in 0x30 to 0x32, w/ lsb in 0x32 msb in
0x30
;input value stored in 0x20, ie OCTTOVERT
TO_OCTAL:
clrc ;clear carry
movlw
B'00000111' ;bit mask field
andwf
OCTTOVERT, W ;and with ADRESH
addlw
0x30 ;make this into an
ASCII number
movwf
0x32 ;store result away
;octal byte 2
movlw
B'00111000' ;bit mask field
andwf
OCTTOVERT, W ;get 2nd set of 3
bits
movwf
0x31 ;place back into file
rrf 0x31 ;rotate to LSB, need 3x
rrf 0x31
rrf 0x31
movf
0x31, W ;now get it back
addlw
0x30 ;and add 30h to make
a ascii number
movwf
0x31 ;store again
;octal byte 3
clrc ;clear carry
movlw
B'11000000' ;bit mask field
andwf
OCTTOVERT, W ;get last 2 bits
movwf
0x30
rrf 0x30
rrf 0x30
rrf 0x30
rrf 0x30
rrf 0x30
rrf 0x30
;btfsc
0x21, 0 ;check the LSB
value from the last byte
;bsf
0x31, 2
movf
0x30, W ;get value back
addlw
0x30 ;convert to ASCII
movwf
0x30 ;store it
return
;transmit data on serial port
SENDBYTE:
btfss
TXSTA, TRMT ;check tx buffer
ready bit
goto
SENDBYTE ;try again, keep
polling
movf
TXPREBUF, W ;get data from
pre-buffer
movwf
TXREG ;move data to tx
register and start transmission
return
ORG 0x2100 ;
data EEPROM location
DE 1,2,3,4 ;
define first four EEPROM locations as 1, 2, 3, and 4
END
; directive 'end
of program'
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