Basic Refrigerator Control Using the MC9RS08KA2

Freescale Semiconductor
Application Note
Document Number: AN3409
Rev. 0, 01/2007
Basic Refrigerator Control Using
the MC9RS08KA2
by: Jose Ruiz
RTAC Americas
1
Introduction
Some refrigerators still have a basic electromechanical
circuit that controls the temperature. This application
shows how to implement a low-cost, basic temperature
control for refrigerators using the MC9RS08KA2. This
method can be implemented to control the temperature of
any device using a thermoresistor, a potentiometer,
resistors and a capacitor.
2
Requirements
•
•
•
•
•
•
MC9RS08KA2 microcontroller (MCU)
One potentiometer
One thermoresistor
One ceramic capacitor
Two ¼ watt resistors
CodeWarrior™ 5.1 development tool
© Freescale Semiconductor, Inc., 2007. All rights reserved.
Contents
1
2
3
4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1 Control Value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.2 Temperature Sensor . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Temperature Control Application. . . . . . . . . . . . . . . 9
3.4 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NCP18WB333J03RB Thermistor Range Table. . . . . . . . . 13
Appendix B
Code Implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Implementation
3
Implementation
The temperature control is implemented with a single potentiometer and a capacitor connected to one
MC9RS08KA2 MCU I/O pin. The temperature sensor is a basic voltage divider formed by a resistor and
a thermistor. The output is an I/O pin connected to a relay that switches the supply of the refrigerator.
The flow of the program consists of reading the control wheel value followed by reading the sensor voltage
and, finally, switching the output ON or OFF according to the control and sensor values.
3.1
Control Value
The refrigerator temperature control is a basic RC network connected to an I/O pin. By measuring the
charging time of the RC network, we can determine the potentiometer resistance, and therefore, the value
you entered. The charge curve of the RC network is used to determine the time the curve takes to go from
0 V to the input-high voltage (VIH). This method is used because the MC9RS08KA2 MCU does not have
an integrated analog-to-digital converter (ADC).
Figure 1. Temperature Control Implementation
The first step is configuring the control pin as output. Set the pin value to 0 to discharge the capacitor. After
the capacitor is fully discharged, change the control pin direction to an input. The capacitor starts charging
to VDD.
When the voltage of the capacitor gets to VIH, the pin state changes from 0 to 1.
A variable resistor (potentiometer) is used to modify the time the capacitor takes to reach VIH. Adjusting
its resistance varies that time.
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
2
Freescale Semiconductor
Implementation
Figure 2. Charge Capacitor Waveform
The capacitor voltage is given by the following equation:
t
− ⎞
⎛
Vc = Vdd⎜⎜1 − e rc ⎟⎟
⎠
⎝
•
•
•
•
•
Solving for time
Vc ⎞
⎛
t = −rc ln⎜1 −
⎟
⎝ Vdd ⎠
Vc — Voltage of the capacitor
VDD — Supply voltage of RC network
t — Time (seconds)
r — Resistance
c – Capacitor
A 10 kΩ potentiometer and 33 nF capacitor were used in this application note.
From the MC9RS08KA2 datasheet, we know that when VDD > 2.3 V, the VIH for the inputs is 0.70 x VDD.
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
3
Implementation
If the MC9RS08 MCU is supplied with 3.3 V then:
VIH = 0.70 x VDD = (0.70 x 3.3.) = 2.31 V
Table 1 shows the difference in time using the above with different resistance commercial values.
Table 1. Time Result According Resistance Values
VDD
VIH
R
C
time
3.3
2.31
1k
33nF
3.973E-05
3.3
2.31
3k
33nF
0.0001192
3.3
2.31
5k
33nF
0.0001987
3.3
2.31
7k
33nF
0.0002781
3.3
2.31
10k
33nF
0.0003973
Figure 3. Charge Curve with Different Resistor
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
4
Freescale Semiconductor
Implementation
NOTE
The resistor value must not reach 0, or a short circuit can occur.
3.1.1
Code Implementation
The first step is to configure the control pin as output, and setting a low level on it, (0). Then wait for the
RC network to discharge completely;
Pin_Measure:
bset control,PTADD
bclr control,PTAD
clr ControlValue
lda #$FE
Discharge2:
dbnza Discharge2
; Set control pin as Output
; Discharge RC network
The following step is to configure the control pin as input and increment a counter while pin state is 0:
bclr control,PTADD
; Set Control pin as Input
measure_pin:
inc ControlValue
brclr control,PTAD,measure_pin; Inc value while pin is in low state
rts
The
ControlValue
variable represents the time taken for the capacitor to reach VIH.
After the pin reaches the high level, we know the approximate position of the potentiometer entered by the
user.
3.2
Temperature Sensor
A basic voltage divider with one resistor and one thermoresistor is used to implement the temperature
sensor. The thermoresistor resistance depends on the temperature. For each temperature, we have a
different voltage in the divider. This value is effectively measured with the ADC implemented by software
that uses one resistor, one capacitor, and the analog comparator included in the MC9RS08KA2 MCU.
The voltage divider is composed of the thermoresistor NCP18WB333J03RB and a 82 ohms resistor. It is
better to have a big variation in the output voltage of the sensor with a little variation in the temperature.
The supply voltage of the RC network in this application note is 3.3 V and the output voltage of the sensor
can be calculated with the next equation.
⎛ NTC ⎞
⎛ NTC ⎞
Vout = Vdd ⎜
⎟ = 3.3⎜
⎟
⎝ NTC + R ⎠
⎝ NTC + 82 ⎠
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
5
Implementation
According to the thermoresistor specifications, the resistor range is between 89.61 Ω to 116.16 Ω in a
range of 4 °C to –0.5 °C (Section Appendix A, “NCP18WB333J03RB Thermistor Range Table”). With
those values the following data is calculated:
Table 2. Sensor Output Voltage
Temperature
NTC Value
Resistor
VDD
Sensor Output
–1
119.11
82
3.3
1.9544677
– 0.5
116.16
82
3.3
1.9344368
0
113.21
82
3.3
1.9138005
0.5
110.26
82
3.3
1.8925309
1
107.31
82
3.3
1.8705985
1.5
104.36
82
3.3
1.8479717
2
101.41
82
3.3
1.824617
2.5
98.46
82
3.3
1.8004987
3
95.51
82
3.3
1.7755788
3.5
92.56
82
3.3
1.7498167
4
89.61
82
3.3
1.7231688
Instead of having an ADC module, the MC9RS08KA2 MCU has a basic ADC implemented by software
using the analog comparator module. This software ADC is basically composed by a RC network and the
analog voltage to be measured. The software measures the time taken by the RC network to reach the
sensor input voltage. This ADC by software is fully detailed in the RS08 Quick Reference Guide
(RS08QRUG). Download the document at http://www.freescale.com
Figure 4. Sensor Value Input (ADC by Software)
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
6
Freescale Semiconductor
Implementation
The formula to calculate the time taken for the capacitor to charge is the same as the temperature control
formula :
t
− ⎞
⎛
Vc = Vdd ⎜⎜1 − e rc ⎟⎟
⎝
⎠
Solving for time
Vc ⎞
⎛
t = −rc ln⎜1 −
⎟
⎝ Vdd ⎠
But, for the ADC by software the RC network is fixed. In this case, the resistor value is 10 kΩ. The
capacitor is 0.1 μF.
If the sensor values and the capacitor charging curve are graphed together the result is the time the RC
network takes to reach the sensor output voltage.
Figure 5. Capacitor Charge Versus Sensor Output Voltage
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
7
Implementation
Based on the bus speed (8 MHz for this application), it is effective to build a table with the timer value
according the sensor voltage.
To calculate the timer counts of each sensor voltage the next formula must be applied:
⎛ BusClock ⎞
⎟⎟
TimerCounts = VIH time ⎜⎜
⎝ prescaler ⎠
Table 3. Temperature, Sensor Output, and Microcontroller Counts
3.2.1
Temperature
V sensor
VIH Time
-0.5°C
0°C
0.5°C
1°C
1.5°C
2°C
2.5°C
3°C
3.5°C
4°C
1.93444
1.9138
1.89253
1.8706
1.84797
1.82462
1.8005
1.77558
1.74982
1.72317
0.0008824
0.0008674
0.0008521
0.0008367
0.000821
0.000805
0.0007888
0.0007723
0.0007555
0.0007385
Timer
counts
(Bus/32)
220.5889
216.8392
213.0323
209.1667
205.2403
201.2512
197.1975
193.0769
188.8873
184.6263
Timer
counts
(Bus/32)
110.2944
108.4196
106.5162
104.5833
102.6201
100.6256
98.59874
96.53846
94.44366
92.31315
Code Implementation:
ADC_Single_conversion:
; Discharge Capacitor
bset 1,PTADD
bclr 1,PTAD
lda #$FE
waste:
dbnza waste
mov #ACMP_ENABLE,ACMPSC
mov #MTIM_ENABLE,MTIMSC
wait
bset 4,MTIMSC
lda MTIMCNT
sta ADCValue
mov #MTIM_STOP_RESET,MTIMSC
; ACMP Enabled
; Timer Counter Enabled
; Wait for Analog Comparator Interrupt
; Stop MTIM
; read counter timer value
;store counter value
;Stop and reset counter
mov #HIGH_6_13(SIP1), PAGESEL
brset 3, MAP_ADDR_6(SIP1),Conv_OK ; branch if ACMP interrupt arrives
bra ADC_Single_conversion
Conv_OK:
mov #ACMP_DISABLED, ACMPSC
rts
; ACMP Disabled, Clear Interrupt flag
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
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Freescale Semiconductor
Implementation
3.3
Temperature Control Application
The refrigerator’s temperature control has four positions, the range of each one is:
• Position 4: 0 °C – 1 °C
• Position 3: 1 °C – 2 °C
• Position 2: 2 °C – 3 °C
• Position 1: 3 °C – 4 °C
The control switches on the relay when the temperature is over range. It switches it off when the
temperature reaches the window value.
Because of temperature inertia, the window temperature is 1.5 °C. Figure 6 shows the window and the
values from it.
Figure 6. Temperature Control Range
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
9
Implementation
For example, when the temperature position is 1, if the temperature is higher than 4 °C, the relay is closed,
and the refrigerator compressor is on. Next, when the temperature reaches 2.5 °C, the application opens
the relay and the compressor stops.
This guarantees that the temperature is stable for long periods of time between the ranges and, no matter
what; the temperature is never more than 4 °C.
Each temperature limit can be easily changed in the definition part of the main code.
; Variable definitions
; Prescaler /64
TEMP1_ON
SET 92
TEMP1_OFF
SET 99
TEMP2_ON
SET 97
TEMP2_OFF
SET 103
TEMP3_ON
SET 100
TEMP3_OFF
SET 107
TEMP4_ON
SET 105
TEMP4_OFF
SET 111
The definition_ON is the value that closes the relay, and definition_OFF opens the relay. And the
resolution of these values can be adjusted with the timer prescaler.
3.3.1
Code Implementation
;**************************************************************
;*
Comparation (Control vs Temp)
*
;**************************************************************
comparation:
lda ControlValue
cmp #65
blo Temp1_4
cmp #130
blo Temp2_4
cmp #195
blo Temp3_4
mov
lda
cmp
blo
cmp
bhs
rts
#04,ControlValue
ADCValue
#TEMP4_ON
Compresor_ON
#TEMP4_OFF
Compresor_OFF
; selector = 4 (Coldest)
mov
lda
cmp
blo
cmp
bhs
rts
#03,ControlValue
ADCValue
#TEMP3_ON
Compresor_ON
#TEMP3_OFF
Compresor_OFF
; selector = 3 (Mid-Low)
Temp3_4:
Temp2_4:
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
10
Freescale Semiconductor
Implementation
mov
lda
cmp
blo
cmp
bhs
rts
#02,ControlValue
ADCValue
#TEMP2_ON
Compresor_ON
#TEMP2_OFF
Compresor_OFF
; selector = 2 (Mid-High)
mov
lda
cmp
blo
cmp
bhs
rts
#01,ControlValue
ADCValue
#TEMP1_ON
Compresor_ON
#TEMP1_OFF
Compresor_OFF
; selector = 1 (Hot)
Temp1_4:
Compresor_ON:
bset output,PTAD
rts
Compresor_OFF:
bclr output,PTAD
rts
; Compresor ON
; Compresor OFF
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
11
Conclusion
3.4
Schematic
Figure 7. Hardware Schematic
4
Conclusion
This application note shows how to implement a simple on-off control system with a low-end 8-bit
microcontroller.
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
12
Freescale Semiconductor
Conclusion
Appendix A NCP18WB333J03RB Thermistor Range Table
Temp (°C)
Resistance (K)
-40
1227.263
-35
874.449
-30
630.851
-25
460.457
-20
339.797
-15
253.363
-10
190.766
-5
144.964
0
111.087
5
85.842
10
66.861
15
52.470
20
41.471
25
33.000
30
26.430
35
21.298
40
17.266
45
14.076
50
11.538
55
9.506
60
7.870
65
6.549
70
5.475
75
4.595
80
3.874
85
3.282
90
2.789
95
2.379
100
2.038
105
1.751
110
1.509
115
1.306
120
1.134
125
0.987
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
13
Conclusion
Appendix B Code Implementation
INCLUDE 'derivative.inc'
; Include derivative-specific definitions
; export symbols
XDEF _Startup
ABSENTRY _Startup
; Variable declarations
ACMP_ENABLE
SET
$92
ACMP_DISABLED
SET
$20
MTIM_INIT
SET
$50
MTIM_ENABLE
SET
$40
MTIM_STOP_RESET SET
$30
MTIM_64_DIV
SET
$06
FREE_RUN
SET
$00
DEBUG_MODE
SET
$00
RUN_MODE
SET
$01
control
output
SET $04
SET $05
TEMP1_ON
TEMP1_OFF
TEMP2_ON
TEMP2_OFF
TEMP3_ON
TEMP3_OFF
TEMP4_ON
TEMP4_OFF
SET
SET
SET
SET
SET
SET
SET
SET
MODE:
EQU
94
100
98
104
102
108
106
112
DEBUG_MODE
; variable/data section
ORG
RAMStart
ADCValue:
counter
ControlValue
DS.B 1
DS.B 1
DS.B 1
; code section
ORG
ROMStart
;**************************************************************
;*
MACRO DECLARATION
*
;**************************************************************
TRIM_ICS: MACRO
mov #$FF,PAGESEL
lda #$FA
tax
lda ,x
sta ICSTRM
; Macro used to configure the ICS with TRIM
;change to last page
; load the content which TRIM value is store
; move A content to X
; read D[X]
; Store TRIM value
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
14
Freescale Semiconductor
Conclusion
ENDM
ACK_RTI: MACRO
mov #HIGH_6_13(SRTISC), PAGESEL
bset 6,MAP_ADDR_6(SRTISC)
ENDM
;**************************************************************
;*
Comparation (Control vs Temp)
*
;**************************************************************
comparation:
lda ControlValue
cmp #65
blo Temp1_4
cmp #130
blo Temp2_4
cmp #195
blo Temp3_4
mov
lda
cmp
blo
cmp
bhs
rts
#04,ControlValue ; selector = 4 (Coldest)
ADCValue
#TEMP4_ON
Compresor_ON
#TEMP4_OFF
Compresor_OFF
Temp3_4:
mov #03,ControlValue; selector = 3 (Mid-Low)
lda ADCValue
cmp #TEMP3_ON
blo Compresor_ON
cmp #TEMP3_OFF
bhs Compresor_OFF
rts
Temp2_4:
mov #02,ControlValue
lda ADCValue
cmp #TEMP2_ON
blo Compresor_ON
cmp #TEMP2_OFF
bhs Compresor_OFF
rts
; selector = 2 (Mid-High)
Temp1_4:
mov #01,ControlValue; selector = 1 (Hot)
lda ADCValue
cmp #TEMP1_ON
blo Compresor_ON
cmp #TEMP1_OFF
bhs Compresor_OFF
rts
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
15
Conclusion
Compresor_ON:
bset output,PTAD
rts
Compresor_OFF:
bclr output,PTAD
rts
; Compresor ON
; Compresor OFF
;**************************************************************
;*
CONFIGURES SYSTEM CONTROL
*
;**************************************************************
Init_mc:
mov #HIGH_6_13(SOPT), PAGESEL
mov #$E3, MAP_ADDR_6(SOPT)
; Enable STOP mode and COP with long timeout period
clr ICSC1
TRIM_ICS
clr ICSC2
bset output,PTADD
rts
; FLL is selected as Bus Clock
; Enable PTA5 as output
;**************************************************************
;*
Modulus Timer Configuration for ADC
*
;**************************************************************
MTIM_ADC_Init:
mov #MTIM_64_DIV,MTIMCLK
; Select bus clock as reference, Set prescaler with 64
mov #FREE_RUN,MTIMMOD
; Configure Timer as free running
mov #MTIM_STOP_RESET,MTIMSC
rts
;**************************************************************
;*
ADC Single Conversion
*
;**************************************************************
ADC_Single_conversion:
; Discharge Capacitor
bset 1,PTADD
bclr 1,PTAD
lda #$FE
waste:
dbnza waste
; Start Conversion
mov #ACMP_ENABLE,ACMPSC
Rising edges detections
mov #MTIM_ENABLE,MTIMSC
wait
signals)
bset 4,MTIMSC
lda MTIMCNT
sta ADCValue
mov #MTIM_STOP_RESET,MTIMSC
; ACMP Enabled, ACMP+ pin active, Interrupt enabled,
; Timer Counter Enabled
; Wait for Analog Comparator Interrupt (match
; Stop MTIM
; read counter timer value
; store counter value
; Stop and reset counter
mov #HIGH_6_13(SIP1), PAGESEL
brset 3, MAP_ADDR_6(SIP1),Conv_OK ; branch if ACMP interrupt arrives
bra ADC_Single_conversion
; Comparator Interrupt OK
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
16
Freescale Semiconductor
Conclusion
Conv_OK:
mov #ACMP_DISABLED, ACMPSC
rts
; ACMP Disabled, Clear Interrupt flag
;**************************************************************
;*
Control Value
*
;**************************************************************
Pin_Measure:
bset control,PTADD
; Set control pin as Output
bclr control,PTAD
; Discharge RC network
clr ControlValue
lda #$FE
Discharge2:
dbnza Discharge2
bclr control,PTADD
; Set Control pin as Input
measure_pin:
inc ControlValue
brclr control,PTAD,measure_pin; Inc value while pin is in low state
rts
;**************************************************************
;*
RTI Module Configuration
*
;**************************************************************
Init_RTI:
mov #HIGH_6_13(SRTISC), PAGESEL
mov #$37, MAP_ADDR_6(SRTISC)
; Enable RTI (1 sec period)
rts
;**************************************************************
;*
MAIN
*
;**************************************************************
_Startup:
bsr Init_mc
bsr Init_RTI
bsr MTIM_ADC_Init
; Configure MITM for ADC module
; Application Loop
mainLoop:
feed_watchdog
; Clear COP timer
bsr ADC_Single_conversion ; ADC Conversion
bsr Pin_Measure
; Control Measure
jsr comparation
; Comparation
stop
; Enter in STOP mode
ACK_RTI
; Ack for RTI Interrupt
bra mainLoop
;**************************************************************
;*
Startup Vector
*
;**************************************************************
ORG
$3FFD
JMP _Startup
; Reset
Basic Refrigerator Control Using the MC9RS08KA2, Rev. 0
Freescale Semiconductor
17
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Document Number: AN3409
Rev. 0
01/2007
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For information on Freescale’s Environmental Products program, go to
http://www.freescale.com/epp.