AN003 - IC Microsystems

AN003
ICmic
APPLICATION NOTES
IC MICROSYSTEMS
AN003
TEMPERATURE SENSING APPLICATION USING ICmic DAC
(ICM7363/7343/7323)
Shahab A. Najmi
ICM7363, ICM7343 and ICM7323 are 12-, 10- and 8- bit low power Digital to Analog Converters
(DAC). This application note describes the usage of these DAC to acquire low frequency analog
data. This provides an alternative method to acquiring data without using an expensive Analog to
Digital Converter (ADC).
This option is worth considering in many applications that are typically implemented with an ADC
since it can result in lower power consumption, reduced system cost and more flexibility. This
approach is particularly useful if one or more of the following are true.
-
Repetitive high bandwidth transient signal is to be measured. The advantage in such a
case is that we can replace an expensive, high bandwidth ADC with a much cheaper
DAC and comparator combination. Only the comparator has to be able to sustain the
entire bandwidth of the signal while the DAC can have a long settling time without
affecting the quality of the measurement. However if the signal is single shot event then
the ADC method is preferable.
-
DAC is already being used in the system for some other application and it can be utilized
to double as an ADC when it is not being used as a DAC, thus removing the need for a
separate ADC. This would help reduce the system cost and form factor.
-
Low frequency data is to be measured with high accuracy and low system cost. Using a
DAC might be desirable for the reason that it can sometimes reduce the amount of
overhead on the processor if the processing part can be moved in the analog domain. An
example of this case is when low battery alarm has to be generated, it might be desirable
to use a simple DAC to provide reference (negative input) and a comparator comparing
the battery voltage (positive input). The processor needs to react only in the event that
the comparator output goes low thus triggering the low battery alarm.
Apart from the above mentioned cases and examples there are many cases where using a DAC
is a better option than the conventional method of using an ADC. This application note gives an
example of persuading the DAC to act as an accurate ADC for temperature sensing.
Temperature Sensing using a DAC
This is an application that is typically implemented with an analog to digital converter (ADC).
However this note provides an alternative method of implementing such a design. This approach
is particularly suitable in many cases, some of which have been mentioned in the previous
section.
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AN003
We can implement this system as in the block diagram below.
Temp
Sensor
(LM235)
Analog V out
Digital in
Comparator
(LM339)
Analog out
Microcontroller
(8051)
DAC
(ICM7363)
Serial port
Digital in
Figure 1: Digitizing temperature sensor output using ICM7363 DAC
The DAC / comparator combination can act as a successive approximation ADC. This can be
done in a number of ways but the quickest is by using binary trial of weights as described below.
Successive Approximation Method
In order to digitize an analog value each bit starting from the most significant one is first set to 1
and the corresponding analog value is compared with the actual value. If the actual value is
greater than the resulting value then the bit is set to one otherwise it is reset to 0. The process is
repeated for all the bits till we reach the least significant bit. The algorithm for this conversion is
given below.
Table 1: Algorithm for Digital conversion of data
Begin:
Make the most significant bit high;
Loop:
Convert using the current value;
Wait to allow the DAC to settle;
If comparator input is high
Leave the bit high;
Else
Make the bit low;
Make the next bit high;
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Loop till all bits are tested;
Value after the loop would be the converted value;
Thus this process makes the minimum time for one complete conversion for the resulting
successive approximation “ADC” equal to the conversion time of the DAC times the number of
bits in DAC.
Total Conversion time (min.) = Settling time for DAC x Number of bits in DAC
Thus for 8-bit digital conversion the maximum rate for the conversion is equal to 1/8 of the DAC
conversion rate. This is fast enough for many low frequency applications like the one discussed in
this application note.
Voltage (V)
A simplified illustration of the operation of this algorithm is shown in the figure below.
Digital to Analog Converter output
Half scale
(V)
Quarter
Scale
(V)
Analog value
1st conversion
4th conversion
8th conversion
Time (t)
Figure 2: DAC output zeroing on analog value
Implementation for 8051 based microcontroller
The following code is for the conversion of the analog data into digital utilizing a 8051 based
microcontroller.
This example uses ICM7363 which is a 12-bit low power DAC to get a 12 bit digital value for the
input voltage provided by the temperature sensor.
IC MICROSYSTEMS 2001-2004. All rights reserved.
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Table 2: Sample code for successive approximation method for ICM7363
; Description
;
;
;
Sample code for digital conversion of analog data using ICM7363 with 8051 based microcontroller
This example uses port 2 to communicate with the DAC. The code provides output in ASCII character 1, 0
which has to be interpreted as a voltage according to the formula 2 x (Vref,in x D)/2^n
;
;
;
;
MCS51 pins
P2.0
P2.2
P2.3
;
;
;
Author: Shahab A. Najmi
Sairan Sakrani
Feb, 2004
$MOD51
SCK
SDI
CSB
; Contains Assembler specific directives
;============================================================================
; NOTES:
; *> AT89S8252 Memory
;
(Data) RAM: 0000H – 0100H 256 bytes
;
(Data) EEPROM: 0000H – 07FFH 2K bytes (need to set EEMEN=1 with MOVX)
;
(Pgm) Flash: 0000H – 2000H 8K bytes
;
; *> AT89S8252 implements 256 byte RAM. RAM addresses overlapping SFR are
; accessed using indirect addressing.
;
; *> MCS-51 RAM bit addresses
;
RAM byte 0020H: 00H(LSB) – 07H(MSB)
;
RAM byte 0021H: 08H(LSB) – 0FH(MSB)
;
…
;
RAM byte 002FH: 78H(LSB) – 7FH(MSB)
;
; *> MCS-51 Hardware bit address
;
As defined in MOD51 file
;
; *> This firmware architecture
;
RAM 0000H – 0007H ( 7 bytes) R0 thru R7
;
RAM 0008H – 001FH ( 24 bytes) Stack
;
RAM 0020H – 0100H (224 bytes) Variables
;============================================================================
;Hardware addresses
; SERIAL COMMUNICATION VARIABLES
P_MOSI
P_MISO
P_CLK
SPIF
BIT P1.5
BIT P1.6
BIT
P1.7
EQU 10000000B
P_SCK
P_CLRB
P_SDI
P_CSB
EQU
EQU
EQU
EQU
P2.0
P2.1
P2.2
P2.3
;P2.0 – SCK
;P2.1 - CLRB
;P2.2 - SDI
;P2.3 - CSB
;Variables – Data For Port 0 signals
P0_D0 BIT
000H
P0_D1 BIT
001H
;RAM 20.0
;RAM 20.1
;Variables – Data For Port 1 signals
ICLK
BIT
008H
IPLB
BIT
009H
;RAM 21.0
;RAM 21.1
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AN003
IDS
mSDO
P1_D4
P1_D5
P1_D6
P1_D7
BIT
BIT
BIT
BIT
BIT
BIT
00AH
00BH
00CH
00DH
00EH
00FH
;RAM 21.2
;RAM 21.3
;RAM 21.4
;RAM 21.5
;RAM 21.6
;RAM 21.7
;Variables – Data For Port 2 signals
mSCK
BIT
010H
mCLRB BIT
011H
mSDI
BIT
012H
mCSB
BIT
013H
ORLY
BIT
014H
P2_D5 BIT
015H
P2_D6 BIT
016H
P2_D7 BIT
017H
;RAM 22.0
;RAM 22.1
;RAM 22.2
;RAM 22.3
;RAM 22.4
;RAM 22.5
;RAM 22.6
;RAM 22.7
;Variables – Data For Port 3 signals
P3_D0 BIT
018H
P3_D1 BIT
019H
P3_D2 BIT
01AH
P3_D3 BIT
01BH
P3_D4 BIT
01CH
P3_D5 BIT
01DH
P3_D6 BIT
01EH
P3_D7 BIT
01FH
;RAM 23.0
;RAM 23.1
;RAM 23.2
;RAM 23.3
;RAM 23.4
;RAM 23.5
;RAM 23.6
;RAM 23.7
;Variables – Bit addressable flags
BITCOUNT
EQU
020H
;Variables – DAC codes
DACCL EQU
030H
DACCH EQU
031H
;RAM 24.0
;DAC code LOW byte
;DAC code HIGH byte
;Temporary Variables
BLKVAR
EQU
032H
TEMPVAL
EQU
033H
;Temporary value holder
TEMPCNT
EQU
034H
;Temporary control sequence holder
COMPRESULT
BIT
035H
;Result of comparison
WDLYH
EQU
036H
;Delay – high byte
WDLYL
EQU
037H
;Delay – low byte
BLKVAR2
EQU
038H
SENDBUF
EQU
039H
TEMPBUF
EQU
03AH
TEMPBUF2
EQU
03BH
;============================================================================
;Interrupt Vectors
ORG
0000H
LJMP
BEGIN
;0000H - RESET vector
ORG
0003H
LJMP
ISEXT0
;0003H - External 0
ORG
000BH
LJMP
ISTMR0
;000BH - Timer/Counter 0
ORG
0013H
LJMP
ISEXT1
;0013H – External 1
ORG
001BH
LJMP
ISTMR1
;001BH – Timer/Counter 1
ORG
0023H
LJMP
ISSER
;0023H – Serial Port
ISEXT0:
ISTMR0:
ISEXT1:
ISTMR1:
ISSER: RETI
;Empty interrupt services
;============================================================================
;Beginning of Main Routine
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;============================================================================
ORG
0100H
BEGIN:
MOV
IE,#00H
;Disable interrupts
;----- Initialize interface to DACs
CLR
P_CLRB
SETB
P_CSB
CLR
P_SCK
CLR
P_SDI
SETB
P_CLRB
SETB P3.2
MOV A, #0
MOV BLKVAR, #0
MOV BLKVAR2, #0
MOV DACCL, #0FFH
MOV DACCH, #08FH
LCALL SNDDAC
MOV
MOV
;CLRB=LOW
;CSB(P2.3)=HIGH
;SCK(P2.0)=LOW
;SDI(P2.2)=LOW
;CLRB(P2.1)=HIGH
;TO BE USED AN INPUT
DACCL, #00H
DACCH, #0E0H
;Initialize DAC lower byte
BITCOUNT, #0BH
;Initialize the bit counter
CONVERSION:
MOV
CONVHS:
MOV
MOV
MOV
MOV
MOV
A, #00H
TEMPCNT, #08H
TEMPVAL, #00H
DACCL, #00H
DACCH, #050H
;Conversion High Byte setup
;Initialize DAC lower byte
;Initialize DAC Upper byte
CONVH:
NRESTH:
MOV
MOV
MOV
ORL
MOV
LCALL
LCALL
LCALL
LCALL
LCALL
LCALL
B,#02H
;To divide by this to move bits
TEMPVAL, DACCH ;Saves the old value temp
A, DACCH
A, TEMPCNT
DACCH, A
SNDDAC
DL1MS
DL1MS
DL1MS
DL1MS
DL1MS
JB
MOV
P3.2, NRESTH
;if (pin 12==1){NRESTH}
DACCH, TEMPVAL
MOV
DIV
MOV
JZ
LJMP
A, TEMPCNT
AB
TEMPCNT, A
CONVLS
CONVH
MOV
MOV
A, #80H
TEMPCNT, A
CONVLS:
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AN003
MOV
TEMPVAL, #00H
MOV
MOV
MOV
ORL
MOV
B,#02H
;To divide by this to move bits
TEMPVAL, DACCL ;Saves the old value temp
A, DACCL
A, TEMPCNT
DACCL, A
LCALL
LCALL
LCALL
LCALL
LCALL
LCALL
SNDDAC
DL1MS
DL1MS
DL1MS
DL1MS
DL1MS
JB
MOV
P3.2, NRESTL
;if (pin 12==0){NRESTL}
DACCL, TEMPVAL
MOV
DIV
MOV
JZ
LJMP
A, TEMPCNT
AB
TEMPCNT, A
ENDCONV
CONVL
CONVL:
NRESTL:
ENDCONV:
MOV SENDBUF, DACCH
LCALL SERIALTRANS
MOV SENDBUF, DACCL
LCALL SERIALTRANS
MOV SENDBUF, #10
LCALL RS232
MOV SENDBUF, #13
LCALL RS232
LJMP CONVERSION
;============================================================================
;End of Main Routine
;============================================================================
;----;
;
;
;
;
;
Send code to all DACs and update simultaneously
LCALL SNDDAC
;Send the high and low bytes to DAC
MOV
CJNE
MOV
CJNE
A,DPL
A,TBENDL,SWND1
A,DPH
;[12cyc]
A,TBENDH,SWND2 ;[24cyc]
;
;
;
;
;
;
CLR
SETB
CLR
SETB
DJNZ
RET
P2.7
;[12cyc]
P2.7
;[12cyc]
P3.6
;[12cyc]
P3.6
;[12cyc]
WAVEPER,SWRPT ;[24cyc]
;SWRPT:
;
MOV
;
LJMP
;SWND1:
;
NOP
DPTR,#SNTBL1+1 ;[24cyc]
SWLUP
;[24cyc]
;[12cyc]
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AN003
;
;
;
;SWND2:
;
;
;
;
;
;
;
;
;;
NOP
NOP
;[12cyc]
;[12cyc]
NOP
NOP
NOP
NOP
NOP
NOP
NOP
NOP
LJMP
;[12cyc]
;[12cyc]
;[12cyc]
;[12cyc]
;[12cyc]
;[12cyc]
;[12cyc]
;[12cyc]
;[24cyc]
SWLUP
;============================================================================
;Subroutine:
SNDDAC
;Description:
Send 16 bits of data to DAC.
;
First, CSB is set to LOW, then 16 bits are sent,
;
then, CSB is brought back HIGH.
;Arguments: DACCH = Data high byte (Bit15 thru Bit8)
;
DACCL = Data low byte (Bit7 thru Bit0)
;Trashed: Whatever trashed by subr SN8BIT
;******************************
SNDDAC:
SETB
P_CSB
;CSB should already HIGH initially, do it anyhow
CLR
P_CSB
;CSB=LOW, initiate the data transfe
MOV
A,DACCH
LCALL SN8BIT
;Send high byte – note the subr do the POP
MOV
A,DACCL
LCALL SN8BIT
;Send low byte
SETB
P_CSB
;CSB=HIGH, end the data transfer
RET
;Done
;============================================================================
;============================================================================
;Subroutine:
SN8BIT
;Description:
Send 8 bits of data on P2.2 (SDI)
;
Clock the P2.0 (SCK) as needed.
;Arguments: A = data to be sent
;Trashed: C,R0
;******************************
SN8BIT: MOV
R0, #8
;Init loop counter
SN8LUP: CLR
P_SCK
;CLK=L
RLC
A
;Set C=bit to be sent
MOV
P_SDI,C
;Set SDI=bit to be sent
SETB
P_SCK
;CLK=H, ICM7363 latches data on SDI at +ve edge
DJNZ
R0, SN8LUP
;Decrement counter, loop if more bits to sent
CLR
P_SCK
;CLK=L
RET
;Done
;============================================================================
;============================================================================
;Subroutine:
DL1MS
;Description:
Delay about 1ms ( for crystal @7.3728 MHz )
;
Each ms needs about 0.001sec/7.3728e+6 = 7372.8 = 7372 cycles
;Arguments: None
;Trashed: R5, R6, R7
;******************************
DL1MS: MOV
R5,#150
;12 cyc; Init delay counter
MOV
R6,#3
;12 cyc
MOV
R7,#1
;12 cyc
;
DL1MSL: DJNZ
R5, DL1MSL
;24 cyc
(0 + (3+0*150-1)*150) x 24
DJNZ
R6, DL1MSL
;24 cyc
(3 + 0*150) x 24
DJNZ
R7, DL1MSL
;24 cyc
(1) x 24
RET
;============================================================================
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AN003
;============================================================================
;Subroutine:
WVDLY
;Description:
Delay about N x 1ms ( for crystal @7.3728 MHz )
;
Each ms needs about 0.001sec/7.3728e+6 = 7372.8 = 7372 cycles
;Arguments: WDLYH, WDLYL – delay count in ms
;Trashed: R5, R6, R7
;******************************
WVDLY:
MOV
A,WDLYL
JNZ
WVDLYL
MOV
A,WDLYH
JZ
WVDLYE
WVDLYL: LCALL
DEC
MOV
JNZ
DL1MS
WDLYL
A,WDLYL
WVDLYL
MOV
JZ
DEC
A,WDLYH
WVDLYE
WDLYH
SJMP
WVDLYL
WVDLYE:
RET
;============================================================================
;===================================================================
; Subroutine:
SERIALOUT
; Description:
WRITES SERIAL DATA OUT THROUGH SCL, MOSI, MISO
;
A HAS INPUT DATA
; Trashes:
A, SPDR
;===================================================================
SERIALOUT:
SETB P_MOSI
SETB P_MISO
SETB P_CLK
MOV SPCR, #01010111B
;MOV A, SENDBUF
BBB:
MOV SPDR, SENDBUF
MOV A, SPSR
ANL A, #SPIF
JZ BBB
RET
;WRITE TO SERIAL SBUF WITH RS232
;Subroutine RS232
RS232:
TRANS:
HERE:
MOV TMOD, #20H
MOV TH1, #-2
MOV SCON, #50H
SETB TR1
MOV SBUF, SENDBUF
;MOV SBUF, #'A'
JNB TI, HERE
CLR TI
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AN003
RET
;============================================================================
; Subroutine:
SERIALTRANS
; Description:
Transmits each bit of the conversion separately in ASCII to the serial buffer
; Trashes:
A, Whatever is trashed by RS232
;============================================================================
SERIALTRANS:
MOV TEMPBUF, SENDBUF
SERIALTRANS0:
MOV A, TEMPBUF
ANL A, #10000000B
JZ CALLTRANSZERO0
LJMP CALLTRANSONE0
CALLTRANSZERO0:
LCALL TRANSZERO
LJMP SERIALTRANS1
CALLTRANSONE0:
LCALL TRANSONE
LJMP SERIALTRANS1
SERIALTRANS1:
MOV A, TEMPBUF
ANL A, #01000000B
JZ CALLTRANSZERO1
LJMP CALLTRANSONE1
CALLTRANSZERO1:
LCALL TRANSZERO
LJMP SERIALTRANS2
CALLTRANSONE1:
LCALL TRANSONE
LJMP SERIALTRANS2
SERIALTRANS2:
MOV A, TEMPBUF
ANL A, #00100000B
JZ CALLTRANSZERO2
LJMP CALLTRANSONE2
CALLTRANSZERO2:
LCALL TRANSZERO
LJMP SERIALTRANS3
CALLTRANSONE2:
LCALL TRANSONE
LJMP SERIALTRANS3
SERIALTRANS3:
MOV A, TEMPBUF
ANL A, #00010000B
JZ CALLTRANSZERO3
LJMP CALLTRANSONE3
CALLTRANSZERO3:
LCALL TRANSZERO
LJMP SERIALTRANS4
CALLTRANSONE3:
LCALL TRANSONE
LJMP SERIALTRANS4
SERIALTRANS4:
MOV A, TEMPBUF
ANL A, #00001000B
JZ CALLTRANSZERO4
LJMP CALLTRANSONE4
CALLTRANSZERO4:
LCALL TRANSZERO
LJMP SERIALTRANS5
CALLTRANSONE4:
LCALL TRANSONE
LJMP SERIALTRANS5
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AN003
SERIALTRANS5:
MOV A, TEMPBUF
ANL A, #00000100B
JZ CALLTRANSZERO5
LJMP CALLTRANSONE5
CALLTRANSZERO5:
LCALL TRANSZERO
LJMP SERIALTRANS6
CALLTRANSONE5:
LCALL TRANSONE
LJMP SERIALTRANS6
SERIALTRANS6:
MOV A, TEMPBUF
ANL A, #00000010B
JZ CALLTRANSZERO6
LJMP CALLTRANSONE6
CALLTRANSZERO6:
LCALL TRANSZERO
LJMP SERIALTRANS7
CALLTRANSONE6:
LCALL TRANSONE
LJMP SERIALTRANS7
SERIALTRANS7:
MOV A, TEMPBUF
ANL A, #00000001B
JZ CALLTRANSZERO7
LJMP CALLTRANSONE7
CALLTRANSZERO7:
LCALL TRANSZERO
LJMP SERIALTRANS8
CALLTRANSONE7:
LCALL TRANSONE
LJMP SERIALTRANS8
SERIALTRANS8:
MOV SENDBUF, TEMPBUF
RET
;=================================================================
; Subroutines to transmit an ASCII 0 and 1
; Descrition: If a zero is to be transmitted then this subroutine sends out an ASCII zero
;
and a ASCII 1 in case of a one
; Trashes: SENDBUF
;-=================================================================
TRANSZERO:
MOV SENDBUF, #’0’
LCALL RS232
RET
TRANSONE:
MOV SENDBUF, #’1’
LCALL RS232
RET
END
IC MICROSYSTEMS 2001-2004. All rights reserved.
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Contact Information
For any other questions / queries please feel free to contact us at support@icmic.com or contact
us at the following address.
IC MICROSYSTEMS Sdn. Bhd.
Suite S06, 2320 Century Square,
63000 Cyberjaya, Selangor D.E.,
Malaysia
Tel: (603) 8319 1919
Fax: (603) 8319 1918
Disclaimer
IC MICRSOYSTEMS (ICmic) reserve the right to make corrections, modifications or changes to
any of its products and services at any time and to discontinue any of its product or service
without notice.
The information in this document is believed to be accurate and correct. However ICmic does not
assume any liability for any loss or damage caused to any party due to the usage of the
information in this document. Contents of this document are not to be released to any third party
without written consent of the owner.
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