Introduction to MSP430 ADCs

40 Pages·2007·0.73 MB·English

Introduction to MSP430 ADCs
Lane Westlund
MSP430 Applications Engineer
Texas Instruments
© 2006 Texas Instruments Inc, Slide 1
Agenda
• Analog measurements with the MSP430
(cid:131) Comparator, ADC10, ADC12, SD16, SD16_A
• Hands-on lab with ADC12
• Summary
© 2006 Texas Instruments Inc, Slide 2
Comparator_A
0V VCC
• References
CAEX
usable internally
CA0
and externally
CCI1B
+
+
• Low-pass filter
CA1 -- CAOUT
selectable by set CAIFG
Flag
0V VCC
software
CAON
• Input terminal
CAREF
multiplexer
• One interrupt
0.5xVCC
vector with enable
0.25xVCC
D
G
S
© 2006 Texas Instruments Inc, Slide 3
Comparator-Based Slope ADC
• 10-bit+ accuracy
• Resistive sensors
• Low battery detect
• Very low cost
• App note SLAA038
VCAREF
t_x = R_x x C x ln
Vcc
. . .
t_NTC
R_NTC = 10k x
t_10k
© 2006 Texas Instruments Inc, Slide 4
Example: Thermistor
• R = 10K, R = NTC
REF M
• V = V *e(-t/RC)
CAREF CC
• Relationship simplifies
to single multiply &
divide operations
t_NTC
V
CAREF
C x ln
Vcc t_NTC
R_NTC
R_NTC = 10k x
=
t_10k
10k t_10k
V
CAREF
C x ln
Vcc
© 2006 Texas Instruments Inc, Slide 5
Slope Resistance Considerations
• Measurement as accurate as R
REF
• V independent
CC
• Resolution based on number of max counts possible
• Precharge of C impacts accuracy
M
• Slope measurement time duration a function of RC
© 2006 Texas Instruments Inc, Slide 6
Integrating A/D Voltage Measurement
• V range is near full scale
IN
• P toggling creates a
X.Y
1-bit DAC at V MSP430
OUT
SD-Type
• Match V to V
OUT IN
P
• SLAA104
X.Y
V Software
OUT
CA
V Integration
IN
Used for voltage sensors, 10-bit+ resolution as accurate as Vcc
© 2006 Texas Instruments Inc, Slide 7
Integrating A/D Considerations
• Resolution determined by times through S/W loop
• Inherently excellent noise immunity
• V must be known
CC
• DAC pulse symmetry required
• Select RC values for < +/- 1LSB V ripple
OUT
• Reference: SLAA104
© 2006 Texas Instruments Inc, Slide 8
ADC10
• 200ksps+
• Autoscan
• Single
Sequence
Repeat-single
Repeat-sequence
• Int/ext reference
• TA SOC triggers
• Data transfer
controller
• 30us ref settling, No
decoupling required
© 2006 Texas Instruments Inc, Slide 9
Why Is Autoscan + DTC Important?
O
Data2
T
ADC
U Data1
A Data0
DTC
Data2
// Software // Autoscan + DTC
// Software // Autoscan + DTC
Res[pRes++] = ADC10MEM; _BIS_SR(CPUOFF);
Res[pRes++] = ADC10MEM; _BIS_SR(CPUOFF);
ADC10CTL0 &= ~ENC;
ADC10CTL0 &= ~ENC;
if (pRes < NR_CONV)
if (pRes < NR_CONV)
{
{
CurrINCH++;
CurrINCH++;
if (CurrINCH == 3)
if (CurrINCH == 3)
CurrINCH = 0;
CurrINCH = 0;
ADC10CTL1 &= ~INCH_3;
ADC10CTL1 &= ~INCH_3;
ADC10CTL1 |= CurrINCH;
ADC10CTL1 |= CurrINCH;
ADC10CTL0 |= ENC+ADC10SC;
ADC10CTL0 |= ENC+ADC10SC;
}
}
Fully Automatic
70 cycles/Sample
© 2006 Texas Instruments Inc, Slide 10