Texas Instruments | DN101 -- Using the ADC to Measure Supply Voltage (Rev. A) | Application notes | Texas Instruments DN101 -- Using the ADC to Measure Supply Voltage (Rev. A) Application notes

Texas Instruments DN101 -- Using the ADC to Measure Supply Voltage (Rev. A) Application notes
Design Note DN101
Using the ADC to Measure Supply Voltage
By Kjetil Aamodt
Keywords
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•
•
•
•
1
•
•
•
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ADC
VDD
Supply Voltage
CC1110
CC1111
CC2430
CC2431
CC2510
CC2511
Introduction
The above mentioned SoCs contain a
feature for sampling VDD/3 using the
ADC. By setting the ADC to sample
VDD/3 with 1.25 V as internal voltage
reference, VDD can easily be calculated.
Figure 1: ADC Block Diagram
SWRA100A
Page 1 of 4
Design Note DN101
Table of Contents
KEYWORDS.............................................................................................................................. 1
1
INTRODUCTION............................................................................................................. 1
2
VOLTAGE MEASUREMENT.......................................................................................... 3
3
GENERAL INFORMATION ............................................................................................ 4
3.1 DOCUMENT HISTORY ......................................................................................................... 4
SWRA100A
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Design Note DN101
2
Voltage Measurement
The following code sets up the ADC to perform voltage measurement of VDD/3.
/* Reference voltage:
Internal 1.25 V,
Resolution: 12 bits,
ADC input: VDD/3 (VDD is the battery voltage) */
#define SAMPLE_BATTERY_VOLTAGE(v)
do {
ADCCON2 = 0x3F;
ADCCON1 = 0x73;
while(!(ADCCON1 & 0x80));
v = ADCL;
v |= (((unsigned int)ADCH) << 8);
} while(0)
//
//
//
//
\
\
\
\
\
\
\
Max ADC input voltage = reference voltage =>
(VDD/3) max = 1.25 V => max VDD = 3.75 V
12 bits resolution means that max ADC value = 0x07FF = 2047 (dec)
(the ADC value is 2’s complement)
// Battery voltage, VDD = adc value * (3.75 / 2047)
// To avoid using a float, the below function will return the battery voltage * 10
// Battery voltage * 10 = adc value * (3.75 / 2047) * 10
#define CONST 0.0183195 // (3.75 / 2047) * 10
unsigned char getBatteryVoltage(void) {
unsigned int adcValue;
SAMPLE_BATTERY_VOLTAGE(adcValue);
// Note that the conversion result always resides in MSB section of ADCH:ADCL
adcValue >>= 4; // Shift 4 due to 12 bits resolution
return CONST * adcValue;
}
SWRA100A
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Design Note DN101
3
General Information
3.1
Document History
Revision
SWRA100A
Date
2007.10.22
SWRA100
2006.07.06
Description/Changes
Updated code example (12 bits resolution instead of 14).
Added reference to CC1111.
Changes to Figure 1.
Initial release.
SWRA100A
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