Mcu/Digital System And Maintain Low Current Drain For Long Battery Life - Renesas RL78/I1D User Manual

DETECT-IT RL78/I1D detector boards kit
(chemical equations can be seen in full Technical Information for TGS5042 document)
• DC bias amplifier (Ch1 operational amplifier)
At MCU power startup Ch0 and Ch1 operational amplifiers are initialized in low power mode. Ch1 is biased at
VDD/11, where VDD depends on the CR123 lithium battery voltage (2.7V to ~3.1V) or slightly higher if E1 debugger
is used for power (often around 3.3V) . The resistor bias network for Ch1 plus (+) input draws less than 0.7µA over the
possible VDD power supply range. However the buffered output of Ch1 operational amplifier presents a low impedance
and constant voltage even as the CO Sensor sources possibly >10µA when CO Gas concentration of 10,000ppm or
more. Since the resistor bias network voltage can vary slightly with resistor tolerance, the ~VDD/11 bias level output on
Ch1 AMP1O output is measured by ADC input ANI7 to serve as the baseline for CO sensor current-to-voltage
measurements.
• Current-to-voltage trans-impedance amplifier (Ch0 operational amplifier)
As the CO gas concentration increases, the CO sensor current proportionally increases and flows into the path of 909K
feedback resistor causing a linear voltage increase on the operational amplifier output. The Ch0 operational amplifier
output is a voltage level is established by the equation:
AMP0O voltage =~ VDD/11 + (CO current x 909000
Note : CO current is in nA/PPM.
This resulting output is periodically measured by ADC conversion of input ANI4. The alarm is sounded when the
AMP0O voltage exceeds the AMP1O voltage by an increase representing 100ppm. This voltage increase depends on
the CO sensor calibration level printed on the canister. For example, with a CO sensor having calibration value of 1425
(1.425 nA/PPM), the voltage increase threshold =
100ppm x 1.425E-9 nA/ppm x 909000
The method to determine 0.1295V increase above the VDD/11 bias point is described in the ADC section below.
6.2.2

MCU/digital system and maintain low current drain for long battery life

• Low power interval timers like Real Time Counter, 12-bit Interval Timer, 2 units of 8-bit Interval Timer:
Real Time Counter interrupt at 1 second rate is used to wake up MCU to take ADC voltage measurements on 3 different
values:
 AMP1O (DC bias = VDD/11),
 AMP0O (DC bias + CO sensor increase)
 internal VBGR = 1.45V.
• ADC conversion with internal VBGR = 1.45V (nominal) reference voltage:
ADC conversions are performed both on the ~VDD/11 bias level (on AMP1O) and the total voltage on AMP0O. With
the CO sensor calibration number of 1425 (1.425 nA/ppm), the voltage increase with 100ppm is 0.1295V. Since the
full-scale ADC reference voltage is based on VDD, but VDD can be variable from 2.7V to 3.5V, the ADC conversion
codes are ratio-metric with VDD. That means that the ~VDD/11 voltage level measure at AMP1O will be a constant
digital code. However, the CO sensor voltage increase will be a constant value at 100ppm. That also means the
AMP0O output will have a larger ADC conversion code when VDD = 2.7V compared to when VDD = 3.5V. To
counter the ratio-metric ADC conversion when using ADC voltage reference = VDD, we also take an ADC conversion
of VBGR = 1.45V and scale the AMP0O conversion accordingly. This conversion can be seen in the conditional
equation to trigger the CO alarm:
Note : where :
adc_co = ADC conversion digital code of AMP0O
adc_bias = ADC conversion digital code of AMP1O
adc_adiss = ADC conversion digital code of internal VBGR = 1.45V (nominal)
And the constants are 1.425 nA/ppm x 100ppm, 909kΩ and 1.45V
UM-YDETECT-IT-RL78 V1.30
If(adc_co > (adc_bias + adc_adiss * (0.1425*0.909/1.45)),
)
= 0.1295V
Kit User Manual
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