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Low Power Microcontrollers for
Temperature, Humidity & Carbon
Monoxide Instrumentation
White Paper
ROHM MarketingUSA
Presented by ROHM Semiconductor
Using Low Cost Microcontrollers for
Temperature, Carbon Monoxide, and Humidity
Measurement, Display, and Control
In today’s energy consumption and safety-conscious
world, collecting and using accurate information about
ambient temperature and humidity and the concentration of carbon monoxide (CO) is of critical importance. It
is equally important to make measurements and provide
control information or actions in a cost-effective manner.
Advances in single-chip low-power-consuming microcontrollers that utilize a unique RC-type analog-to-digital
converter (RC-ADC) now allow designers to meet these
challenges in creating products that are used to monitor
temperature or humidity and detect dangerous levels of
carbon monoxide. These new microcontrollers enable
product designers to create products and systems that
record, display, and use the data to manage environmental control units and energy consumption as well as
monitor perishable product quality—all in real time. In
many of these applications the activity of collecting and
using environmental data, in the -40°C to 85°C range,
is performed by battery operated handheld or remotely
installed products. The rising demand for low power
consumption in products that monitor the environment
and harvest energy from their local environment are
perfect applications for these devices due to their low
power consumption.
Applications requiring environmental
monitoring
Environmental monitoring is critical in building controls,
test and measurement, and in the transportation and
storage of perishable products such as food and pharmaceuticals. In the building controls industry, monitoring
temperature, humidity and CO is a key function that has
to be performed reliably with a high level of precision
and very low power consumption. Without providing
low power sensors that collect these types of data, the
ability to control the environment in a building is virtually
impossible. In the test and measurement industry, end
products need to provide accurate temperature and/
or humidity information as their principal function or use
ambient temperature data to accurately perform various
measurements. These measurements are typically conducted using a handheld, battery operated device that
requires high accuracy utilizing a simple system design
with low power consumption and multiple display and
communication options. In the transportation and storage of food and pharmaceutical industry, commonly
referred to as “cold chain monitoring,” it is critical to
monitor the environment that the products are exposed
to from the source to the consumer, thus guaranteeing
Designers of environmental monitoring products and
the quality of products that are delivered to the public.
systems require that their products:
Cold chain monitors are typically battery operated, need
n
n
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monitor temperature, humidity or CO concentration
while consuming little power
to provide high accuracy measurements, display infor-
provide a means of communicating the monitored
data, either wired or wirelessly, and
variations in measurements outside of a predefined
mation, log data to be read and used later, and indicate
range.
have a high level of integration and a wide variety
of associated features to allow designers maximum
flexibility in device selection
LAPIS Semiconductor
http://www.rohm.com/us/low-power-micro.html 1
Features of microcontroller-based
environmental monitors
Environmental sensing and monitoring products typically
which in many cases would require even more external
discrete components than the microcontroller and dedicated semiconductor device.
require a microcontroller to make measurements, man-
Temperature / Humidity / CO Sensors –
age collected data, display information, manage user
Temperature, humidity, or CO measurements can be
inputs and provide communication of recorded data.
made by measuring the value of a discrete component
that has a predictable value at given environmental conditions. These components can be monitored in a variety of ways including a bridge circuit, a simple voltage
divider, or electrically determining how the variable value
component affects a circuit. A typical way to do this is to
use an RC circuit.
Each of these methods has its advantages and disadvantages. One of the typical challenges to making these
measurements is precise control over excitation circuits,
either current, voltage, or frequency. Accurate measuring circuits and coordination of the resources associated with making measurements are also challenging.
RC measurements in particular require the coordination
of many discrete functions such as counters, stimulus
circuits, and calibration. The ideal solution would be self
calibrating, highly integrated, and provide repeatable
performance over the entire operating voltage and temperature ranges of the product.
Figure 1. A microcontroller featuring an RC analog-to-digital converter is a cost-effective means of calculating temperature
and humidity in portable instruments, weather stations and
thermostats.
RC-Type ADC - Accurate temperature,
humidity, and CO measurements made
easy with less power
Measurement Methods
The most common method to easily measure tempera-
There are a number of ways that allow a microcontroller-
use a variable resistance component. An RTD is an
based system to monitor temperature, humidity and/or
example of a component that can be used to determine
CO for these types of applications. The following are a
temperature, as RTDs have a well defined resistance-
few of the more commonly used methods.
temperature curve. Figure 2 shows a typical Resistance
ture, humidity, or CO with a discrete component is to
Dedicated Semiconductor Sensors – A designer
can add a dedicated sensing device to their circuit. A
separate microcontroller would be used to manage and
monitor the sensor. Implementing a system with a discrete measuring device results in a multiple chip solution
LAPIS Semiconductor
vs. Temperature curve for an RTD. There are similar
devices that have well-defined variable resistive values
over humidity and CO concentrations that would be
used to measure and monitor those environmental conditions.
http://www.rohm.com/us/low-power-micro.html 2
a fixed value resistor. The product of the values of the
RTD Resistance-Temperature Curve
resistance and capacitance affects the rate at which
the signal oscillates. The amount of time that it takes to
charge CS1 is recorded. The ADC then discharges CS1
and charges it with the same oscillating signal through
the variable value component, RS1. In this case, RS1 is
a component whose resistive value is dependent upon
an environmental condition. The amount of time it takes
-40 -30 -20 -10
0
10
20
30
40
50
60
70
80
80 100
Figure 2. Typical Resistance vs. Temperature curve of an RTD
to charge the cap to the same predetermined level
through RS1 is recorded. A temperature, humidity or
CO concentration sensitive device could easily be used
The major functional blocks required by an RC-ADC are
two counters, one driven by an internal clock and one
by a variable rate counter whose frequency is based on
the RC loading on specific pins of the device. Reaching
terminal count for either of these counters would generate an interrupt to the CPU. When TC is reached the
other counter is stopped and its count is recorded. The
count of the second counter is proportional to the value
being measured, the issue to be solved is relating that
value to an accurate measurement.
The advantage of using this type of ADC is that it can
be designed so that it uses a known value component
to calibrate the circuit at the time of measurement, then
measure the variable value component to determine
as RS1 to make the associated measurement.
Since the frequency of the charging signal is a function
of the product of the resistors and CS1, the exact value
of CS1 factors out of the equation:
K * CS1 * RT1 = Time Ref -> RT1 = Time Ref / (K * CS1)
K * CS1 * RS1 = Time Variable -> RS1 = Time Variable /
(K * CS1)
RS1/RT1 = (Time Variable / (K * CS1)) / (Time Ref /(K *
CS1)) = Time Variable /Time Ref
RS1/RT1 = Time Variable /Time Ref
RS1 = (Time Variable /Time Ref) * RT1
the relative effect of the two components on the circuit.
As you can see, the ratio of the amount of time it takes
The challenge to designers is implementing this method
to charge through RS1 compared to RT1 is the ratio of
of measurement and calibration without a dedicated
the values of RS1 to RT1, and since RT1 is a known
resource on a microcontroller.
value, RS1 can be calculated. As such, the tempera-
Figure 3 shows a block diagram of how the RC-ADC
ture (or humidity or CO concentration) at RS1 is known
interfaces with external components. As you can see,
external component count is minimal and the types of
components required are passive and small. The typical
RT1
RT1
range for CS is 700 to 1000 pF. The power consumed
RS1
RS1
CS1
CS1
while using the RC-ADC is normally no more than 0.90
mW (0.30 mA at 3.0V Vdd) and a measurement takes
IN1
less than 2 seconds.
Capacitor CS1 is discharged and then is charged by
passing a load-sensitive oscillating signal through RT1,
LAPIS Semiconductor
Figure 3. Simple external circuitry is the key to the low cost of the
RC analog-to-digital converter technique.
http://www.rohm.com/us/low-power-micro.html 3
which provide high accuracy temperature, humidity, or
method the variability of the value of the cap CS1, dif-
CO monitoring using very little power and few external
ferences between devices, or variations due to the
components. The devices include many of the features
voltage applied to the device are taken out of the equa-
required for low power environmental monitors. The lat-
tion. In other words, this self-calibrating method takes
est additions to the LAPIS Semiconductor ML610400
out component and process variability. Implementing
microcontroller family give designers even more options
an RC-ADC of this type provides the ability to design
to design products and systems that monitor, record,
monitoring products with relatively high precision and
display, and communicate temperature, humidity, or
accuracy.
CO data while consuming little power using few components.
Features of LAPIS Semiconductor’s Low
Power Microcontrollers
Dedicated RC-ADCs are unique in that they are
As shown above, there are many ways to monitor tem-
and LAPIS Semiconductor provides sample software (in
perature, humidity, or CO. The key issue is how to do it
the C language) that customers can use as-is or modify
with the lowest power consumption, the fewest number
as necessary.
of components, with an acceptable level of accuracy,
and use the information collected appropriately?
designed specifically for these types of measurements
In addition to the low power consuming architecture of
the 8 bit RISC CPU core at the heart of the ML610400
Lapis Semiconductor’s Low Power Microcontrollers
devices, the family includes a wide selection of non-
offer a variety of solutions that have integrated RC-ADCs
volatile memory configurations and a range of LCD
Product Concept for Low Power MCU
Conventional LP-MPU
Lower
system costs
Lapis Semiconductor’s
Ultra-Low Power MCU
System
miniaturization
Flash
MCU
Reduces
environmental
impact
Flash
MCU
1.8V~3.6V
Typ. 1.5µ[email protected]
Dry Cell
Dry Cell
Lithium battery Flash
3V
MCU
|
Dry batteryx2 Flash
MCU
3V
Dry Cell
because the value of RS1 is determined. Using this
Dry batteryx1
1.5V
Solar
panels
Subs idiary
power supply
1.1V~3.6V
Typ. 0.5µ[email protected]
Further reduction in
power consumption
Figure 4. ROHM’s Ultra-Low Power Microcontrollers can operate from a low battery voltage or solar
panel and have significantly lower power consumption in Halt and Stop modes.
LAPIS Semiconductor
http://www.rohm.com/us/low-power-micro.html 4
controllers to choose from. All devices in the Low
tion notes that accompanies the Demo Kit. Customers
Power Microcontroller family are based on a low leak-
can purchase a Demo Kit to evaluate the features of
age process (see Figure 4), allowing for as low as 0.5uA
the ML610400 family of devices. Information about the
consumption in Halt mode and 0.15uA in Stop mode.
Demo Kit is also available at http://www.rohm.com/us/
Almost all include an on-board LCD controller. The LCD
low-power-micro.html
controllers available span from as few as 45 LCD segments up to 1536 segments. All have non-volatile memory, ranging from 8 KB to 128KB. Devices with FLASH
memory are designated by a “Q” in the part number; the
lack of a Q indicates that there is a ROM option available.
Summary
LAPIS Semiconductor Low Power Microcontrollers are
ideal for all types of battery operated products, especially those that require high accuracy temperature, humidity, or CO measurement. This is because the devices
All devices have background debugging capabilities,
are inherently low power consuming and include among
accessible by a USB-based debugging pod referred to
their many peripherals RC-ADCs which are specially
as uEASE, and the same code development and debug-
designed to measure variable value components such
ging tools are used for all devices in the Low Power
as RTD, Humidity and CO Sensors.
Microcontroller family. More details about the individual
products in the family and a complete Product Selector
Guide can be found online:
http://www.rohm.com/us/low-power-micro.html
There are a number of ways that an RC-ADC can
To learn more about Low Power Microcontrollers, please
contact:
http://www.rohm.com/us/low-power-micro.html
http://www.rohm.com/us/contact_us/lapissemi/USASales.html
be used to determine the value of a variable value
component, and those methods are described in the
User’s Manuals which can be found on the Low Power
Microcontroller website listed above and in the applica-
Figure 5. The Demo Kit provides an effective means
of evaluating the features of the ML610400
family of devices.
LAPIS Semiconductor
Figure 6. The ML610Qxx uEASE Tool Kit.
http://www.rohm.com/us/low-power-micro.html 5
LAPIS Semiconductor
is a ROHM Group
company
LAPIS Semiconductor
6815 Flanders Drive, Suite 150
San Diego, CA 92121
http://www.lapis-semi.com/en | 1.888.775.7646
NOTE: For the most current product information, contact a LAPIS Semiconductor sales representative in your area.
LAPIS Semiconductor assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other
right, and makes no representations that the circuits are free from patent infringement. Specifications subject to change without notice for
the purpose of improvement.
The products listed in this catalog are designed to be used with ordinary electronic equipment or devices (such as audio visual equipment,
office-automation equipment, communications devices, electrical appliances and electronic toys). Should you intend to use these products
with equipment or devices which require an extremely high level of reliability and the malfunction of which would directly endanger human
life (such as medical instruments, transportation equipment, aerospace machinery, nuclear-reactor controllers, fuel controllers and other
safety devices), please be sure to consult with our sales representative in advance.
© 2011 LAPIS Semiconductor Co. Ltd. Although every effort has been made to ensure accuracy, LAPIS Semiconductor accepts no
responsibility for errors or omissions. Specifications and product availability may be revised without notice. No part of this document
represents an offer or contract. Industry part numbers, where specified, are given as an approximate comparative guide to circuit function
only. Consult LAPIS Semiconductor prior to use of components in safety, health or life-critical systems. All trademarks acknowledged.
1.888.775.7646
http://www.rohm.com/us/low-power-micro.html
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