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Keywords:
Temperature sensor, LED driver, micro controller
APPLICATION NOTE 6647
HOW TO BUILD A HIGH-RESOLUTION
TEMPERATURE MONITORING SYSTEM
Abstract: This application note introduces a temperature monitoring system with LED display based on the MAXQ2000
microcontroller, the MAX31875 high-resolution local temperature sensor, and the MAX7219 eight-digit LED display
drivers.
Introduction
The
MAX31875 is a ±2°C-accurate local temperature sensor with an I C/SMBus interface. The device is available in a 4-
bump, wafer-level package, uses <10µA average supply current, and can operate over -20°C to +150°C. With its tiny
package, low power consumption, and excellent accuracy, the MAX31875 can be used in various temperature-
measurement applications, such as portable equipment, handheld electronics, and industrial equipment. This application
note presents an idea for a temperature-monitoring system. The system is based on the
MAXQ2000 microcontroller,
MAX31875 high-resolution local temperature sensor, and
MAX7219 8-digit LED display drivers. The MAXQ2000 reads
temperatures through the I
C interface of the MAX31875, and an LED display is accomplished using the MAX7219 LED
driver through a serial peripheral interface (SPI).
Overview
The temperature-monitoring system uses the MAXQ2000 as a controller. It works as an interface between the
temperature sensor and LED display. The MAXQ2000 receives temperature information through the I
C interface from
the MAX31875 and creates an SPI to the MAX7219, which drives the LED display. The I
C interfaces that the
MAXQ2000 offers run at 400kHz or 100kHz, and SPI commands can be sent at up to 8MHz. The MAXQ2000 keeps
checking environment temperature through the I
C interface from the MAX31875. An eight-digit LED displays the
MAX31875's local temperature in Celsius. The MAX7219 is a compact, serial input/output common-cathode display
driver. Simply connect it to the eight-digit, seven-segment numeric LED displays and send temperature information
through the SPI from the microcontroller.
Hardware Setup
The test program runs on the Maxim Command Module (
CMAXQUSB). It works as an interface to receive commands
from the PC and creates an SPI or SMBus/I
C-compatible interface to send commands to the MAX31875 and
MAX7219. An on-board level translator converts the MAXQ2000 2.5V logic signals to the external VDD logic level. The
VDD logic level is jumper-selectable for 2.5V, 3.3V, or 5V, provided by an on board ultra low-noise LDO
MAX8511. The
MAX3373
is a bidirectional level translator specially designed for the I C bus. Optional user-switchable SCL/SDA pullup
resistors are provided on-board. The MAX31875 EV kit with VDD, GND, SCL, and SDA test point header is connected to
connector P7 on the CMAXQUSB. The MAX7219 is used to drive the 8-digit, 7-segment LED display. The MAX7219's
SPI is connected to connector P8 on the CMAXQUSB. The firmware is loaded into the MAXQ2000 through connector
P5. The hardware configuration is shown as in Figure 1. An actual setup is shown in Figure 2.
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