Application of Digital Temperature Monitor MAX6622 in Temperature Measurement and Control

1 Introduction

As computer performance continues to improve, more and more functions are integrated into computers. Therefore, the processing capacity of computers is increasing day by day. In order to meet the needs of large amounts of data processing, more and more chipsets are integrated into the host, and the operating frequency of the CPU and chipsets is constantly increasing. More chipsets and faster clock frequencies will generate more heat. In a limited space, system heat dissipation is a difficult problem. How to balance system performance, system comfort (including computer case temperature, noise generated by fan rotation), and system running time is an important topic in computer design. Therefore, this article proposes a temperature measurement and control application design based on MAX6622.

2 MAX6622 Introduction

MAX6622 is a precision multi-channel temperature monitor launched by MAXIM. It integrates A/D converter, register and interface circuit, and can directly output digital signal. The device has the advantages of high precision, fast response speed, small size, low power consumption and convenient software interface control. It is widely used in temperature monitoring systems such as desktops, notebooks, servers and workstations.

The MAX6622 precision multichannel temperature sensor can monitor local temperature and 4 remote temperatures. All temperature channels have programmable alarm thresholds. When the measured channel temperature exceeds its threshold, the corresponding status bit in the status register is set. Two open-drain outputs, ALERT and OVERT, are set low according to the state of the corresponding bit in the status register. The 2-wire serial interface uses the standard system management bus to execute write byte, read byte, send byte, and receive byte commands. It can also be used to read temperature data and set alarm thresholds. The MAX6622 operates over a -40°C to +125°C temperature range and is packaged in a 16-pin TSSOP.

2.1 Main Features

2.2 Pin Configuration and Function Description

The MAX6622 pin configuration is shown in Figure 1, and the pin function descriptions are shown in Table 1.

2.3 Working Mode

The MAX6622 is a 5-channel precision temperature monitor with 1 local temperature sensor channel and 4 remote temperature sensor channels. Each temperature channel has a programmable alarm threshold, and Channel 1 and Channel 4 also have programmable over-temperature thresholds. Communication with the MAX6622 is achieved through the SMBus serial interface and a dedicated alarm output. The alarm outputs ALERT and OVERT are used to confirm whether the temperature threshold set by the software is exceeded. The typical application of ALERT is an interrupt, and OVERT is connected to a fan, system switch, or other temperature control circuit.

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2.3.1 ADC conversion method

In the default conversion mode, the MAX6622 first measures channel 1, then measures channel 2, channel 3, local channel, channel 4, and starts conversion. The conversion results are stored in the corresponding temperature data register.

2.3.2 Low power standby mode

Software standby mode is entered by setting the STOP bit in Configuration Register 1 to 1. Hardware standby mode is entered by setting STBY low. Software standby mode stops the ADC and reduces the supply current to approximately 30 mA, while hardware standby mode interrupts the ADC clock and the supply current is approximately 350 mA. In software or hardware standby mode, the data is retained in memory and the SMBus interface is activated, listening for SMBus commands. If a start signal is recognized by the SMBus, a timeout interval is activated. After the SMBus is activated, the supply current increases. If a standby command is received while a conversion is in progress, the conversion cycle is interrupted and the temperature register is not updated, the previous data is unchanged and can still be used.

2.3.3 ALERT interrupt mode

When the internal or external temperature data exceeds the upper temperature limit, an ALERT interrupt is generated. ALERT is an open-drain output, so multiple devices share a normal interrupt line. All ALERT interrupts can be masked by setting register 3.

2.3.4 OVERT Over-temperature alarm

The MAX6622 has two overtemperature registers for storing remote alarm threshold data, which are used for OVERT output. When the measured channel temperature is greater than the temperature stored in the corresponding threshold register, an OVERT signal is generated. An overtemperature output can be used to activate a cooling fan, send a warning, initiate a clock reduction, or shut down the system to prevent component damage.

3 Typical application circuit

The CPU is the largest heat source in the computer. In addition, the graphics processor (GPU), DDR memory, hard disk and optical drive all require temperature detection and control components. These components contain diodes for remote temperature detection. The temperature sensor can directly detect the temperature of the CPU's internal die and accurately control its temperature.

The typical application circuit of MAX6622 is shown in Figure 2. The accuracy of remote temperature measurement depends on the ideal factor of the remote diode. The temperature diode at the bottom of the IC is usually a PNP type. DXP_ must be connected to the emitter of the transistor, and DXN_ must be connected to the base. If the temperature of a channel is not used, the DXP_ and DXN_ of that channel are left floating.

The ADC has a good suppression effect on low-frequency signals. When electromagnetic interference is large, a 2200 pF capacitor should be connected between DXP_ and DXN_ to help filter, but it should not exceed 3300 pF because the rise time of the conversion current causes errors.

4. Software Design

First, set the configuration register to make the MAX6622 work in normal mode. Set the upper temperature limit in the alarm threshold register. When the measured channel temperature is greater than the corresponding threshold register temperature, the OVERT bit is enabled to activate the cooling fan, send a warning, initialize the clock, or shut down the system to prevent damage to the components. The OVERT bit remains valid until the current temperature drops below the set upper limit temperature minus the hysteresis value of 4°C, so that the ambient temperature will be maintained below the set temperature. In this working mode, the MAX6622 can be separated from the system microprocessor as an independent temperature monitor to achieve automatic temperature control function. Its main program flow is shown in Figure 3.

The software design uses C language programming, which greatly improves the efficiency of development and debugging; it is also easy to understand the generated documents and facilitates porting. The C language compilation environment uses the integrated debugging environment Workbench and C430 language debugger C-SPY provided by IAR to compile, directly download to the on-chip Flash memory, and run offline. The entire user interface is friendly. During the debugging process, the contents of each register can be seen in the upper-level software and modified online. It supports single-step operation and online observation of the real-time values ​​of each defined variable. The following is part of the code.

5 Conclusion

The temperature detection function, working mode and typical application circuit of MAX6622 are introduced in detail. When MAX6622 works in normal mode, it can automatically drive the fan to adjust the temperature. It can greatly reduce power consumption in notebooks, desktops, servers, workstations and other application fields. It has the advantages of strong anti-interference ability and high temperature acquisition accuracy. Therefore, MAX6622 can be widely used in occasions that require temperature control.