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ADT7411 Datasheet(PDF) 13 Page - Analog Devices |
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ADT7411 Datasheet(HTML) 13 Page - Analog Devices |
13 / 36 page ADT7411 Rev. A | Page 13 of 36 THEORY OF OPERATION Directly after the power-up calibration routine, the ADT7411 goes into idle mode. In this mode, the device is not performing any measurements and is fully powered up. To begin monitoring, write to the Control Configuration 1 register (Address 18h) and set Bit C0 = 1. The ADT7411 goes into its power-up default measurement mode, which is round robin. The device proceeds to take measurements on the VDD channel, internal temperature sensor channel, external temp- erature sensor channel, or AIN1 and AIN2, AIN3, AIN4, AIN5, AIN6, AIN7, and finally AIN8. Once it finishes taking measure- ments on the AIN8 channel, the device immediately loops back to start taking measurements on the VDD channel and repeats the same cycle as before. This loop continues until the mon- itoring is stopped by resetting Bit C0 of the Control Config- uration 1 register to 0. It is also possible to continue monitoring as well as switching to single-channel mode by writing to the Control Configuration 2 register (Address 19h) and setting Bit C4 = 1. Further explanations of the single-channel and round robin measurement modes are given in later sections. All measurement channels have averaging enabled on them at power-up. Averaging forces the device to take an average of 16 readings before giving a final measured result. To disable aver- aging and consequently decrease the conversion time by a factor of 16, set C5 = 1 in the Control Configuration 2 register. There are eight single-ended analog input channels on the ADT7411: AIN1 to AIN8. AIN1 and AIN2 are multiplexed with the external temperature sensors D+ and D− terminals. Bits C1 and C2 of the Control Configuration 1 register (Address 18h) are used to select between AIN1/2 and the external temperature sensor. The input range on the analog input channels is dependent on whether the ADC reference used is the internal VREF or VDD. To meet linearity specifications, it is recommended that the maximum VDD value is 5 V. Bit C4 of the Control Configuration 3 register is used to select between the internal reference and VDD as the analog inputs’ ADC reference. The dual serial interface defaults to the I2C protocol on power- up. To select and lock in the SPI protocol, follow the selection process as described in the Serial Interface Selection section. The I2C protocol cannot be locked in, while the SPI protocol on selection is automatically locked in. The interface can only be switched back to I2C when the device is powered off and on. When using I2C, the CS pin should be tied to either VDD or GND. There are a number of different operating modes on the ADT7411 devices and all of them can be controlled by the configuration registers. These features consist of enabling and disabling interrupts, polarity of the INT/INT pin, enabling and disabling the averaging on the measurement channels, SMBus timeout, and software reset. POWER-UP CALIBRATION It is recommended that no communication to the part is initiated until approximately 5 ms after VDD has settled to within 10% of its final value. It is generally accepted that most systems take a maximum of 50 ms to power up. Power-up time is directly related to the amount of decoupling on the voltage supply line. During the 5 ms after VDD has settled the part is performing a calibration routine; any communication to the device will interrupt this routine and could cause erroneous temperature measurements. If it is not possible to have VDD at its nominal value by the time 50 ms has elapsed or that communication to the device has started prior to VDD settling, then it is recom- mended that a measurement be taken on the VDD channel before a temperature measurement is taken. The VDD measure- ment is used to calibrate out any temperature measurement error due to different supply voltage values. CONVERSION SPEED The internal oscillator circuit used by the ADC has the capability to output two different clock frequencies. This means that the ADC is capable of running at two different speeds when doing a conversion on a measurement channel. Thus the time taken to perform a conversion on a channel can be reduced by setting C0 of the Control Configuration 3 register (Address 1Ah). This increases the ADC clock speed from 1.4 kHz to 22 kHz. At the higher clock speed, the analog filters on the D+ and D− input pins (external temperature sensor) are switched off. This is why the power-up default setting is to have the ADC working at the slow speed. The typical times for fast and slow ADC speeds are given in the specification pages. The ADT7411 powers up with averaging on. This means every channel is measured 16 times and internally averaged to reduce noise. The conversion time can also be reduced by turning the averaging off. This is done by setting Bit C5 of the Control Configuration 2 register (Address 19h) to a 1. |
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