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ADT7421ARZ-REEL Datasheet(PDF) 8 Page - ON Semiconductor |
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ADT7421ARZ-REEL Datasheet(HTML) 8 Page - ON Semiconductor |
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8 / 20 page  ADT7421 http://onsemi.com 8 The user can switch between measurement ranges at any time. Switching the range likewise switches the data format. The next temperature result following the switching is reported back to the register in the new format. However, the contents of the limit registers do not change. It is up to the user to ensure that when the data format changes, the limit registers are reprogrammed as necessary. More information on this is found in the Limit Registers section. ADT7421 Registers The ADT7421 contains 22, 8−bit registers in total. These registers store the results of remote and local temperature measurements, high and low temperature limits, and configure and control the device. See the section through the Consecutive ALERT Register section of this data sheet for more information on the ADT7421 registers. Additional details are shown in Table 3 through Table 7. The entire register map is available in Table 8. Address Pointer Register The address pointer register itself does not have, nor does it require, an address because the first byte of every write operation is automatically written to this register. The data in this first byte always contains the address of another register on the ADT7421 that is stored in the address pointer register. It is to this register address that the second byte of a write operation is written, or to which a subsequent read operation is performed. The power−on default value of the address pointer register is 0x00. Therefore, if a read operation is performed immediately after power−on, without first writing to the address pointer, the value of the local temperature is returned because its register address is 0x00. Temperature Value Registers The ADT7421 has three registers to store the results of local and remote temperature measurements. These registers can only be written to by the ADC and can be read by the user over the SMBus. The local temperature value register is at Address 0x00. The external temperature value high byte register is at Address 0x01, with the low byte register at Address 0x10. The power−on default for all three registers is 0x00. Configuration Register The configuration register is Address 0x03 at read and Address 0x09 at write. Its power−on default is 0x08. Only five bits of the configuration register are used. Bit 0, Bit 1, and Bit 3 are reserved; the user does not write to them. Bit 7 of the configuration register masks the ALERT output. If Bit 7 is 0, the ALERT output is enabled. This is the power−on default. If Bit 7 is set to 1, the ALERT output is disabled. This applies only if Pin 6 is configured as ALERT. If Pin 6 is configured as THERM2, then the value of Bit 7 has no effect. If Bit 6 is set to 0, which is power−on default, the device is in operating mode with ADC converting. If Bit 6 is set to 1, the device is in standby mode and the ADC does not convert. The SMBus does, however, remain active in standby mode; therefore, values can be read from or written to the ADT7421 via the SMBus. The ALERT and THERM outputs are also active in standby mode. Changes made to the registers in standby mode that affect the THERM or ALERT outputs cause these signals to be updated. Bit 4 switches beta cancellation on and off. With Bit 4 set to zero beta cancellation is set on. If using a discrete transistor as the sensing element, Beta Cancellation should be switched off by setting Bit 4 to 1. Bit 5 determines the configuration of Pin 6 on the ADT7421. If Bit 5 is 0 (default), then Pin 6 is configured as an ALERT output. If Bit 5 is 1, then Pin 6 is configured as a THERM2 output. Bit 7, the ALERT mask bit, is only active when Pin 6 is configured as an ALERT output. If Pin 6 is set up as a THERM2 output, then Bit 7 has no effect. Bit 2 sets the temperature measurement range. If Bit 2 is 0 (default value), the temperature measurement range is set between 0 °C to +125°C. Setting Bit 2 to 1 sets the measurement range to the extended temperature range (−40 °C to +125°C). Table 3. Configuration Register Bit Assignments Bit Name Function Power−On Default 7 MASK1 0 = ALERT Enabled 1 = ALERT Masked 0 6 RUN/STOP 0 = Run 1 = Standby 0 5 ALERT/ THERM2 0 = ALERT 1 = THERM2 0 4 Beta Enable 0 = Beta Compensation On 1 = Beta Compensation Off 0 3 Reserved Reserved 1 2 Temperature Range Select 0 = 0°C to 125°C 1 = Extended Range 0 1 Reserved Reserved 0 0 Reserved Reserved 0 Conversion Rate Register The conversion rate register is Address 0x04 at read and Address 0x0A at write. The lowest four bits of this register are used to program the conversion rate. For example, a conversion rate of five conversions per second means that beginning at 200 ms intervals, the device performs a conversion on the internal and the external temperature channels. The conversion rate register can be written to and read back over the SMBus. The higher four bits of this register are unused and must be set to 0. The default value of this register is 0x06, giving a rate of 4 conversions per second. Use of slower conversion times greatly reduces the device power consumption. |
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