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AD7417ARU-REEL7 Datasheet(PDF) 14 Page - Analog Devices |
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AD7417ARU-REEL7 Datasheet(HTML) 14 Page - Analog Devices |
14 / 20 page REV. G –14– AD7416/AD7417/AD7418 Please note that when reading back from the ADT7416/ ADT7417/ADT7418, no more than three bytes of data must be read back. A STOP command must be inserted at the end of the read communication. If a STOP command is not inserted by the master and the ADT7416/ADT7417/ADT7418 receives more SCL cycles than the maximum needed for three bytes of data, then the I 2C interface on the ADT7416/ADT7417/ ADT7418 will pull the SDA line low and prevent it from going high again. To recover the ADT7416/ADT7417/ADT7418 interface the part must be powered off and on again. Reference the application note, AN-686, on the Analog Devices website for more information on I 2C interfaces. OTI OUTPUT The OTI output has two operating modes, which are selected by Bit D1 of the Configuration Register. In the comparator mode, (D1 = 0), the OTI output becomes active when the temperature exceeds TOTI and remains active until the tempera- ture falls below THYST. This mode allows the AD7416/AD7417/ AD7418 to be used as a thermostat, for example, to control the operation of a cooling fan. TOTI THYST OTI OUTPUT COMPARATOR MODE OTI OUTPUT INTERRUPT MODE READ* READ* READ* READ* READ* READ* READ* IN INTERRUPT MODE, A READ OPERATION OR SHUTDOWN RESETS THE OTI OUTPUT; OTHERWISE THE OTI OUTPUT REMAINS ACTIVE INDEFINITELY, ONCE TRIGGERED. * Figure 13. Operation of OTI Output (Shown Active Low) The open-drain configuration of OTI allows the OTI outputs of several AD7416/AD7417/AD7418s to be wire-ANDed together when in active low mode. The OTI output is used to indicate that an out-of-limit tem- perature excursion has occurred. OTI is an open-drain output that can be programmed to be active low by setting Bit D2 of the Configuration Register to 0 or active high by setting Bit D2 of the Configuration Register to 1. In the interrupt mode (D1 = 1), the OTI output becomes active when the temperature exceeds TOTI and remains active even if the temperature falls below THYST, until it is reset by a read opera- tion. Once OTI has become active by the temperature exceeding TOTI, and has then been reset, it will remain inactive even if the temperature remains, or subsequently rises again, above TOTI. It will not become active again until the temperature falls below THYST. It will then remain active until reset by a read opera- tion. Once OTI has become active by the temperature falling below THYST and then reset, it will remain inactive even if the temperature remains, or subsequently falls again, below THYST. OTI is also reset when the AD7416/AD7417/AD7418 is placed in shutdown mode by setting Bit D0 of the Configuration Regis- ter to 1. The OTI output requires an external pull-up resistor. This can be connected to a voltage different from VDD (for example, to allow interfacing between 5 V and 3.3 V systems) provided that the maximum voltage rating of the OTI output is not exceeded. The value of the pull-up resistor depends on the application but should be as large as possible to avoid excessive sink currents at the OTI output, which can heat the chip and affect the tempera- ture reading. The maximum value of the pull-up resistor that will meet the output high current specification of the OTI out- put is 30 k Ω, but higher values may be used if a lower output current is required. For most applications, a value of 10 k Ω will prove suitable. FAULT QUEUE To avoid false triggering of the AD7416/AD7417/AD7418 in noisy environments, a fault queue counter is provided that can be programmed by Bits D3 and D4 of the Configuration Regis- ter (see Table V) to count 1, 2, 4, or 6 fault events before OTI becomes active. In order to trigger OTI, the faults must occur consecutively. For example, if the fault queue is set to 4, then four consecutive temperature measurements greater than TOTI (or less than THYST) must occur. Any reading that breaks the sequence will reset the fault queue counter, so if there are three readings greater than TOTI followed by a reading less than TOTI, the fault queue counter will be reset without triggering OTI. POWER-ON DEFAULTS The AD7416/AD7417/AD7418 always powers up with the following defaults. Address pointer pointing to Temperature Value Register com- parator mode TOTI = 80 °C THYST = 75 °C OTI Active LOW Fault Queue = 1 These default settings allow the AD7416/AD7417/AD7418 to be used as a standalone thermostat without any connection to a serial bus. OPERATING MODES The AD7416/AD7417/AD7418 has two possible modes of operation depending on the value of D0 in the Configuration Register. Mode 1 Normal operation of the AD7416/AD7417/AD7418 occurs when D0 = 0. In this active mode, a conversion takes place every 400 µs. Once the conversion has taken place, the part partially powers down, consuming typically 350 µA of the current until the next conversion occurs. Two situations can arise in this mode on the request of a tempera- ture read. If a read occurs during a conversion, the conversion aborts and a new one starts on the stop/repeat start condition. The temperature value that is read is that of the previous com- pleted conversion. The next conversion will typically occur 400 µs after the new conversion has begun. If a read is called between conversions, a conversion is initiated on the stop/repeat start condition. After this conversion, the part returns to performing a conversion every 400 µs. |
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