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HI-8428 Datasheet(PDF) 5 Page - Holt Integrated Circuits |
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HI-8428 Datasheet(HTML) 5 Page - Holt Integrated Circuits |
5 / 16 page FUNCTIONAL DESCRIPTION (cont.) HOLT INTEGRATED CIRCUITS 5 WETTING CURRENT In Ground/Open mode a current of approximately 1mA is sourced from the SENSE pin when it is grounded and VDD is powered at 12.0V. The wetting current serves to provide current through switches or relay contacts to prevent dry contacts and improve switch contact reliability. When programmed as Supply/Open sensors, CRn is set to a logic 0. Referring to Figure 1, Supply/Open thresholds are set at >12V for the supply state and <6V for the open state, providing 3 - 6V of hysteresis / noise rejection. For the Supply/Open case the wetting current into the sense input is simply the current sunk by the effective 30k to GND. For V = 28V, I is 1ma. See Figure 2. SUPPLY/OPEN SENSING WETTING CURRENT Ω SENSE WET a switch in series with a diode is closed to provide a pull down to ground of 30k . Ω BUILT-IN TEST Writing a high in Control Register bit CR9 puts the HI-8428 into the Built-In Test (BIT) mode. Referring to Figure 1, when in the test mode each of the internal inputs to the sense comparators are set to either a high or low. Since the input sense pin is isolated by a 680k resistor, this test mode will not disturb the actual status of the input pin. When in BIT mode, setting CRn high for a particular sensor forces the comparator inputs high. A zero in CRn forces the comparator input low. To verify correct comparator behavior, the user must read from the Data Register and compare with the value written to CR1-8. Ω NOTE: but while still in BIT mode Certain flight applications require periodic sensor testing during flight. To ensure seamless transition between BIT mode and normal operation mode, the following steps should be followed: 1) The host should read and record the Configuration Register value for normal mode operation. 2) The host should read and record the last value of the Data Register before enabling BIT mode (CR9 = 1). 3) Following test completion, , the host should set the sensor outputs to their pre-test values by writing bits CR8 - CR1 with their corresponding Data Register pre-test values recorded in step 2) above. 4) Normal operation (CR9 = 0) is restored by writing the Configuration Register with its pre-test value stored in step 1). HI-8428 |
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