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DS2460S+ Datasheet(PDF) 7 Page - Dallas Semiconductor |
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DS2460S+ Datasheet(HTML) 7 Page - Dallas Semiconductor |
7 / 8 page Abridged Data Sheet DS2460 7 of 8 Read and Write This section discusses the read and write behavior of the various registers and the EEPROM. Please refer to the full data sheet for details. SHA-1 Engine Control This section describes the user’s view of the SHA-1 engine and how to operate it. For details refer to the full data sheet (includes Figures 7 to 9 and Tables 1 and 2). SHA-1 COMPUTATION ALGORITHM This description of the SHA computation is adapted from the Secure Hash Standard SHA-1 document that can be downloaded from the NIST website (www.itl.nist.gov/fipspubs/fip180-1.htm). Further details are found in the full version of the data sheet. Application Information SDA and SCL Pullup Resistors SDA is an open-drain output on the DS2460 that requires a pullup resistor (Figure 10) to realize high logic levels. Because the DS2460 uses SCL only as input (no clock stretching) the master can drive SCL either through an open-drain/collector output with a pullup resistor or a push-pull output. Figure 10. Application Schematic VCC SDA SCL µC GND VCC SDA AD0 SCL AD1 AD2 GND RP RP VCC To additional 2-wire devices Example slave address = 100 0101 Pullup Resistor RP Sizing According to the I²C specification, a slave device must be able to sink at least 3mA at a VOL of 0.4V. This DC condition determines the minimum value of the pullup resistor: R PMIN = (VCC - 0.4V)/3mA. With an operating voltage of 5.5V, the minimum value for the pullup resistor is 1.7k Ω. The "Minimum RP" line in Figure 11 shows how the minimum pullup resistor changes with the operating (pull-up) voltage. For I²C systems, the rise time and fall time are measured from 30% to 70% of the pullup voltage. The maximum bus capacitance CB is 400pF. The maximum rise time must not exceed 300ns. Assuming maximum rise time, the maximum resistor value at any given capacitance CB is calculated as: RPMAX = 300ns/(CB*ln(7/3)). For a bus capacitance of 400pF the maximum pullup resistor would be 885 Ω. |
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