LtFNTunIumo/U6qLzbQcHgI/AAAAAAAAQOo/COvH_VX-ZCI/s1600/WlMqm71403685766.jpg' alt='Pull Up And Pull Down Resistors Pdf' title='Pull Up And Pull Down Resistors Pdf' />Design calculations for robust I2. C communications. Chris Parris, Senior Applications Engineer and Jonathan Dillon, Senior Applications Engineer, Memory Products Division, Microchip Technology Inc. April 1. 8, 2. 01. Many systems use an I2. C bus for internal communications between devices, such as microprocessors, microcontrollers, memories, and other digitally controlled devices. SMSC Ethernet Physical Layer Layout Guidelines SMSC AN18. Revision 0. 8 102708 APPLICATION NOTE Decouple ground planes as practical, as shown below in Figure 2. In electronic logic circuits, a pullup resistor is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as. This bus topology relies on correctly sized resistance pull ups for reliable, robust communications. Incorrectly sizing these resistors can lead to erroneous bus conditions and transmission errors caused by noise or changes in temperature and operating voltages, and by variations between devices. I2. C is a two wire synchronous bus with the SCL line used as a clock, produced by the bus master. The SDA line is used for bi directional data transfer. The data line is modified while the clock is in specific states, to indicate the start and stop of transmissions, and avoid additional lines. The I2. C bus is built around open collector outputs, where a device can pull a line low through a transistor to ground, as shown in Figure 1. This allows easy arbitration over control of the bus, enabling the implementation of bi directional communications on a single data line and multi master support. Pull Up And Pull Down Resistors Pdf' title='Pull Up And Pull Down Resistors Pdf' />As shown in Figure 1, each line has an external resistor to Vdd, which pulls the line high when released or idle. The three considerations when determining the pull up resistor values Rp are Supply voltage VddTotal bus capacitance CBUSTotal high level input current IIH. Calculating the ideal pull up resistor values for the following example conditions Supply voltage Vdd of 5. VClock frequency of 4. KHz. Bus capacitance of 1. FSupply Voltage Vdd. OuHfEQLgg/Uevm54jUsAI/AAAAAAAABGk/Ve0CRyJKyxY/s1600/Pull-down+resistors.png' alt='Pull Up And Pull Down Resistors Pdf' title='Pull Up And Pull Down Resistors Pdf' />The I2. C specification defines a voltage below VIL, or 3. VIH, or 7. 0 of the supply voltage, as a logical high, as shown in Figure 2. A voltage between these two levels leads to an undefined logic level. In reality, the pin will read either logical high or low in this range, but it may vary between devices, with temperatures, voltages, noise sources and other environmental factors influencing the logic levels. The supply voltage limits the minimum Rp value for which the bus can be pulled low. A strong pull up will prevent a device from being able to bring the line sufficiently low, to ensure a logical low is detected. This is caused by the potential divider formed between the pull up resistor and the on resistance of the transistor to ground, as shown in Figure 3. The on resistance of the transistor is not typically specified. Now that you have assembled, tested, and installed your MegaSquirt, you need to get your engine started and tuned. This is not too difficult if you work methodically. MDP 01, 03, 05 www. Vishay Dale Revision 12Sep13 3 Document Number 31511 For technical questions, contact ff2aresistorsvishay. THIS DOCUMENT IS. Sony VFETs in PushPull Class A Part 1 Common Source Mode, Transformer Coupled By Nelson Pass Introduction This article is the first of a series presenting fairly. When I first got involved in digital electronics, it took me awhile to understand the concepts of pullup and pulldown resistors and when to use up or dow. Instead, a maximum sink current IOL is given for which the voltage drop across the transistor is below the output logical low voltage level VOL. Applying Ohms Law yields Equation 1. For Microchips I2. C EEPROM devices, the VOL specification is a maximum of 0. V at an IOL of 3 m. A, with other manufacturers devices in a similar range. Equation 1 Minimum pull up resistance, allowing the bus to be pulled low. For multiple devices on the bus, the minimum Rp is determined by the device with the lowest sink current. Total Bus Capacitance CBUSOn the SCL and SDA lines, the capacitance includes all pins, connections, PCB traces and wire. Combined, this is referred to as the bus capacitance and, for long traces and cabling, this can be significant. The open collector topology requires the external resistor to pull the line high when released. The pull up resistor, coupled with the bus capacitance, has an RC time constant, which limits the rise time. Software Fritz Box 7240 here. This becomes significant with increasing clock frequencies, as less time is available for the line to rise. If the selected resistor value is too high, the line may not rise to a logical high before it is next pulled low. This is an important consideration for designs that feature many devices on a single bus, which often have higher bus capacitance. Bus capacitance can be calculated from PCB trace lengths and published pin capacitance, or measured using capacitance probes or smart tweezers. If a precise calculation or measurement of the bus capacitance is not possible, an overestimated worst case reading should provide a safe maximum resistance value. Equation 2 is the general equation used to determine the voltage across a charging capacitive load, as a function of time. This allows for the calculation of the time required for the bus voltage to rise to a particular value, for a specific pull up resistance and bus capacitance. Equation 2 General equation of charging a capacitor through a resistor. We can then calculate the time T1 for the voltage to rise to VIL the time T2 to rise to VIH and, critically, the time between these two levels TR, as shown in Figure 4. Since both VIL and VIH are products of Vdd, the equation is independent of supply voltage, since the Vdd terms cancel out. The maximum rise time for a variety of operating voltages is specified by the I2. C standard, and is determined by the pull up resistance. From this time and the bus capacitance, we can calculate the maximum allowable pull up resistance Rp. For a 4. 00 k. Hz clock frequency at 5. V, the specified maximum rise time, TR, is 3. CBUS of 1. 00 p. F. Equation 3 Minimum pull up resistance value to meet I2. C rise time standard. Total High Level Input Current IIH. Even when no device is pulling down the line and it is a logical high, current continues to flow through the pull up resistors. This current is caused by the leakage of the digital inputs of the devices on the bus, from low quality PCB materials and possibly from soldering residues. Some of these cannot be foreseen, but, assuming quality materials and good manufacturing practices, the input pin leakage is dominant. From Figure 2, the line needs to be above VIH to be regarded as logical high, when there are no devices pulling the bus low. The leakage current limits the maximum value of Rp, such that the voltage drop across it does not prevent the line from being pulled above VIH. It is also prudent to allow some guard margin on the VIH specification, to prevent noise spikes from bringing the voltage below the VIH level. For robust operation in a high noise environment, the I2. C specification recommends 0. Vdd as a suitable margin above VIH. Equation 4 Additional margin over logical high input level. The leakage of digital inputs is normally given in the datasheet of devices and, for Microchips I2. C EEPROM devices, the maximum input leakage current Il. IEE is 1 A. The minimum components for a system are a microcontroller I2. C master and an I2. C slave device. For this example, assuming a microcontroller with 1 A input leakage Il. IMCU and four I2. C EEPROM devices, and allowing 1. IIH is 1. 0 A. Equation 5. Leakage current due to pin leakages for defined bus. Applying Ohms law, we can determine the maximum value for Rp that will meet these specifications. Equation 6 Minimum pull up resistance value to ensure logical high. Resistor Value Calculation. From the supply voltage, the bus capacitance and the leakage calculations, we have a range of values for RP. The 5. 0 K maximum caused by the leakage current can be discarded, since the bus capacitance dominates. As a result, the range of acceptable resistor values is. Designers should choose a value near the middle of the range, to provide as much guard banding as possible. For this example, a 2. K pull up resistor would be ideal. Bus Speed vs. Power Consumption.