Hello I need to communicate with a device using SMBus. However, I could not choose correct value for pull up resistors. Vdd is 4.9V and I use SMBus. In same conditions for I2C communications I used 2k ohm resistors and it worked, but I am not sure the proper value for SMBus. Any ideas?
In its datasheet it says this one, but I could not understand it totally. I think it says for 5 V I need 15 k resistors. However, if I use 20 k resistors with 4.9V Vdd, would it also work?
This quote is from "Application Note: Implementing The SMBus Interface For Use With Bren-Tronics Batteries":
One of the features of SMBus is its ability to interface systems with different supply voltages, through its implementation of an open-drain interface with absolute (as opposed to ratiometric, relative to the supply voltage, like I2C) high/low voltage thresholds. Vih of the SMBus interface is 2.1V; the upper bound of logic-high voltage is 5.5V – facilitating operation with 3.V/3.3V/5V logic. The pull-up devices for the open drain interface (one for each line – clock and data) are simply connected to the supply voltage in the host system.
The “letter of the law” in SMBus is that the “steady-state” output-low current (Iol) must be limited to 350 microamperes or less. This translates into pull-up resistors of 15K ohms in a 5V system, for the simplest of SMBus interfaces. However, the stray capacitances present in wiring and/or ESD/EMI-suppression devices of bus-connected devices can sometimes extend rise times beyond the 1.0 microsecond specification limit if resistive pull-ups that comply with the Iol limit are used.
There are a number of options that can be used to address these limitations.
Active pull-up devices, that provide higher dynamic current during the low-to-high transition while maintaining the steady-state limit elsewhere, can be implemented as shown in Figure 2.3 of the specification.
A simpler approach is to use lower-value resistive pull-ups, if currents higher than the Iol limit can be tolerated by ALL bus-connected devices (present and future) without violating the 0.8Vdc Vil specification limit. This is often the case, as this is a technique frequently used in commercial system designs. In this regard, Bren-Tronics batteries go beyond the specification, with the ability to sink SMBus current at levels up to 700 microamperes or more.