I²C communication is widely used due to its simplicity, but when the system grows (more devices, longer traces, different voltage domains), communication issues may appear.
One common root cause is the incorrect use of I²C buffers or level translators.
This post explains:
How I²C logic levels really work
Why buffers are needed
Types of I²C Buffer Technologies
How Static Voltage Offset (SVO) works
Common design mistakes to avoid
How I²C logic levels really work
I²C does not use fixed voltages like 0 V or 5 V to define logic states. Instead, it uses thresholds relative to VCC:
Logic 0 (LOW): below ~30% of VCC
Logic 1 (HIGH): above ~70% of VCC
Undefined region: between 30%–70% → signal is unreliable
In real systems, signal levels may not behave ideally.
Due to:
Different driver strengths
Load variations
Buffer offset behavior
You may observe three voltage levels, even though the protocol only defines two logic states.
This can lead to misinterpretation of signals.
We will explore this behavior in more detail in the following sections, especially when analyzing how I²C buffers and offset mechanisms work.
Why buffers are needed
You should consider an I²C buffer when:
Bus capacitance exceeds ~400 pF
Long PCB traces or cables are used
Multiple devices are connected
Different voltage domains are required
Isolation between sections is needed
Buffers divide the system into smaller segments and improve signal integrity.
For a deeper understanding, refer to the training: The Ins and Outs of I²C Bus Buffers
Types of I²C Buffer Technologies
Different buffer architectures solve these challenges in different ways:
Static Offset: Introduces a fixed voltage offset on one side of the bus, enabling direction detection.
Incremental Offset: Adds a small dynamic voltage shift (~100 mV), allowing symmetrical bidirectional operation.
Amplifier: Boosts sink current instead of shifting voltage, helping weak drivers control heavier loads.
In this post, we will focus specifically on Static Offset buffers, as they are widely used and often misunderstood in system design.
Static Offset Buffer Characteristics
Static Offset buffers are commonly used due to their robust behavior:
Fully bidirectional and support multi-master operation, including clock stretching and do not generate any ‘glitches’.
Isolate bus segments electrically, preventing capacitance and loading on one side from affecting the other side.
Each side operates independently with its own rise time and pull-up network.
Regenerate signals, producing clean LOW levels independent of the input LOW amplitude.
Use a fixed Static Voltage Offset (SVO), creating a slightly higher LOW level on one side of the buffer. This offset allows the buffer to determine the origin of a LOW signal and prevents latching.
Weaknesses of Static Offset Buffers
Require specific design rules due to their special offset voltage levels.
Static Offset ports cannot be connected together on the same bus segment.
Only one offset port should exist per bus segment.
Add propagation delay, which must be considered in timing analysis. Multiple buffers can reduce the maximum achievable I²C bus speed
The main challenge in I²C buffers is that the bus is bidirectional. Both master and slave can pull the line LOW. By introducing a fixed voltage offset, the buffer can distinguish which side initiated the LOW condition and correctly propagate the signal across the bus.
How Static Voltage Offset (SVO) works
Static Voltage Offset introduces an intentional voltage offset, so a LOW level is not always represented by 0 V. Instead, the buffer creates two valid LOW levels, both within the I²C specification. To achieve this, the input and output logic levels on the special side of the buffer are intentionally designed to be different.
The key concept is that the special output LOW voltage (VOL) is set slightly higher than the corresponding input HIGH threshold (VIH):
For example:
Special output VOL ≈ 0.7 V
Special input VIH ≈ 0.55 V
When the buffer drives the special side LOW at approximately 0.7 V, that voltage is still considered a valid LOW by other I²C devices on the bus. However, for the buffer's own special input, 0.7 V is interpreted as a HIGH because it is above its VIH threshold.
As a result, the buffer's output cannot feed back into its own input, preventing the latching condition commonly found in simple bidirectional buffers.
This enables the buffer to distinguish between two conditions:
State 1:
The voltage is between the special input threshold (VIL) and the offset level (VOL/SVO).
This indicates that the LOW originated from one side of the buffer.
State 2:
The voltage is pulled much closer to 0 V, below the special threshold.
This indicates that the LOW originated from the opposite side of the buffer.
By monitoring these voltage levels, the buffer can determine where the LOW signal originated (Side A or Side B). Once the direction is identified, the buffer can correctly propagate the signal to the other side while avoiding feedback, latching, and communication conflicts.
But, How Does the Buffer Determine the Signal Direction?
The Buffer uses an internal voltage offset to automatically determine the direction of I²C signal propagation.
The offset appears on the opposite side of the device that is actively pulling the bus LOW, allowing the buffer to identify the signal source and maintain proper bidirectional operation.
The buffer continuously monitors the voltage present on both sides and compares it against the internal offset thresholds.
Case 1: LOW originated from the Non-Offset side (A → B)
A device on Side A pulls the line LOW.
The buffer propagates this LOW to Side B while adding the static offset:
Side A = LOW Side B = LOW + Offset
As a result:
Input = LOW
Output = OFFSET
The presence of the offset voltage indicates that the signal originated from the Non-Offset side and is being propagated toward the Offset side.
Case 2: LOW originated from the Offset side (B → A)
A device on Side B pulls the line LOW.
The buffer recognizes that the voltage has been driven below the offset threshold and propagates a true LOW toward Side A:
Side B ≈ LOW Side A ≈ LOW
As a result:
Input = LOW
Output = LOW
The absence of an offset on the output indicates that the signal originated from the Offset side and is being propagated toward the Non-Offset side.
INPUT
OUTPUT
DIRECCITION
LOW
LOW
B SIDE TO A SIDE
LOW
OFFSET
A SIDE TO B SIDE
Common design mistakes to avoid
When using I²C buffers that implement Static Voltage Offset (SVO), it is important to carefully consider both device selection and system-level design. This is because not all buffers use the same offset value. Typically, the SVO can vary between approximately 0.1 V and 0.6 V, depending on the internal design of each device. This variation directly affects how LOW levels are interpreted and how signals are propagated between segments of the bus, which can ultimately impact the reliability of bidirectional communication.
Another important aspect is that the offset is not always located on the same side of the buffer. In some devices, the offset is applied on Side B, while in others it is on Side A. There are even cases where the offset may dynamically change sides depending on the operating conditions. For this reason, it is essential to fully understand the internal behavior of the selected buffer in order to avoid unexpected system behavior.
The easiest way to verify this is by reviewing the device datasheet and identifying whether it specifies a parameter such as the VOL–VILC difference, which represents the difference between the LOW-level output voltage and the LOW-level input voltage during a contention condition. Voltage contention occurs when both sides of the buffer attempt to pull the line LOW at the same time but with different voltage levels or drive strengths.
Due to this variability, there is a very important design rule: SVO-based buffers must never be directly connected to each other, whether in series or in parallel. This means that you should not connect two buffer sides that both introduce a voltage offset, as their LOW levels will not align correctly.
The reason for this limitation is that it can lead to errors in LOW-level propagation. In such cases, a LOW signal may propagate correctly in one direction but fail in the opposite direction, breaking the bidirectional behavior of the I²C bus, which is essential for proper communication.
These diagrams show correct implementations of SVO-based I²C buffers. In these configurations, only one side of the system introduces a static voltage offset, allowing the buffer to properly distinguish the origin of the LOW signal. This ensures correct bidirectional communication, stable signal propagation, and prevents contention or oscillation issues.
These diagrams illustrate incorrect configurations where two SVO-based buffers are directly connected. In this case, both sides introduce different LOW offset levels, preventing proper alignment of logic thresholds. As a result, the buffer may fail to correctly detect the signal origin, leading to incomplete LOW propagation, loss of bidirectional behavior, and potential communication failures.
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