SLVUCP8 September 2024 TPS26750
The Liquid Detection Config register contains all the configurable settings for liquid detection including number of samples, timing between sampling, hysteresis range for entering and exiting liquid detected state, and the enable bits for Liquid Detection and Corrosion Mitigation. This section goes through the register field in more detail based on the TPS26750 Technical Reference Manual.
The Sleep Time fields allows for customizable delay between each sampling period. Higher sleep time can result in less power consumption but can take more time to enter or exit the liquid detected state depending on when the short occurred and when the sampling takes place. Likewise, shorter sleep time allows the TPS26750 to detect liquid faster, but increases power consumption.
Sleep Time in Seconds No Liquid [15:0] - The sleep timer or delay between each interval of sampling (in units of seconds) when there is no liquid or short detected on the USB Type-C connector. During this timer or delay, VBUS and CC are active and the PD is in normal operation.
Sleep Time in Seconds Liquid [31:16] - The sleep timer or delay between each interval of sampling (in units of seconds) when liquid/short is detected on the USB Type-C connector. During this timer or delay, VBUS and CC are disabled (if corrosion mitigation is enabled) and the TPS26750 is in the Liquid Detected state.
Number of Samples [39:32] - The number of samples to take for both high and low measurements. This field value needs to be in terms of 2n samples. For example, if this field is configured to be 0x3, the TPS26750 calculates this as 23 = 8 samples and takes 8 high samples (when SBU lines are pulled high) and 8 low samples (when SBU lines are pulled low) during the sampling intervals.
After the TPS26750 has collected 2n samples for both high and low samples, the TPS26750 computes two average values, one for the high and one for the low measurements, and compares against the set high and low thresholds. The TPS26750 allows the user to configure the high and low threshold to enter and exit the Liquid Detected state. The high measurement average is compared against the High Threshold to see if there are any shorts to ground. The low measurement average is compared against the Low Threshold to see if there any shorts to positive voltage. The high and low threshold value ranges from 0V to 3.3V for the register fields.
The Application Customization GUI already has liquid detection threshold values that have been tested with the TPS26750EVM and that are automatically set when the Liquid Detection feature is enabled. However, when setting up the high and low thresholds for custom designs, take into consideration hardware implementation (resistor value, board impedance, layout) and how this implementation aligns with the configurable thresholds. Once the custom design is complete, comparing the firmware voltage threshold against actual analog readings is important. If set incorrectly, then the TPS26750 can be stuck in an infinite cycle of falsely detecting liquid. For example, if the high threshold voltage is set to 3.0V, but the analog voltage on the SBU line (in non-liquid condition) is actually 2.7V, then this leads to the TPS26750 detecting a false Liquid Detection event.
Low Threshold ADC No Liquid [47:40] - The low voltage threshold to exit the Liquid Detected state. The TPS26750 uses this value to compare against the low samples to determine if there are shorts to positive bias voltage pins. This field needs to be less than or equal to the Low Threshold ADC Liquid field, otherwise the TPS26750 is not able to exit the Liquid Detected state.
High Threshold ADC No Liquid [55:48] - The high voltage threshold to exit the Liquid Detected state. The TPS26750 uses this value to compare against the high samples to determine if there are shorts to ground level pins (0V). This field needs to be greater than or equal to the High Threshold ADC Liquid field, otherwise the TPS26750 is not able to exit the Liquid Detected state.
Low Threshold ADC Liquid [63:56] - The low voltage threshold to enter the Liquid Detected state. The TPS26750 uses this value to compare against the low samples to determine if there are shorts to positive bias voltage pins. This field needs to be greater than or equal to the Low Threshold ADC No Liquid field, otherwise the TPS26750 is not able to enter the Liquid Detected state.
High Threshold ADC Liquid [71:64] - The high voltage threshold to enter the Liquid Detected state. The TPS26750 uses this value to compare against the high samples to determine if there are shorts to ground level pins (0V). This field needs to be less than or equal to the High Threshold ADC No Liquid field, otherwise the TPS26750 is not able to enter the Liquid Detected state.
The Sample Time field allows for customizable delays before the TPS26750 takes the voltage measurement from high to low and low to high. Increased sample time allows for the voltage reading to settle and can produce more accurate measurements but takes more time for the sampling interval to complete. Likewise, low sample time allows for faster sampling intervals but can produce less accurate measurements if the voltage hasn’t fully settled. The sample timing needs to be taking into consideration with the number of samples. For example, if sample time is set to 10ms with 23 samples (8 high and 8 low samples), the total time for each sampling interval can be calculated using the following equation:
Sample Time in 10ms No Liquid [75:72] - The time for each pulse to settle (configurable in 10ms steps) before the TPS26750 samples when there is no liquid or short is detected on the USB Type-C connector. This value applies to both high and low pulse sample durations.
Sample Time in 10ms Liquid [79:76] - The time for each pulse to settle (configurable in 10ms steps) before the TPS26750 samples when liquid or short has been detected on the USB Type-C connector. This applies to both high and low pulse sample durations.
Liquid Detection State [80] - Read only bit to inform if the TPS26750 is in a Liquid Detected state. ‘1’ represents that liquid is present, ‘0’ represents that no liquid is present. This bit acts the same as the GPIO event LIQUID_DETECTED [157] and the I2C interrupt Liquid Detection [60].
Enable Corrosion Mitigation [81] - If enabled, the TPS26750 automatically handles corrosion mitigation. When liquid is detected, the TPS26750 disconnects and disables the port by removing VBUS and pulling down the CC pins. If this bit is disabled, the TPS26750 continues normal operation if liquid is detected on the USB Type-C connector. An external microcontroller can enable or disable this feature on the fly when the TPS26750 is operating. Any modification to this field only takes effect the next time the appropriate policy is invoked in the Type-C policy engine.
Enable Liquid Detection [82] - The main control bit to enable the Liquid Detection feature. An external microcontroller can enable or disable this feature on the fly when the TPS26750 is operating. Any modification to this field only takes effect the next time the appropriate policy is invoked in the Type-C policy engine.