Timing a Strong Pullup for Parasite Powered 1 Wire Operations

A parasite-powered 1 Wire device can communicate successfully yet fail during an operation that requires more current. The ordinary pullup resistor may establish logic levels without supplying enough power for conversion or a memory operation. Validate the strong-pullup interval and the voltage delivered at the remote device.

Identify operations that need power support

The Analog Devices DS18B20 data sheet describes strong-pullup requirements for parasite-powered temperature conversion and copying scratchpad data to nonvolatile memory. The required timing and duration depend on the operation and configuration.

Record the exact device, power mode, command, conversion setting, and number of devices active at once. Do not assume that every 1 Wire product has identical power requirements.

Check the delivered voltage

Measure the bus voltage near the remote device during the complete powered interval. Include the strong-pullup switch drop, cable resistance, connector resistance, and total current demand.

For example, the controller end can remain near its supply voltage while a long cable produces a lower voltage at the sensor. A successful logic-high measurement at the controller does not establish adequate remote operating voltage.

Capture the command-to-power transition

Observe the last command bit, strong-pullup control, and bus voltage on one timeline. Verify that power support begins within the device’s specified interval and remains active for the necessary operation time.

Check the worst supported conversion duration rather than ending the interval based on one fast sample. If the device cannot communicate during the strong-pullup state, use the documented completion method for that power mode.

Release the bus before communication

Confirm that the strong pullup is disabled before another device needs to pull the line low. A low-impedance source left enabled can prevent a valid response and create contention.

Test repeated operations and command failures so the control software does not leave the power switch active indefinitely or release it too early. Include controller reset during the operation and verify the resulting default switch state.

For multiple devices, budget simultaneous demand explicitly and measure the supply response when the selected group begins its operation together. A command that starts conversion on every device can require more current than a test addressing only one sensor.

Save the command sequence, power interval, remote voltage, node count, and operation result. The completed evidence should show that the bus supplies both communication levels and operation energy at the intended location. A valid presence pulse or readable identifier demonstrates basic communication, not successful completion of every parasite-powered operation.