TMP125 SPI Sensor: 7 Checks for Ten-Bit Data and Shutdown
TMP125 SPI Sensor: 7 Checks for Ten-Bit Data and Shutdown
Decode TMP125 ten-bit signed temperature correctly and review SPI framing, SI shutdown control, one-shot operation and the freshness of conversion results. This is a documentation-based planning guide, not a report of independent hands-on testing.
1. Identify TMP125 rather than borrowing a nearby driver
TMP125 is a temperature sensor with a ten-bit result and 0.25 degree Celsius code resolution. It is not the thirteen-bit TMP123, nor a drop-in register-compatible replacement simply because both use SPI-style communication. Verify the six-pin package and distinguish SI from SO. Check the documented 2.7 V to 5.5 V supply range and host logic compatibility. Resolution describes code spacing, not measurement accuracy or the temperature of an object located elsewhere on the board. Manufacturer documentation.
2. Decode only the documented temperature field
The readout is a sixteen-bit word, but the temperature is contained in bits D14 through D5 as a ten-bit signed twos-complement value. D15 is a leading zero, and the lower five bits repeat the least significant temperature bit rather than adding resolution. Extract the ten-bit field, sign-extend that field, and multiply by 0.25 degrees Celsius. Test the decoder against positive, zero and negative examples; a generic signed-sixteen-bit conversion is not equivalent. Manufacturer documentation.
3. Establish the serial frame before interpreting numbers
Pulling CS low latches the last completed conversion into the shift register. Follow the manufacturer timing diagram for clock edges, setup, hold and chip-select behavior rather than selecting an SPI mode by trial alone. A complete transaction also matters for input-side control, so verify the full sixteen clocks with a logic analyzer when available. Keep transaction errors separate from valid temperature values. A plausible room-temperature number is only one test vector, not proof of correct framing. Manufacturer documentation.
4. Treat SI as a control signal, not an unused serial pin
The power-down control is sampled from SI on the documented third rising clock edge, and all sixteen bits must be clocked for shutdown to take effect. A normal read can therefore change the operating state if the transmitted pattern is careless. For applications not using shutdown, follow the SI connection shown in the typical circuit. Keep the transmit pattern explicit in firmware and verify the physical pin: input SI and output SO have different jobs. Manufacturer documentation.
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5. Keep one-shot control distinct from returned data
The one-shot sequence requests a conversion and then returns the device to shutdown. The transaction used to request it does not magically contain the newly completed result. Wait for the documented conversion interval before reading that result, and retain the desired power-down control during the read if another conversion is not wanted. Check the separate one-shot timing diagram rather than inventing a generic configuration register. Record state transitions so a delayed measurement is not mislabeled with a later acquisition time. Manufacturer documentation.
6. Track conversion freshness and the physical thermal path
After power-up or reset the result can read zero until the first conversion completes; zero is therefore not automatically a measured temperature. The documented continuous conversion period is about 120 ms, so faster repeated reads can return the same completed sample. Timestamp the acquisition policy rather than treating every SPI transaction as a new conversion. Remember that the sensing element is the chip itself, with package leads and board copper influencing its thermal response. Manufacturer documentation.
7. Validate signed decoding and low-power recovery together
Use a stable, safe temperature reference and allow the assembly to equilibrate. Verify raw frames, extracted codes, temperature scaling and the delay between wakeup and accepted data. Exercise shutdown and one-shot recovery without assuming the TMP123 chip-select conversion behavior applies to TMP125. Log supply, SPI settings, control bits and sampling interval. Manufacturer accuracy limits and reference-instrument uncertainty remain part of the error budget; this documentation-based guide does not claim independent calibration or suitability for safety-critical thermal protection. Manufacturer documentation.
Related reading: TMP123 SPI Sensor: 7 Checks for Chip-Select Conversion Freshness.
Catalog note: The exact-part search did not return a matching Epro listing during preparation. Check the current catalog; this guide makes no price, availability or included-accessory claim.