The PULSE Inertial Measurement Unit (IMU) has been designed for demanding navigation (INS) and stabilization applications. Special care has been taken to ensure perfect and known synchronization between accelerometers and gyroscopes measurements.
The motion to output delay is also limited as much as possible to enable high-end stabilization applications. Time stamping precisely the IMU measurements is also critical to ensure correct integration in Inertial Navigation System (INS) solutions.
Internal Clock Accuracy
The PULSE uses a high accuracy and stable crystal to drive its internal clock. If no external clock reference is provided to the PULSE, the output data rate (ODR) and Sync Out signal accuracy is directly driven by the internal PULSE crystal accuracy.
The table below details the PULSE clock accuracy when running on its internal clock:
| Frequency Tolerance at 25°C | +/- 20 ppm |
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| Frequency Stability Over Temperature | +/- 20 ppm |
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| Specified Temperature | -40 to 85°C |
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Timing & Delays
The diagram below details the general sampling, processing and output scheme. In this diagram, the PULSE is setup with a 2 Mbit/s UART and an ODR (Output Data Rate) of 2 kHz.
The table below details the PULSE timing and delay for the same ODR of 2 kHz and 2Mbit/s baudrate:
| Power On To First Valid Data | < 800 ms |
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| Gyroscopes Delay | < 2.0 ms |
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| Accelerometers Delay | < 2.0 ms |
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| Transmit Time (41 bytes) | 205 µs |
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| Sync Out To Start of Transmit | 216 µs ± 50µs |
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| Sync Out To End of Transmit | 420 µs ± 50µs |
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| TxEn High to Start of Transmit | 500ns ±500ns |
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| TxEn Low to End of Transmit | 1.5 µs ±500ns |
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Sensor Delays
The accelerometers and gyroscopes delays are expressed as total delays ie motion to output. The accelerometers and gyroscopes channels have been carefully time synchronized for best Inertial Navigation accuracy.
Sync Out Signal
The PULSE can output a synchronization output signal that can be used to precisely timestamp IMU data. The Sync Out signal is generated exactly when the accelerometers and gyroscopes values are sampled. The sync out signal period, polarity and pulse width can be configured by the user.
| Signal Level | TTL 3.3V |
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| Signal Shape | Pulse or Toggle |
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| Signal Polarity | Rising or Falling Edge |
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| Pulse Width | 1 µs to 90% signal period |
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| Rise Time (10% to 90%) | < 100 ns |
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| Fall Time (90% to 10%) | < 100 ns |
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| Frequency | 1 Hz to 2 kHz |
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| Clock Accuracy (1σ) | ± 40 ppm over temperature |
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| Signal Activation Jitter | < 12 µs |
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| Signal De-activation Jitter | < 64 µs |
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Sync Out / Clock Input
The Sync Out and Clock Input are multiplexed on the same signal. The PULSE can thus only output a Sync Out signal when using the internal clock mode.
Tx Enable Signal
The Tx Enable signal (TxEn) is idle low and goes high as soon as the PULSE starts transmitting data over the UART. The signal stays high until the last byte has been sent by the PULSE. The Tx Enable signal then returns to the low state indicating the PULSE is not transmitting anymore.
This signal can be used by the host application to easily know when to start reading data from the PULSE and thus avoid polling or blocking read. The TxEn signal is also very helpful if you would like to implement RS-485 signal conversions.
