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Sensors 2019, 19, 959
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floating-point moving-average filter. Even when the fixed-point implementations are compared, our
lowpass architecture decreases runtime.
0
1
2
3
4
0
1
2
3
Time (s)
Amplitude (V)
Short Strong Contraction
Weak Contraction
Relaxed Muscle
Threshold
Figure 16. Rectified and smoothed EMG signal for various movements with the threshold for prosthesis
drive activation indicated.
Table 5. Runtime per sample of filters in C implementation at a 48 MHz clock.
(µs)
Comb Filter (2nd order IIR)
8.17
Highpass Filter (2nd order IIR)
5.88
Lowpass Filter (1st order IIR)
1.28
Rectification and Smoothing
2.62
Total Signal Processing Chain Runtime
17.95
Table 6.
Comparison of runtime per sample of various lowpass-filter C implementations at
a 48 MHz clock.
(µs)
(i) Selected Lowpass Filter (1st order IIR)
1.28
(ii) Fixed-point Lowpass Filter (1st order IIR, direct form II)
1.67
(iii) Floating-Point Lowpass Filter (1st order IIR, direct form II)
20.12
(iv) Floating-Point Lowpass Filter (5th order FIR)
49.80
(v) Floating-Point Lowpass Filter (8th order FIR)
73.60
4.7. Power Consumption
The current consumption of the sensor system, as presented by Roland et al. [11] was measured
to be 8.5 mA at a supply voltage of 3.7 V (=31.5 mW). This includes the µC, which requires 5.2 mA.
For this measurement, the controller was set to 48 MHz clock frequency in PL2 mode and the ADC,
DAC, brown out detector and IO-pins were activated. The current consumption of the controller
includes also the integrated OpAmps for gain adjustment, the non-volatile memory and the DSP
software. The BLE module would increase the current consumption at 3.7 V by 0.9 mA in advertising
mode and by 8.9 mA in send/receive mode [45]. The send/receive mode is only activated in case of
sensor configuration or data transfer to the PC. In deep sleep mode, the power consumption of the
BLE module is negligible. However, the high clock frequency was selected to avoid limitations in
the experiments, which comprise also floating point implementations. The relation between µC clock
frequency and power consumption is given in Table 7. Due to the fact that the ADC clock is connected
to the µC clock, the ADC settings have to be checked and recalculated accordingly.