Increased sampling frequency

Due to some legacy code from the first MIMU3333 test boards, the inter­nal sam­pling fre­quency of the board was pre­vi­ously set to around 400Hz while the inter­nal sam­pling fre­quency of the IMUs was a 1000Hz. The main rea­sons for the low sam­pling fre­quency was that the IMUs (of the MIMU333 test board) were orig­i­nally con­nected to two dif­fer­ent ports on the uC and that the uC was run­ning on a sin­gle inter­nal PLL, which lim­ited the inter­nal clock fre­quency since the USB needed 48MHz. These hur­dles have now been cleared and the sam­pling fre­quency of the board is now set to 1000Hz, just like the inter­nal IMU sam­pling frequency.

Unfor­tu­nately, the 1000Hz sam­pling fre­quency is not with­out prob­lems. For the MIMU22BT boards every­thing works fine. How­ever, for the MIMU4444 array boards, the sam­pling fre­quency works fine inter­nally but the USB can­not keep up in case you want all raw data from all 32 IMUs. If you try to out­put all raw data from all IMUs at every sam­pling instant, the sam­pling fre­quency will drop to around 500Hz since the uC will be wait­ing for the data to be sent. I’m not sure whether this is a lim­i­ta­tion of the USB pro­file, the USB stack, or our code.

The mech­a­niza­tion and the fil­ter­ing of the aided iner­tial nav­i­ga­tion can be run at 1000Hz. This means that the clock fre­quency, i.e. power con­sump­tion, can be traded for sam­pling fre­quency. Cur­rently, the uC makes up around 2/3 of the power (cur­rent) con­sump­tion of around 100mA. Its power con­sump­tion will scale rather lin­early with the clock fre­quency. Con­se­quently, the power con­sump­tion could, in prin­ci­ple, for exam­ple, be reduced by 1/3 by halv­ing the clock fre­quency at the cost of halv­ing the sam­pling frequency.

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