An XMOS-based Polarimeter
A polarimeter shield developed for the XMOS Startkit is shown below. The shield plugs into the Startkit’s J7/J8 headers and contains just 2 IC’s - an L6470 microstepping motor control IC, and an LTC1865 analog-to-digital converter IC. The L6470 is a complete, single chip solution for driving stepper motors in micro-stepping mode - at up to 128 steps per normal step, as has been described earlier.
Leads from the stepper motor are connected at the two green blocks designated MOT A and MOT B on the left hand side of the shield. A 3 pin detector such as the TAOS TSL25x is connected at the 3 pin header J4 at the bottom left of the PCB. On the right hand side of the shield are connector blocks for AC power input (18-24V) - top and DC power out for an external high brightness LED (bottom).
During operation, the L6470’s absolute position register is polled to generate a synchronisation pulse at the start of each motor revolution. ADC readings from the detector (situated behind the spinning analyzing polariser) are processed in the same manner as described above for the Propeller-based polarimeter.