RoseOnRails – Return from Warsaw

Hi everyone !

Tiring and cold week in Warsaw, I hope this cold doesn’t make me too sick.

This morning I explore the datasheet of the four Hall captors we’ve at our disposal thanks to Alexis in Telecom. They have differents characteristics :

First, three kinds of them are General Purpose Chopper Stabilized Switches  (A112X) and one kind of General Purpose Stabilized Latches (A122X). The major differences between these kinds are shortly explained here :

Hall-effect switches are most commonly used in sensing absolute position, because they require a south pole of sufficient strength to turn the output on and then removal of that field to turn the output off. ”

Hall-effect latches are most commonly used in sensing multi-pole ring magnets for motor commutation, because they require a south pole of sufficient strength to turn the output on and a north pole of sufficient strength to turn the output off. ”

Others differences intrinsic to each classification are with me composant we’ve the hysteris function of the captor and if the sensor is mounted (LH package) or not (UA package).

The parts number we’ve have are :

Part Number             Operate Point(G)             Release Point (G)         Hysterisis(G)

A1120(LH)               <50 (typ 35)                   >5 (typ 25)                    Typical 10

A1121(UA)                 <120                                >40                                  >10 Typical 25

A1122(UA)               <205                                >105                                 >10 Typical 25

A1220(UA)               5 to 40                             -40 to -5                          Typical 45


G unit stands for 1 Gauss = 10^-5 Tesla.

With Gleison we’re going to test which is more convenient for us according to the magnetic field we received from the magnet under the rail.

Another urgent matter was on our agenda. Find another way than SPI in order to control our LED strips. We conclude following Sam advice we could control our LED strip by only using timers and GPIOs.

We’ve make one calculation :

Number of LEDs * time to refresh one LED + duration of the reset signal
900 *(1.25*24)+50 = 28.2 ms

The persistence of the vision is commonly evaluated to 1/25 second = 40 ms. So we should be able to refresh our fast enough using one thread on STM32.

Why are we using only one thread because if we were using multi-thread there would be some interruptions and the accuracy of the timing to control the LEDs wouldn’t be precise enough.

The STM32 we’re going to use is STM32F405RGT6 because he was already in the library on Expedition PCB and meets our needs (enough GPIOs, at least one timer, UART available to communicate with the nrf51822 on the same PCB). We also assume it would be better if we use PWM to have a processor frequency of at least 100MHz, in order to use for example a PWM clock frequency of 100MHz and a round number of ticks of 125. The difference of price with the last one (STM32F207VET6) which could have met our criteria was inferior to 2 euros.

I also start the schema of our LEDs PCB. Today we’re going to evaluate precisely how many GPIOs we’re going to use in order to place the corresponding number of pads on our LEDs’ PCB.


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