topic Re: FTM0 Quadrature decoder in Kinetis Microcontrollers

FTM0 Quadrature decoder

dcantero — Mon, 24 Nov 2014 14:22:49 GMT

Hi everybody!!

I´m involved in a proyect with kinetis K60f120 to control 3 DC motors with encoder. I have used the FTM3 to generate PWM signals and the other 3 FTM modules (FTM0, FTM1 and FTM2) to connect the motor encoders. I have used the same code to initialize all FTM modules, but two of them work properly (FTM1 and FTM2) but the third (FTM0) dont work.

FTM0 is more general FTM module than the others, so could be necesary something else for correct initialization???

This is the code I have used:

// enable the clock for FTM1

SIM_SCGC6 |= SIM_SCGC6_FTM0_MASK;

//enable the counter

FTM0_MODE |= FTM_MODE_FTMEN_MASK;

FTM0_CONF |= FTM_CONF_BDMMODE(3); // counter running in BDM mode

FTM0_MOD = 3999;

FTM0_CNTIN = 0;

FTM0_CNT = 0;

FTM0_QDCTRL |= FTM_QDCTRL_QUADEN_MASK | FTM_QDCTRL_QUADMODE_MASK; // configuring for quadrature mode

// start the timer clock, source is the external clock

FTM0_SC |= FTM_SC_CLKS(3);

//configuring the input pins:

PORTC_PCR1 = PORT_PCR_MUX(4); // FTM0 CH0

PORTC_PCR2 = PORT_PCR_MUX(4); // FTM0 CH1

Any suggestion??????

Thanks in advance!!!!!!

David.

Re: FTM0 Quadrature decoder

chris_brown — Mon, 24 Nov 2014 19:43:09 GMT

Hi David,

Please refer to the FlexTimer section in Chapter 3 of your reference manual. I don't believe FTM0 supports the Quadrature decoder feature (even though the control registers for that feature are present).

Which means you will need to create your own quadrature decoder software functions or use the help of external hardware for the third decoder.

Regards,

Chris

Re: FTM0 Quadrature decoder

dcantero — Tue, 25 Nov 2014 08:46:14 GMT

Hi Christopher!!

Unfortunately I think you are right!! Channels FTM0 and FTM3 dont integrate quadrature decoder.

I need to implerment a decoder by software.

has anybody an example of software implemented quadrature decoder for kinetis?

I´m looking for a code example or application note in freescale but I dont found anythig!!!

Thanks!!!!!!

David

Re: FTM0 Quadrature decoder

egoodii — Wed, 26 Nov 2014 03:46:55 GMT

What maximum edge rate (minimum width between edges) will you need to support?

Re: FTM0 Quadrature decoder

egoodii — Tue, 02 Dec 2014 16:27:48 GMT

The SAFEST way to handle quadrature-input decoding is a 'sampled' system, where the two inputs are simultaneously sampled and the 'difference' from a previous sample gives you the necessary counter-update info, like this, where ENC0_PT is defined as the encoder-GPIO port, and ENC0_A (and _B) are the associated bit-masks:

Diff_Enc0 = (ENC0_PT & (ENC0_A | ENC0_B)) ^ Last_Enc0; //Get encoder bit changes-must be atomic!

if( Diff_Enc0 != 0 )

{

if( (~Last_Enc0 & Diff_Enc & ENC0_A) ) //First look for A edges

{

Last_Enc0 ^= Diff_Enc; //Update the 'current' pattern

if( Last_Enc0 & ENC0_B )

Steps0--; //Current 'B' state defines CW or CCW

else

Steps0++; //Increments and decrements must be atomic

}else if( (Last_Enc0 & Diff_Enc & ENC0_A) )

Last_Enc0 ^= Diff_Enc; //Update the 'current' pattern

if( Last_Enc0 & ENC0_B ) //Opposite 'B' decode for CW or CCW

Steps0++;

else

Steps0--;

}else if( (~Last_Enc0 & Diff_Enc & ENC0_B) ) //And now B edges

{

Last_Enc0 ^= Diff_Enc; //Update the 'current' pattern

if( Last_Enc0 & ENC0_A )

Steps0++; //Current 'A' state defines CW or CCW

else

Steps0--;

}else if( (Last_Enc0 & Diff_Enc & ENC0_B) )

{

Last_Enc0 ^= Diff_Enc; //Update the 'current' pattern

if( Last_Enc0 & ENC0_A ) //Opposite 'A' decode for CW or CCW

Steps0--;

else

Steps0++;

}else //Ignore any non-tested changes

Last_Enc0 ^= Diff_Enc; //Update the 'current' pattern, ignoring confused change

}

The 'limitation' of this process is that there MUST be at most one 'useful' (not just bounce) edge-input changing per 'sample' -- otherwise no 'correct' change can be made, and the last 'else' clause kicks-in and just 'skips' the change. The Kinetis FTM1 and/or FTM2 'quadrature mode' run this kind of state machine in hardware using the bus clock for sampling, and is thus capable of edge-rates up to 2/3rds of the bus clock (based on experiments and the datasheet parameter under 'General Switching Specs' called 'GPIO pin interrupt pulse width', and note 1, of 1.5 bus-clock-cycles 'minimum' pulse widths). In software, you will be MUCH more limited in the rate you can stand -- I might see you running this on a 10KHz timer-interrupt, but personally I wouldn't expect to drive an interrupt rate much in excess of that, for a minimum of 100us between 'valid' edge changes. This kind of sampling will ignore 'bounce' on any one input, as just one edge changing 'back and forth' at the sample-times will just increment and decrement the counter each time (and 'bounces' between samples ignored entirely!), and the final 'settled' value will be correct for the total step count. And that is the 'fundamental advantage' over, say, just trying to 'interrupt' on either input-edge and decode THOSE, as the interrupt-turnaround time will define a 'window' where you may LOSE a fast edge change, and thus miscount. This method CANNOT get lost on such bounces of any one edge at a time -- it is just limited in that the valid A/B/A/B/A... interleaved rising/falling edge sequences CANNOT occur faster than the sample-rate.