STM32F4 Encoder count is changing when it should not

Question

I am currently using the STM32F4 with the STM32F429ZI Nucleo-144 Board. I am looking to use this microcontroller to evaluate the position of a rotary encoder via a quadrature encoder interface. Looking at the documentation, this is done with the timers. I have the A/B encoder outputs hooked up to PA6 and PC7 on the micro, but I have noticed that the counts appear to be drifting.

During the debugging, I noticed that if I disconnect one of the encoder outputs to the microcontroller and I move the motor, the counts still increment/decrement even though only one of the encoder lines are connected. Since I am counting on both the TI1 and TI2 edges, this should not be happening. If I am reading the below diagram correctly, since one of my lines is held high using the internal pull-up, clock pulses on the other input should be going up/down/up/down and really just cycling between two different counts. However, if I am rotating the encoder, the counts keep incrementing or decrementing depending on the direction.

Why is the encoder count changing with only one encoder input connected? I also have the scope trace attached to prove that only one count is is active, as well as the code.

EDIT: I have also tried changing the polarity from BOTH EDGE to RISING EDGE, with no perceived benefit.

#include "stm32f4xx_hal.h"
#include "encoder_test.h"

GPIO_InitTypeDef  GPIO_InitStruct;
TIM_HandleTypeDef Timer_InitStruct;
TIM_Encoder_InitTypeDef Encoder_InitStruct;

void EncoderTest_Init()
{
    __HAL_RCC_GPIOA_CLK_ENABLE();
    __HAL_RCC_GPIOC_CLK_ENABLE();
    __HAL_RCC_TIM3_CLK_ENABLE();

    /**TIM3 GPIO Configuration
    PA6     ------> TIM3_CH1
    PC7     ------> TIM3_CH2
    */

    GPIO_InitStruct.Pin = GPIO_PIN_6;
    GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
    GPIO_InitStruct.Pull = GPIO_PULLUP;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_MEDIUM;
    GPIO_InitStruct.Alternate = GPIO_AF2_TIM3;
    HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

    GPIO_InitStruct.Pin = GPIO_PIN_7;
    GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
    GPIO_InitStruct.Pull = GPIO_PULLUP;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_MEDIUM;
    GPIO_InitStruct.Alternate = GPIO_AF2_TIM3;
    HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

    Timer_InitStruct.Instance = TIM3;
    Timer_InitStruct.Init.Period = 0xFFFF;
    Timer_InitStruct.Init.CounterMode = TIM_COUNTERMODE_UP;
    Timer_InitStruct.Init.Prescaler = 1;
    Timer_InitStruct.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;

    Encoder_InitStruct.EncoderMode = TIM_ENCODERMODE_TI12;
    Encoder_InitStruct.IC1Filter = 0x00;
    Encoder_InitStruct.IC1Polarity = TIM_INPUTCHANNELPOLARITY_BOTHEDGE;
    Encoder_InitStruct.IC1Prescaler = TIM_ICPSC_DIV1;
    Encoder_InitStruct.IC1Selection = TIM_ICSELECTION_DIRECTTI;
    Encoder_InitStruct.IC2Filter = 0x00;
    Encoder_InitStruct.IC2Polarity = TIM_INPUTCHANNELPOLARITY_BOTHEDGE;
    Encoder_InitStruct.IC2Prescaler = TIM_ICPSC_DIV1;
    Encoder_InitStruct.IC2Selection = TIM_ICSELECTION_DIRECTTI;

    if (HAL_TIM_Encoder_Init(&Timer_InitStruct, &Encoder_InitStruct) != HAL_OK)
    {
        while (1);
    }

    if (HAL_TIM_Encoder_Start_IT(&Timer_InitStruct, TIM_CHANNEL_1) != HAL_OK)
    {
        while (1);
    }
}


void TIM3_IRQHandler()
{
    HAL_TIM_IRQHandler(&Timer_InitStruct);
}

Answer 1

Upon further investigation, it appears that the issue is due to the prescaler. The prescaler does not work in encoder mode when you provide even values. Since prescaler is the entered value + 1, using the STM32F4 HAL, the entered prescaler must be even.

I found confirmation that I am not the only person with this issue at this forum post. There is some discussion at the post that prescalers may not be compatible with encoder mode, but this has not yet been confirmed. I have sent an email to ST to get to the bottom of it. It is safe to enter a prescaler value of 0 if it is not supported.

Here is the working code below:

#include "stm32f4xx_hal.h"
#include "encoder_test.h"

GPIO_InitTypeDef  GPIO_InitStruct;

TIM_HandleTypeDef Timer3_InitStruct;
TIM_Encoder_InitTypeDef EncoderTim3_InitStruct;

void EncoderTest_Init_Tim3()
{
    __HAL_RCC_GPIOA_CLK_ENABLE();
    __HAL_RCC_GPIOC_CLK_ENABLE();
    __HAL_RCC_TIM3_CLK_ENABLE();

    /**TIM3 GPIO Configuration
    PA6     ------> TIM3_CH1
    PC7     ------> TIM3_CH2
    */

    GPIO_InitStruct.Pin = GPIO_PIN_6;
    GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
    GPIO_InitStruct.Pull = GPIO_PULLDOWN;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_MEDIUM;
    GPIO_InitStruct.Alternate = GPIO_AF2_TIM3;
    HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

    GPIO_InitStruct.Pin = GPIO_PIN_7;
    GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
    GPIO_InitStruct.Pull = GPIO_PULLDOWN;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_MEDIUM;
    GPIO_InitStruct.Alternate = GPIO_AF2_TIM3;
    HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

    Timer3_InitStruct.Instance = TIM3;
    Timer3_InitStruct.Init.Period = 0xFFFF;
    Timer3_InitStruct.Init.CounterMode = TIM_COUNTERMODE_UP;
    Timer3_InitStruct.Init.Prescaler = 10;
    Timer3_InitStruct.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;

    EncoderTim3_InitStruct.EncoderMode = TIM_ENCODERMODE_TI12;
    EncoderTim3_InitStruct.IC1Filter = 0x00;
    EncoderTim3_InitStruct.IC1Polarity = TIM_INPUTCHANNELPOLARITY_RISING;
    EncoderTim3_InitStruct.IC1Prescaler = TIM_ICPSC_DIV4;
    EncoderTim3_InitStruct.IC1Selection = TIM_ICSELECTION_DIRECTTI;
    EncoderTim3_InitStruct.IC2Filter = 0x00;
    EncoderTim3_InitStruct.IC2Polarity = TIM_INPUTCHANNELPOLARITY_RISING;
    EncoderTim3_InitStruct.IC2Prescaler = TIM_ICPSC_DIV4;
    EncoderTim3_InitStruct.IC2Selection = TIM_ICSELECTION_DIRECTTI;

    if (HAL_TIM_Encoder_Init(&Timer3_InitStruct, &EncoderTim3_InitStruct) != HAL_OK)
    {
        while (1);
    }

    if (HAL_TIM_Encoder_Start_IT(&Timer3_InitStruct, TIM_CHANNEL_1) != HAL_OK)
    {
        while (1);
    }
}



void TIM3_IRQHandler()
{
    HAL_TIM_IRQHandler(&Timer3_InitStruct);
}

EDIT: After speaking with ST tech support, the encoder interface was not intended to be used with a prescaler value, even OR odd. I have pasted their response below, but even with using a prescaler value that appears to work, it seems possible that the encoder counts drift over time.

Another solution is to use no prescalers, but instead extend the 16 bit value into the 32 bit space using the approach suggested here. I have reprinted the approach here in case the link goes dead:

---

From user goosen.kobus.001 on 11/19/2013 on ST's forum:

In my experience using an overflow interrupt to scale up an encoder is not reliable, especially when you have high resolution encoders: it happens from time to time that the encoder value will change sign in the instant you enter the interrupt, causing the system to increment the upper word when it should have decremented etc. This is especially true if the encoder is supposed to be stalled at 0, like a servo motor commanded to go to encoder position 0.

The best approach I have found is to do it manually. this is my procedure:

1. Ensure that the control loop that reads the encoder value is run often, (i.e. that if your encoder is rotating at full speed, the encoder value is still read at least 10-20 times between overflows. For my servo motor application a 1ms loop interval was sufficient.

2. keep track of the last read encoder value.

3. divide the current and last encoder value into quadrants (the most significant 2 bits). i.e. pos_now &= 0xC000; pos_last &= 0xC000;

4. check to see if the encoder has moved from quadrant 0 to quadrant 3 or 3 to 0 in the last step:

   4.1 if(pos_now == 0 && pos_last == 0xC000) upper_word++;

   4.2 if(pos_now == 0xC000 && pos_last == 0) upper_word--;

this is why I say the encoder read loop needs to be run often; you need to be sure that the value is read often enough that it is impossible to go from quadrant 0->1->2->3 in between reads. It should also be possible to put this logic in another timer interrupt that runs at say 10kHz. that way you have an encoder value which is always up to date.

---

ST RESPONSE:

Hi,

I have got the feedback from the design and the architect of the timers. The encoder interface has been designed to work without prescaler in order to not downgrade the resolution of the encoder.

As you observed, they have confirmed that it cannot work with even prescaler value but only the odd ones. We have a sub-counter for the prescaler which is mono-directional, so not affected by the counter direction and incremented on every rising edge of the timer clock (without the prescaler). The counter direction is updated on every rising edge of the timer clock (without the prescaler) but the counter is incremented only while the sub-counter of the prescaler reach the programmed value and according to the value in the direction bit.

So, in one case, the behavior is the same as without prescaler, because the counter is updated with a different direction (every odd clock cycles number), but in the other case, the direction is always the same when the counter is updated and the encoder interface doesn't work correctly.

So you can use the prescaler but with an odd value.

The recommended use case is without prescaler.

Best regards

ST MCU Tech Support