topic Re: accelerometer vector magnitude in Sensors

accelerometer vector magnitude

meirsudry — Tue, 03 Feb 2026 22:06:50 GMT

Hi i encounter an odd response when trying to use the A_VECM feature in FXOS8700.

i have the following piece of code:

MMA865x_Standby();
IIC_RegWrite(CTRL_REG4,0x02); //Disable Data Ready interrupts, enable A_VECM interrupts
IIC_RegWrite(A_VECM_THS_MSB,0x80);   //update MSB THS, reset when condition fails
print_string("A_VECM_THS_MSB= ",16); print_hex(IIC_RegRead(A_VECM_THS_MSB)); WaitPrintBuffer();
IIC_RegWrite(A_VECM_THS_LSB,Acc_THS);   //update LSB THS
print_string("A_VECM_THS_LSB= ",16); print_hex(IIC_RegRead(A_VECM_THS_LSB)); WaitPrintBuffer();
IIC_RegWrite(A_VECM_CNT,Acc_COUNT); //update counter
print_string("A_VECM_CNT= ",13); print_hex(IIC_RegRead(A_VECM_CNT)); printENTER; WaitPrintBuffer();
IIC_RegWrite(A_VECM_CFG,0x78); //0111 1000 Latch enable, A_VECM_INIT, don't update reference when event
IIC_RegWrite(M_CTRL_REG1,((IIC_RegRead(M_CTRL_REG1))&(~0x03))); //Enable Accelerometer_Only
Data_Mode= Accelerometer_Only; //notify EX0 interrupt to read Accelerometer data
IIC_RegWrite(M_CTRL_REG2,((IIC_RegRead(M_CTRL_REG2))&(~0x20))); //Disable autoincrement mode
//    IntSource=IIC_RegRead(TRANSIENT_SRC); //Read 0x1E TRANSIENT_SRC register to clear the interrupt
MMA865x_Active(); //ACTIVATE THE SENSOR

with this code the program works fine and every time i move the device A_VECM is asserted.

in this stage i decided to delete the print_string() lines.

after i did that the A_VECM is asserted once and not asserted anymore.

i tried to replace the print_string() lines with simple delay: for(i=0; i<60000; i++);

with this line, again A_VECM is asserted with no problems.

i can read CNTR_REG4 and it is set to 0x02. nevertheless the A_VECM is not asserted.

any idea what is the reason for that?

thanks

Re: accelerometer vector magnitude

TomasVaverka — Mon, 10 Aug 2015 07:51:02 GMT

Hi Meir,

Please look at my example code. You need to read the INT_SOURCE register (0x0C) to clear the the SRC_A_VECM status bit and deassert the INTx pin.

Hope it helps.

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct". Thank you.

Re: accelerometer vector magnitude

meirsudry — Wed, 12 Aug 2015 05:37:52 GMT

Hi Tomas

thanks for your prompt response.

Unfortunately I can’t open the link you sent for your example code.

Indeed I copied paste a previous piece of code from a program deal with Transient interrupt so I read the transient source instead of INT_Source.

The piece of code I attached in my question is the last part of handling the A_VECM interrupt.

Immediately after the interrupt is asserted I do read INT_SOURCE register.

Finally, I changed the line as you suggested

// IntSource=IIC_RegRead(TRANSIENT_SRC); //Read 0x1E TRANSIENT_SRC register to clear the interrupt

and replaced it with

IntSource=IIC_RegRead(INT_SOURCE); //Read 0x0C INT_SOURCE register to clear the interrupt

The problem still exist.

The point in my case is a beat different.

It looks like if I have a delay after the line:

IIC_RegWrite(CTRL_REG4,0x02); //Disable Data Ready interrupts, enable A_VECM interrupts

I do get interrupts.

The delay is either by setting and printing several registers (see the code I sent in my question),

or even by simple for(i=0; i<60000; i++); to implement the delay.

In these cases I do get A_VECM interrupts after the first one, which is always asserted as expected.

If the delay is not implemented I don't get interrupts after the 1st that was asserted.

thanks

Meir SUDRY

Re: accelerometer vector magnitude

TomasVaverka — Wed, 12 Aug 2015 08:47:07 GMT

Hi Meir,

Sorry, the correct link is: https://community.freescale.com/docs/DOC-106050

Please try to reproduce my settings and let me know if the problem persists.

Regards,

Tomas

Re: accelerometer vector magnitude

meirsudry — Wed, 12 Aug 2015 11:08:16 GMT

Hi Tomas

thanks for the correct link.

the problem still exist.

see below more details (code) of my project

Sensor initialization:

MMA865x_Standby();

IIC_RegWrite(CTRL_REG4,0x02);

IIC_RegWrite(A_VECM_THS_MSB,0x80);

IIC_RegWrite(A_VECM_THS_LSB,Acc_THS);

IIC_RegWrite(A_VECM_CNT,Acc_COUNT);

IIC_RegWrite(A_VECM_INITX_MSB, 0x00);

IIC_RegWrite(A_VECM_INITX_LSB, 0x00);

IIC_RegWrite(A_VECM_INITY_MSB, 0x00);

IIC_RegWrite(A_VECM_INITY_LSB, 0x00);

IIC_RegWrite(A_VECM_INITZ_MSB, 0x00);

IIC_RegWrite(A_VECM_INITZ_LSB, 0x00);

IIC_RegWrite(A_VECM_CFG,0x78);

IIC_RegWrite(M_CTRL_REG1,((IIC_RegRead(M_CTRL_REG1))&(~0x03)));

IIC_RegWrite(M_CTRL_REG2,((IIC_RegRead(M_CTRL_REG2))&(~0x20)));

MMA865x_Active();

IntSource=IIC_RegRead(INT_SOURCE);

now I'm polling for A_VECM event

General_IntSource= IIC_RegRead(INT_SOURCE);

if (General_IntSource&0x02) readdata();

and finally the function readdata()

void readdata(void) //

{

IntSource=IIC_RegRead(INT_SOURCE);

MMA865x_Standby();

IIC_RegWrite(M_CTRL_REG1,((IIC_RegRead(M_CTRL_REG1))|0x03)); //hybrid mode

IIC_RegWrite(M_CTRL_REG2,((IIC_RegRead(M_CTRL_REG2))|0x20)); //enable magnetic auto increment

IIC_RegWrite(CTRL_REG4,0x01); //enable data ready interrupts and disable A_VECM interrupts

MMA865x_Active(); //ACTIVATE THE SENSOR

here I read several times the accelerometer and magnetic data using data ready interrupt to retrieve new data

after finalizing reading data I set the parameters to re-accept A_VECM interrupts

MMA865x_Standby();

IIC_RegWrite(CTRL_REG4,0x02); //Disable Data Ready interrupts, enable A_VECM interrupts

for (i=0; i<60000; i++); - if this line is removed the A_VECM interrupt is not asserted anymore unless I restart the system

IIC_RegWrite(M_CTRL_REG1,((IIC_RegRead(M_CTRL_REG1))&(~0x03))); //Enable Accelerometer_Only

IIC_RegWrite(M_CTRL_REG2,((IIC_RegRead(M_CTRL_REG2))&(~0x20))); //Disable autoincrement mode

MMA865x_Active(); //ACTIVATE THE SENSOR

IntSource=IIC_RegRead(INT_SOURCE); //Read 0x0C INT_SOURCE register to clear the interrupt

}

the first interrupt is always asserted and the system works as expected. i get all data readings.

when the delay line is placed also the system is working fine. the question is why is it needed at all.

i got the idea that the delay is needed because i had there several lines of printing for the debugging. once i removed those lines i stopped getting interrupts after the first A_VECM.

after an investigation i made i realized that a delay after setting CTRL_ REG4 is required in order to get the successive interrupts.

i positively know the interrupt in no asserted because in debug mode i can see the General_IntSource variable get the value of 0x00.

I would appreciate if you can look into this case and come with your insights.

thanks

Meir

MMA865x_Standby(); IIC_RegWrite(CTRL_REG4,0x02); IIC_RegWrite(A_VECM_THS_MSB,0x80); IIC_RegWrite(A_VECM_THS_LSB,Acc_THS); IIC_RegWrite(A_VECM_CNT,Acc_COUNT); IIC_RegWrite(A_VECM_INITX_MSB, 0x00); IIC_RegWrite(A_VECM_INITX_LSB, 0x00); IIC_RegWrite(A_VECM_INITY_MSB, 0x00); IIC_RegWrite(A_VECM_INITY_LSB, 0x00); IIC_RegWrite(A_VECM_INITZ_MSB, 0x00); IIC_RegWrite(A_VECM_INITZ_LSB, 0x00); IIC_RegWrite(A_VECM_CFG,0x78);
IIC_RegWrite(M_CTRL_REG1,((IIC_RegRead(M_CTRL_REG1))&(~0x03))); IIC_RegWrite(M_CTRL_REG2,((IIC_RegRead(M_CTRL_REG2))&(~0x20))); MMA865x_Active(); IntSource=IIC_RegRead(INT_SOURCE);