How to Measure Signal Frequency by Using the Camera Sensor Interface of an i.MX

[#1]

This describes how to perform frequency measurements of an external signal by using the Camera Sensor Interface (CSI) of an i.MX21/25/35 processor.

1. Principle:

A way to measure the frequency of a digital signal is to count the number of received rising or falling edges during a known amount of time.

The CSI embeds a 16-bit frame counter. When programmed in non-gated clock mode, this counter increases at any rising edge on the VSYNC signal.

Other signals of this interface could be ignored such: MCLK, PIXEL_CLK, HSYNC, DATA.

2. Software example for the i.MX25:

void CSI_init(void){

      unsigned int tmp_value = 0;

      /* It assumes that the VSYNC I/O is set to CSI mode */

      /* Disable IPG_PER_CSI to save power consumption */

      *((unsigned int *) CCM_CGR0) &= ~(0x1<<0);

      /* HCLK_CSI and IPG_CLK_CSI should be enabled. */

      *((unsigned int *) CCM_CGR0) |= (0x1<<18);

      *((unsigned int *) CCM_CGR1) |= (0x1<<4);

      /* Configuration of CSI_CSICR1 in non-gated clock mode */

      tmp_value = 0;

      tmp_value |= (1<<8);    // sync FIFO clear

      tmp_value |= (1<<30);   // ext vsync enable

      *((unsigned int *) CSI_CSICR1) = tmp_value;

      // Reset frame counter

      *((unsigned int *) CSI_CSICR3) |= (1<<15);

}

Then, every T seconds, the software has to read the register CSI_CSICR3. The 16-bit size field from bit 16 shows the current value of the frame counter (FRMCNT).

This regular or irregular read could be done based on a GPT to have a known time reference.

It is easy to calculate the frequency of the signal: Frequency = FRMCNT / T (Hz).

At any time, the frame counter can be reset thanks to the bit 15 of the register CSI_CSICR3.

NOTES:

• MCLK does not need to be enabled.
• The input frequency should not be higher than what can electrically support the VSYNC input.
• Please, refer to each i.MX datasheet for more information.