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Thread: Crosstalk with many multiple inputs

  1. #1
    Junior Member
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    Aug 2020
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    Crosstalk with many multiple inputs

    Hi everyone,

    I'm seeing some unwanted behavior and I was hoping that someone could advice on what it could be caused by and/or what could I do about it.

    Long story short, I'm working on reading some parallel data stream from a CMOS sensor. For testing purposed, instead of the actual sensor, I've hooked up Teensy 4.1 to a signal generator in a following way
    - pins 10, 12, 11, 13, 8, 7, 36, 37, 6, 9, 35, 34 receive the parallel data - right now it's just a binary counter, clocked anywhere up to 50MHz
    - pin 32 gets the corresponding clock signal
    - pin 4 gets the DATA VALID signal - which is low when the input data should be ignored and high when it should be processed.

    It generally works, with one small caviat: sometime, the DATA VALID line gets high even though I'm outputting low from the signal generator.

    I initially though that it's a problem with the setup of the readout (the docs are pretty convoluted, it's easy to miss something like that), but I managed to simplify the entire setup, where I dont do any readout, I ignore the data completely, I only look for edges in the DATA VALID signal, using XBAR's edge detection combined with some interrupts. The bare-bones test code looks like this:

    Code:
    uint32_t numXbarInterrupts;
    uint32_t lastSeenXbarInterrupt;
    
    
    void xbar_connect(unsigned int input, unsigned int output)
    {
    	if (input >= 88) return;
    	if (output >= 132) return;
    
    	volatile uint16_t *xbar = &XBARA1_SEL0 + (output / 2);
    	uint16_t val = *xbar;
    	if (!(output & 1)) {
    		val = (val & 0xFF00) | input;
    	} else {
    		val = (val & 0x00FF) | (input << 8);
    	}
    	*xbar = val;
    }
    
    
    void xbarInterrupt()
    {
    	XBARA1_CTRL0 |= XBARA_CTRL_STS0;
    
    	asm("DSB");	
    
    	++numXbarInterrupts;
    }
    
    
    void setup()
    {
    	Serial.begin(115200);	
    
    	// enable clock for clock XBAR
    	CCM_CCGR2 |= CCM_CCGR2_XBAR1(CCM_CCGR_ON);
    
    	// set the IOMUX mode to 3, to route it to XBAR
    	IOMUXC_SW_MUX_CTL_PAD_GPIO_EMC_06 = 3;
    
    	//IOMUXC_SW_PAD_CTL_PAD_GPIO_EMC_06 = IOMUXC_PAD_HYS | IOMUXC_PAD_PUS( 0 ) | IOMUXC_PAD_SPEED( 11 );
    	
    	// set XBAR1_IO008 to INPUT
    	IOMUXC_GPR_GPR6 &= ~(IOMUXC_GPR_GPR6_IOMUXC_XBAR_DIR_SEL_8);
    	
    	// daisy chaining - select between EMC06 and SD_B0_04
    	IOMUXC_XBAR1_IN08_SELECT_INPUT = 0;
    	
    	// detect edges, trigger interrupts
    	XBARA1_CTRL0 = XBARA_CTRL_STS0 | XBARA_CTRL_EDGE0(3) | XBARA_CTRL_IEN0;
    
    	// connect the interrupt to the trigger pad to see if it sees these extra transitions
    	xbar_connect( XBARA1_IN_IOMUX_XBAR_INOUT08, XBARA1_OUT_DMA_CH_MUX_REQ30 );
    	
    	attachInterruptVector( IRQ_XBAR1_01, xbarInterrupt );
    	NVIC_ENABLE_IRQ( IRQ_XBAR1_01 );		
    }
    
    
    
    void loop()
    {	
    	delay( 100 );
    
    	if ( lastSeenXbarInterrupt != numXbarInterrupts )
    	{ 
    		Serial.printf( "Num interrupts %d\n",  numXbarInterrupts - lastSeenXbarInterrupt );
    
    		lastSeenXbarInterrupt = numXbarInterrupts;
    	}
    	else
    	{
    		//Serial.printf("Nothing\n" );
    	}
    }
    It hooks up the DATA_VALID line to the XBAR, connects it to the 0th output and sets up edge detection on it, triggering interrupt on each. The handler only increments a variable, so I can see how many times it fired. And with the above code, and all that clocking hooked up to the remaining pins as described above, I'm seeing multiple edges even though the DATA_VALID is (supposedly) low all the time.

    The number of the detected edges highly depends on the clocking of the data signal, I start seeing them around 100kHz, and when I go to 50MHz, it can pick up over 2000 random edges per 100ms. The numbers change when I *touch* other pins on Teensy. And it doesn't happen until the bus is wide enough - when I limit the number of parallel lines to 8, I'm not seeing any of this even when the signal is clocked at 50MHz.

    All that makes me think that's some problem with signal integrity, and in fact there are some spikes in the DATA_VALID signal that cause the microcontroller to interpret them at HIGHs. I tried probing the DATA VALID at the connected pin, but it looks relatively fine on the oscilloscope - sure, there's noise there, but it doesn't go over 200mV-300mV max. I tried hooking the DATA_VALID line through the XBAR to another pin and investigating it there, but it looks similar too. That said, I'm only using Analog Discovery 2 for both signal generation and investigation, its bandwidth is pretty low, something like 30Mhz, so maybe these spikes are simply missed. And all this signal is hooked up through these regular hookup wires - so I'm not really sure how much they affect the signal integrity. I tried messing with the hysteresis setting on that DATA VALID pin, but it doesn't really help - it does however change the numbers of the ticks recorded (whatever that indicates...)

    Have anyone seen similar behavior? Is this normal? What can/should I do to remedy it? I don't have much experience in circuit design, so it's quite possible that it's one of these "oh yeah, you need to do XXXX", and I would be really grateful if someone could point me in some right direction.

  2. #2
    Senior Member
    Join Date
    Jul 2020
    Posts
    918
    At 50MHz this is likely to be a layout issue - can you provide a pic? You have ground returns for all those signals?

  3. #3
    Junior Member
    Join Date
    Aug 2020
    Posts
    18
    There's no PCB, it's just the logic analyzer/signal generator hooked up to Teensy pins. And the same thing happens at lower speeds too. With 12bit bus, I'm seeing these glitches even at 1MHz. And on the other hand, even 50MHz is fine if I only hook up 8 wires with the data instead of 12.

  4. #4
    Senior Member
    Join Date
    Jul 2020
    Posts
    918
    Can you provide a pic of the layout, nonetheless? And do you have ground returns for all the signals?

  5. #5
    Junior Member
    Join Date
    Aug 2020
    Posts
    18
    Sure - just a signal analyzer hooked up straight to Teensy.

    Click image for larger version. 

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    But the ground was a good direction! Thanks for that tip! I did connect ground in few more places rather than only two, and it totally helped. Below 50MHz all these glitches are gone. I'm still seeing them at 50MHz, but I'm guessing now it's just the quality of these ground connections, and if it was all a dedicated PCB, properly soldered, it should be fine too.

    Thanks!

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