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ZYNQ: Adding an AXI Timer to Trigger Periodic Interrupts

In a previous post, we added GPIOs to our WAV player to enable a simple user interface. In that example we triggered an interrupt whenever a key was pressed. However, I found it hard to debounce the keypad as we would have to somehow disable new interrupts for a while after a keypress has been detected. A more elegant way is to set up an interrupt handler which is regularly triggered by a timer, say every 20 milliseconds. The interrupt handler scans the keyboard and fills a keyboard buffer accordingly. It is easy to implement a hold-off period since we know that there are 20ms elapsing between each handler call.

In this tutorial we learn:

  • How to set up an AXI timer.
  • How to connect a third interrupt signal to the ZYNQ fabric.
  • How to add a third interrupt handler.
  • How to query the presence of an SD card

Interfacing to the AXI Timer

  • Use the include file xtmrctr.h.
  • Use the object XTmrCtr to interface to the timer.
  • The first device ID is XPAR_AXI_TIMER_0_DEVICE_ID (defined in xparameters.h).
  • The corresponding interrupt ID is XPAR_FABRIC_AXI_TIMER_0_INTERRUPT_INTR (defined in xparameters.h).
  • Here is an example for setting up a timer in a standalone project:
    #include "xscugic.h"
    #include "xtmrctr.h"
    #include "xparameters.h"
    XScuGic intCtrl;
    XTmrCtr timer;
    // This handler is called every 20 ms
    void timerInterruptHandler(void *userParam, u8 TmrCtrNumber) {
    void init() {
    	int xStatus = XTmrCtr_Initialize(&timer, XPAR_AXI_TIMER_0_DEVICE_ID);
    	if (xStatus != XST_SUCCESS) {
        	fatalError("Could not initialize timer");
    	XTmrCtr_SetHandler(&timer, (XTmrCtr_Handler)timerInterruptHandler, (void*) 0x12345678);
    	// Initialize the interrupt controller driver so that it is ready to use.
    	XScuGic_Config *IntcConfig = XScuGic_LookupConfig(XPAR_SCUGIC_SINGLE_DEVICE_ID);
    	if (NULL == IntcConfig) {
    		fatalError("XScuGic_LookupConfig() failed");
    	int Status = XScuGic_CfgInitialize(&intCtrl, IntcConfig,IntcConfig->CpuBaseAddress);
    	if (Status != XST_SUCCESS) {
    		fatalError("XScuGic_CfgInitialize() failed");
    	XScuGic_SetPriorityTriggerType(&intCtrl, XPAR_FABRIC_AXI_TIMER_0_INTERRUPT_INTR, 0xA0, 0x3);
    	Status = XScuGic_Connect(&intCtrl, XPAR_FABRIC_AXI_TIMER_0_INTERRUPT_INTR,(Xil_InterruptHandler)XTmrCtr_InterruptHandler,&timer);
    	if (Status != XST_SUCCESS) {
    		fatalError("XScuGic_Connect() failed");
    	// Initialize the exception table.
    	// Register the interrupt controller handler with the exception table.
    	Xil_ExceptionRegisterHandler(XIL_EXCEPTION_ID_INT,(Xil_ExceptionHandler)XScuGic_InterruptHandler,(void *)&intCtrl);
    	// Enable exceptions.
    	XTmrCtr_SetOptions(&timer, 0,	XTC_INT_MODE_OPTION | XTC_AUTO_RELOAD_OPTION);
    	XTmrCtr_SetResetValue(&timer, 0, 0xFFE17B80); // 50 Hz
    	XTmrCtr_Start(&timer, 0);

Checking if an SD Card is Present

  • It can be useful to detect if the user has removed or inserted an SD card. Here is how we can check if an SD-card is inserted:
    int isCardPresent() {
    	u32 StatusReg = XSdPs_GetPresentStatusReg(XPAR_XSDPS_0_BASEADDR);
    	if ( (StatusReg & XSDPS_PSR_CARD_INSRT_MASK) != 0U) {
    		return 1;
    	else {
    		return 0;

Open the Main Project

  • Start with the project from this post. Or download the complete project further down.
  • Change the number of inputs to the concat block to 3:

  • Add an AXI Timer IP core and configure it:

  • Run Connection Automation.
  • Connect the interrupt output of the timer to the 3rd input of the concat block.

The Complete Block Diagram

Create the Bitstream

  • Generate the bitstream.
  • Open the Implementation.
  • Export Hardware (including bitstream).

Modify the C-Code

  • Launch the SDK. I encountered problems occasionally that the SDK creates a new system wrapper project. Normally there is a project named design_1_wrapper_hw_platform_0. I then have two of those. The second one named design_1_wrapper_hw_platform_1.
  • Here is the full modified code (helloworld.c):
  • Run the program with an SD card inserted:

  • You can now use the keypad on the Zedboard to navigate up and down. Use left/right to change the volume. Use the middle button to play. Note that the keys are repeated when you hold them down for long enough.
  • If the SD card is removed, the program jumps back to mounting an SD card. It will automatically detect when an SD card is inserted.

Download the Complete Project

  • Here is a WAV file which I have tested (from the Youtube Audio Library):
  • piano.wav
    Piano March by Audionautix is licensed under a Creative Commons Attribution license ( Artist:
  • To upload the software to the Zedboard, open the project in Vivado, then click on “Launch SDK”.
  • There seems to be a bug when updating the SDK project which selects a wrong UART driver (refer to “SDK Auto Update Bug” to fix it).
  • Select the project CodecDMADemoOneshot and click “Run” (the “play” button at the top of the window).

Next: Using the OLED Display

Zedboard: Using the OLED Display




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