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");
	}
	XScuGic_Enable(&intCtrl, XPAR_FABRIC_AXI_TIMER_0_INTERRUPT_INTR);

	// Initialize the exception table.
	Xil_ExceptionInit();

	// Register the interrupt controller handler with the exception table.
	Xil_ExceptionRegisterHandler(XIL_EXCEPTION_ID_INT,(Xil_ExceptionHandler)XScuGic_InterruptHandler,(void *)&intCtrl);

	// Enable exceptions.
	Xil_ExceptionEnable();

	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.
- Read here on how to resolve a duplicated platform project or a misconfigure UART.
- Read here on how to handle a corrupt BSP project.
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 (https://creativecommons.org/licenses/by/4.0/). Artist: http://audionautix.com/
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

ZYNQ: Adding an AXI Timer to Trigger Periodic Interrupts