3 Getting Started with AC

This example shows a basic initialization and setup for the AC peripheral. The application monitors an analog input signal, compares it to a fixed voltage, and notifies the user via interrupt and an output pin every time the input signal crosses the fixed voltage level. The comparator can be used to monitor battery voltage (or any other DC level).

To monitor an external voltage, the AC input must be connected to this voltage using an I/O pin. This pin needs to have the digital input buffer and the pull-up resistor disabled, to have the highest possible input impedance. For the ATmega4809, PORTD pin 2 (PD2/AINP0) is used as AC positive input:

This translates to the following code:

PORTD.PIN2CTRL = PORT_ISC_INPUT_DISABLE_gc;

In the selected application, the AC uses the analog pin as positive input and an internal reference for its negative input. The Voltage Reference (V_REF) peripheral must be configured before using the internal voltage reference on negative input.

The V_REF provides control registers for selecting between multiple internal reference levels. The internal references are generated from an internal band gap.

A value of 1.5V is selected as reference voltage:

VREF.CTRLA = VREF_AC0REFSEL_1V5_gc;

To enable V_REF voltage generation, the output buffer must be enabled after the selection of reference voltage. Do this by setting the AC0REFEN bit from the VREF Control B register:

This translates to the following code:

VREF.CTRLB = VREF_ADC0REFEN_bm;

After configuring the peripherals and the modules required by the AC, the selection of inputs is done using the MUXCTRLA register as follows:

The positive input pin 0 (AINP0) and internal DAC reference are used as AC inputs:

AC0.MUXCTRLA = AC_MUXPOS_PIN0_gc | AC_MUXNEG_DACREF_gc;

#define DACREF_VALUE	         (VDACREF * 256 / VREF)

The output of the comparator is present on the external pin, and the interrupts are enabled on the negative edge to notify the application when the critical level is reached. That is done using the CTRLA register:

These settings translate to the following code:

AC0.CTRLA = AC_ENABLE_bm | AC_INTMODE_NEGEDGE_gc | AC_OUTEN_bm;

The AC peripheral interrupt must be enabled to complete the setup and make the selected interrupt available to the application. Do this by using the CMP bit from the INTCTRL register:

An MCC generated code example for AVR128DA48 with the same functionality as the one described in this section can be found here:

This example demonstrates the hysteresis feature of the AC peripheral that helps in avoiding frequent toggling of the AC when the positive input oscillates very close to the negative input level. This application is similar to the voltage level detector application, but additionally it has the Hysteresis mode enabled.

AC0.CTRLA |= AC_HYSMODE_25mV_gc;

An MCC generated code example for AVR128DA48 with the same functionality as the one described in this section can be found here:

The tinyAVR® 0- and 1-series, megaAVR® 0-series and AVR® Dx devices feature Event System (EVSYS), a simple but powerful system that allows autonomous control of peripherals without any use of interrupts, CPU, or DMA resources. It allows a change in one peripheral (the event generator) to trigger actions in other peripherals (the event users) through event channels. It provides short and predictable response times between peripherals and can reduce the complexity, size, and execution time of the software, to save power.

The application example in this chapter shows an implementation of duration/frequency measurement for an analog input signal, with minimal usage of microcontroller power. It uses the Event System to route the signals from the AC output through an event channel to Timer Counter B (TCB) event input. Configure the Event System properly to do this.

The first step in the Event System configuration is to set the AC output as event generator for channel 0:

For EVSYS channel 0, the channel generator selection register must be loaded with 0x20 to enable analog comparator as event generator:

EVSYS.CHANNEL0 = EVSYS_GENERATOR_AC0_OUT_gc;

To trigger events on TCB input, the TCB event user must be connected to channel 0:

EVSYS.USERTCB0 = EVSYS_CHANNEL_CHANNEL0_gc;

To enable pulse and period measurements, the TCB is configured in Pulse-Width Measurement mode, having the Event System as input. The Event System is used to route AC output through event channel 0 to TCB event input.

In Pulse-Width Measurement mode, the TCB will start counting when a positive edge is detected on the event input signal. On the next falling edge, the count value is captured. The counter stops when the second rising edge of the event input signal is detected, and this will set the Interrupt flag. Reading the capture will clear the Interrupt flag. When the Capture register is read, or the Interrupt flag cleared, the TCB becomes ready for a new capture sequence. Therefore, it is recommended to read the Counter register before the Capture register since it is reset to zero at the next positive edge:

The following code provides basic initialization for TCB in Pulse-Width Measurement mode with Event System as input:

int8_t TIMER0_init()
{

    TCB0.CTRLB = TCB_CNTMODE_FRQPW_gc;		
    TCB0.EVCTRL = TCB_CAPTEI_bm;			

    TCB0.INTCTRL = TCB_CAPT_bm;			
    TCB0.CTRLA = TCB_CLKSEL_CLKDIV2_gc		
               | TCB_ENABLE_bm			
               | TCB_RUNSTDBY_bm;			

}

An MCC generated code example for AVR128DA48 with the same functionality as the one described in this section can be found here: