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Frequency Measurement with Modern AVR Microcontrollers

How to use the AVR64DD32's asynchronous counter to measure frequencies beyond 100 MHz
August 31, 2023
circuit microcontroller

Modern AVR microcontrollers have asynchronous counters that can be externally driven to count pulses from 1 Hz to beyond 100 MHz. Over the years I’ve explored various methods for building frequency counters typically using the SN74LV8154 32-bit counter, but my new favorite method uses the AVR64DD32 microcontroller ($1.52 on Mouser) to directly measure a signal and report its frequency to a PC using a USB serial adapter. I’m working on a special frequency counter project which builds upon this strategy, but I found the core concept to be interesting enough that I decided to write about it in its own article. The following information is a summary of how the strategy can be achieved, but additional information and source code is available on GitHub.

Theory of Operation

1 The AVR64DD32 datasheet suggests EXTCLK can be driven via XTALHF1 pin to a maximum frequency of 32 MHz (Section 12.3.4.2.1, page 93), but this article by sm6vfz demonstrates this strategy produces results accurate to the single Hz up to 150 MHz.

2 The AVR64DD32 datasheet says “an external digital clock can be connected to the XTAL32K1 pin” (section 26.3, page 344) but my read doesn’t clearly indicate what the upper limit of the frequency is that may be clocked in. Although the XTAL32K1 pin in combination with XTAL32K2 are designed for a 32 kHz crystal oscillator, my read does not indicate that 32 kHz is intended to be an upper limit of what may be clocked in externally.

Basic Setup

Microcontroller: The AD64DD32 8-bit AVR does not come in a DIP package, but the VQFN32 package is easy to hand solder to a QFN32/DIP breakout board. It also cannot be programmed with a ICSP programmer, but instead requires a UDPI programmer. See my Programming Modern AVR Microcontrollers article for more information about programming these chips.

Code: Full source code for this project is on GitHub, and the code highlights are shown at the bottom of this article.

PC Connection: I’m using an RS232 breakout board as a USB/serial adapter. It’s Rx pin is connected to the microcontroller’s Tx pin (pin 2).

Test Signal: I’m using a 50 MHz can oscillator as a test signal. It’s been in my junk box for years and it doesn’t surprise me if it has drifted a few kHz from 50 MHz. Note too that there may be some inaccuracy in the gating time base due to the imprecise nature of the AVR’s 24 MHz internal oscillator.

Serial Monitor: I’m using RealTerm to monitor the output of the microcontroller. The code below gates the counter once per second (1 PPS) then displays the count, so the number displayed is the frequency in Hz. This value would be easy to read in a language like Python for applications requiring frequency measurement over time.

Code: Counting EXTCLK pulses with Timer/Counter D

void setup_extclk_counter()
{
	// Enable the highest frequency external clock on pin 30
	CCP = CCP_IOREG_gc; // protected write
	CLKCTRL.XOSCHFCTRLA = CLKCTRL_FRQRANGE_32M_gc | CLKCTRL_SELHF_bm | CLKCTRL_ENABLE_bm;
	
	// Setup TCD to count the external clock
	TCD0.CMPBCLR = 0x0FFF; // count to max (12-bit)
	TCD0.CTRLA = TCD_CLKSEL_EXTCLK_gc; // count external clock input
	TCD0.INTCTRL = TCD_OVF_bm; // Enable overflow interrupt
	while (!(TCD0.STATUS & 0x01)); // Wait for ENRDY before enabling
	TCD0.CTRLA |= TCD_ENABLE_bm; // Enable the counter
}

// Increments the counter every time TCD0 overflows
volatile uint32_t COUNTER;
ISR(TCD0_OVF_vect)
{
	COUNTER+=4096;
	TCD0.INTFLAGS = TCD_OVF_bm;
}

volatile uint32_t COUNT_DISPLAY = 0;
volatile uint32_t COUNT_NOW = 0;
volatile uint32_t COUNT_PREVIOUS = 0;

// Call this method once per second to update the display frequency
void update_display_count()
{
    TCD0.CTRLE = TCD_SCAPTUREA_bm;
    while ((TCD0.STATUS & TCD_CMDRDY_bm) == 0); // synchronized read
    COUNT_NOW = COUNTER + TCD0.CAPTUREA;
    COUNT_DISPLAY = COUNT_NOW - COUNT_PREVIOUS;
    COUNT_PREVIOUS = COUNT_NOW;
}

Code: Gating at 1 Hz using the system clock as a time base

void setup_gate_sysclk(){
	// 24 MHz clock div 256 is 93,750 ticks/second
	TCA0.SINGLE.CTRLA = TCA_SINGLE_CLKSEL_DIV256_gc | TCA_SINGLE_ENABLE_bm;
	
	// enable overflow interrupt
	TCA0.SINGLE.INTCTRL |= TCA_SINGLE_OVF_bm;
	
	// overflow 5 times per second
	TCA0.SINGLE.PER = 18750-1;
}

// this interrupt is called 5 times per second
uint8_t GATE_TICKS = 0;
ISR(TCA0_OVF_vect){
	GATE_TICKS++;
	if (GATE_TICKS == 5){
		GATE_TICKS = 0;
		update_display_count();
	}
	TCA0.SINGLE.INTFLAGS = TCA_SINGLE_OVF_bm;
}

Code: The main block runs an infinite loop and displays the frequency if an updated number is detected. How to send text to the serial port is outside the scope of this article, but see this project’s code on GitHub for more information about how I did it. I did find this function helpful:

void print_with_commas(unsigned long freq){
	int millions = freq / 1000000;
	freq -= millions * 1000000;
	int thousands = freq / 1000;
	freq -= thousands * 1000;
	int ones = freq;
	printf("%d,%03d,%03d\r\n", millions, thousands, ones);
}

Amplify Small Signals

Using an RF amplifier module, I was able to measure the frequency of radio signals using an antenna. I found a convenient RF buffer amplifier board on Amazon based on a TLV3501 comparator. It is powered with 5V and has SMA connectors for RF input and TTL output, and I was able to use this device to measure frequency of various transmitters including my 144 MHz handheld VHF radio.

Use a Prescaler to Measure Higher Frequencies

There are many inexpensive single chip prescalers which can divide-down high frequency input to produce a waveform that slower counters can measure. It appears there are several RF prescaler modules on Amazon with SMA connectors, making them easy to pair with the preamplifier module above. Most of them seem to use a MB506 2.4 GHz prescaler which is not currently available on Mouser.

I’m also noticing a lot of people using the MC12080 1.1 GHz Prescaler for custom frequency counter designs. It’s a little over $4 on Mouser and doesn’t require much supporting circuitry, although I haven’t personally used this chip yet. I also found recommendations for the MC12093 prescaler. If you have experience creating a frequency counter using a prescaler, send me an email and let me know which chip you recommend and why!

Gate with an External 10 MHz Reference

The examples above use the AVR’s system clock to generate the 1 Hz gate, but accuracy can be improved by gating based upon a 10 MHz frequency reference. This strategy passes the 10 MHz into the XTAL32K1 pin and counts it with the RTC counter, generating 5 hz interrupts that can trigger the gating logic.

In this example I’m measuring the 10 MHz signal which is also responsible for the gating, so because of the chick-and-egg problem the measured frequency will always appear to be exactly 10 MHz even if the oscillator drifts. However, this strategy is useful for ensuring the software is written correctly. If the software is incorrect (e.g., the overflow period is off by one) this number will not read exactly 10 Mhz. Note also that the displayed frequency is ±1 which I presume can be attributed to variations in synchronization alignment while reading the asynchronous counter. No counts are “missed”, so a deficit by 1 in one reading will self-correct by rolling over and appearing as as a surplus by 1 in a future reading.

Code: Gate by dividing-down an external 10 Mhz reference to 5 Hz

void setup_gate_rtc(){
	// Enable the RTC
	CCP = CCP_IOREG_gc;

    // External clock on the XTAL32K1 pin, enable
	CLKCTRL.XOSC32KCTRLA = CLKCTRL_SEL_bm | CLKCTRL_ENABLE_bm;
	
	// Setup the RTC at 10 MHz to interrupt periodically
	// 10 MHz with 128 prescaler is 78,125 ticks/sec
	RTC.CTRLA = RTC_PRESCALER_DIV128_gc | RTC_RTCEN_bm;
	RTC.PER = 15624; // 5 overflows per second (78125/5-1)
	RTC.INTCTRL = RTC_OVF_bm;
	RTC.CLKSEL = RTC_CLKSEL_XTAL32K_gc; // clock in XOSC23K pin
}

// this interrupt is called 5 times per second
ISR(RTC_CNT_vect){
	/* same logic as above */
	RTC.INTFLAGS = 0x11;
}

Conclusion

The AVR64DD32 is a versatile chip with an impressive set of peripherals that is currently offered at low cost with high availability. The asynchronous peripherals make it easy to measure frequency independent of the system clock, and in practice frequencies well into the VHF band can be directly measured with this chip. Although it isn’t available in a DIP package, it’s easy to experiment with on a breadboard using a QFN/DIP breakout board, and I hope more people get the opportunity to experiment with this interesting line of modern AVR microcontrollers.

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