AVR Freaks

Hehe - that's exactly what I'm doing as plan B.  I've just noticed there is a 32kHz divided by 64 mentioned in the text so maybe able to get up to 1 minute wakeups

Yes that's line 1 of the interrupt routine :-)

Just to keep you updated, RTC isn't really a one-shot solution so gone back to TCB and hit some a new problem

"There are three steps in the configuration process:
1. Disable the CLK_PER prescaler - The following code snippet will demonstrate how to disable the CLK_PER prescaler:
CPU_CCP = CCP_IOREG_gc;
CLKCTRL.MCLKCTRLB = 0 << CLKCTRL_PEN_bp;"
 

At which point the processor goes gets a burst of 16MHz which is too much for the low voltage VCC I am using and exhibits shall we say anomalous behaviour. 

Going to 32kHz first in the hope that fixes this problem but do they even test these things ?

Code below.  The app note writes to MCLKCTRLB first which means my processor running at 2.8v gets a burst of 16MHz and seems to lose itself.  Swapped them round now.

ISR routine similarly swapped around.  On waking up the processor returns from power_down() and carries on working, usually coming straight back after doing a quick ADC and finding the main battery voltage is still too low.

Not sure why you even have to disable CLK_PER in MCLKCTRLB but seems to be the case.

void power_down()

{

            CPU_CCP = CCP_IOREG_gc;                                // Enable writing to protected register
            CLKCTRL.MCLKCTRLA = CLKCTRL_CLKSEL_OSCULP32K_gc;    // Select 32KHz Internal Ultra Low Power Oscillator (OSCULP32K)
            
            CPU_CCP = CCP_IOREG_gc;                                // Enable writing to protected register
            CLKCTRL.MCLKCTRLB = 0 << CLKCTRL_PEN_bp;            //     Disable CLK_PER Prescaler
            
            while (CLKCTRL.MCLKSTATUS & CLKCTRL_SOSC_bm) {}        // Wait for system oscillator changing to finish
                
            SLPCTRL.CTRLA = SLPCTRL_SMODE_gm | SLPCTRL_SMODE_STDBY_gc; // Enable sleep mode and select Standby mode            
            
            TCB0.CCMP = 32000;                                    // Load the Compare or Capture register with the timeout value
            TCB0.CTRLA = TCB_CLKSEL_CLKDIV1_gc | TCB_ENABLE_bm | TCB_RUNSTDBY_bm; // Enable TCB and set CLK_PER divider to 1 (No Prescaling)
            TCB0.CTRLB = TCB_CNTMODE_TIMEOUT_gc;                // Configure TCB in Periodic Timeout mode
            TCB0.INTCTRL = TCB_CAPT_bm;                            // Enable Capture or Timeout interrupt
            
            sei();
            sleep_mode();

}

ISR(TCB0_INT_vect)
    {
        TCB0.INTFLAGS = TCB_CAPT_bm;                            // Clear the interrupt flag
        
        TCB0.INTCTRL = 0;                                        // Disable Timeout interrupt

        CPU_CCP = CCP_IOREG_gc;                                    // Enable writing to protected register
        CLKCTRL_MCLKCTRLB    = 0b00000001;                        // /2 giving 8MHz                    

        CPU_CCP = CCP_IOREG_gc;                                    // Enable writing to protected register
        CLKCTRL.MCLKCTRLA = CLKCTRL_CLKSEL_OSC20M_gc;            // Select main 16MHz oscillator
        
        while (CLKCTRL.MCLKSTATUS & CLKCTRL_SOSC_bm) {;}        // Wait for system oscillator changing to finish            
    }