i2c clock speed

Official Hardware Nano Family Nano Every
1

Nano-Every: I've been working with the Nano-Every. I have attempted to use Wire.setClock() method but watching on an oscilloscope sees no change in clock rate. It stays at 100khz regardless of what I set. Even for the standard rates. I'm working on an old legacy application that uses a nonstandard max clock speed of 25khz. Any ideas on how I can drive that down? I notice that some other processors have a timer register but I haven't found any for the Nano Every.

2

twi.c is forcing the minimum clock rate to 100kHz:

void TWI_MasterSetBaud(uint32_t frequency){
//        Formula is: BAUD = ((F_CLKPER/frequency) - F_CLKPER*T_RISE - 10)/2;
//        Where T_RISE varies depending on operating frequency...
//            From 1617 DS: 1000ns @ 100kHz / 300ns @ 400kHz / 120ns @ 1MHz

    uint16_t t_rise;
    
    if(frequency < 200000){
        frequency = 100000;
        t_rise = 1000;
        
    } else if (frequency < 800000){
        frequency = 400000;
        t_rise = 300;    

    } else if (frequency < 1200000){
        frequency = 1000000;
        t_rise = 120;
        
    } else {
        frequency = 100000;
        t_rise = 1000;
    }
    
    uint32_t baud = ((F_CPU_CORRECTED/frequency) - (((F_CPU_CORRECTED*t_rise)/1000)/1000)/1000 - 10)/2;
    TWI0.MBAUD = (uint8_t)baud;
}

It's not clear that 25kHz is possible without changing the CPU clock. The divisor for the clock is only 8bits.

3

Wire.setClock() method never works properly.
The result of formula must be an integer (if it is same for UNo, nano, mini-pro)

Formula is: BAUD = ((F_CLKPER/frequency) - F_CLKPER*T_RISE - 10)/2;

Do the calculation on the opposite side.
Use excel, inputs different values for register, the result frequency is inevitably an integer and select the result frequency most near that you want.