USB issues when setting sysclock to lower frequencies (<48MHz)

[reald]

I hope this is the right way to address this issue. For a battery driven project I want so save power by reducing the system clock (clk_sys).

This works flawless until 48 MHz but below the USB serial port is getting unreliable. (clk_usb will stay at 48 MHz all the time.)

Check this code with different positions of set_sys_clock_khz():

#include <pico/stdlib.h>

void setup() {

   Serial.begin(9600);

   set_sys_clock_khz(48000, false); // [1] works
   //set_sys_clock_khz(18000, false); // [2] fails

}

void loop() {
  
  //set_sys_clock_khz(18000, false); // [3] fails

  Serial.println("clk_sys get:   " + (String)clock_get_hz(clk_sys) );
  Serial.println("clk_sys count: " + (String)frequency_count_khz(CLOCKS_FC0_SRC_VALUE_CLK_SYS) );
  
  delay(1000);
  set_sys_clock_khz(18000, false); // [4] works only if 48000 was set at [1] in setup() 

}

• set_sys_clock_khz(48000, false); at position [1] always works
• set_sys_clock_khz(18000, false); at position [2] always fails: USB serial connection is not possible.
• set_sys_clock_khz(18000, false); at position [3] always fails: USB serial connection is not possible.
• set_sys_clock_khz(18000, false); at position [4] fails

dmesg looks like this:

[ 8605.019412] usb 1-7: new full-speed USB device number 63 using xhci_hcd
[ 8606.392345] usb 1-7: device descriptor read/8, error -71
[ 8606.590239] usb 1-7: device descriptor read/8, error -71
[ 8607.083354] usb 1-7: new full-speed USB device number 64 using xhci_hcd

• And to make it even more strange if 48000 kHz was set at position [1] set_sys_clock_khz(18000, false); at position [4] works, too (18 MHz and USB serial ok).

Questions:

• Why?
• What is the right way to have reliable USB serial connection at the smallest possible clock frequency?

Target: RP2040

Thanks for you help.

[earlephilhower]

This really isn't related to the core. You might have better luck at the RPI forums directly.

[RevoAutomation]

the 48 MHz are required for the USB according to the USB specification. Any MCU (STM32 for example) need 48 MHz to implement a USB standard conform USB communication. If not done from the main clock, there is then a PLL generating the 48 MHz for the USB.

For a power saving scenario, like your battery powered app, USBserial should be disabled and use a UART instead. Then use a external USB-Uart converter, if Uart over USB is required. Disable the whole USB part on the chip saves significant power