Configure UART clock before use

teensy4/debugprintf.c

 __attribute__((section(".progmem")))
 void printf_debug_init(void)
 {
-        // turn on Serial4, run using 24 MHz clock (works if PLL3 off or bypassed)
-        CCM_CCGR0 |= CCM_CCGR0_LPUART3(CCM_CCGR_ON);
+        CCM_CCGR0 |= CCM_CCGR0_LPUART3(CCM_CCGR_ON); // turn on Serial4
         IOMUXC_SW_MUX_CTL_PAD_GPIO_AD_B1_06 = 2; // Arduino pin 17
-        CCM_CSCDR1 = (CCM_CSCDR1 & ~CCM_CSCDR1_UART_CLK_PODF(0x3F)) | CCM_CSCDR1_UART_CLK_SEL;
         LPUART3_BAUD = LPUART_BAUD_OSR(25) | LPUART_BAUD_SBR(8); // ~115200 baud
         LPUART3_CTRL = LPUART_CTRL_TE;
 }

teensy4/startup.c

 	for (i=0; i < 176; i++) _VectorsRam[i] = &unused_interrupt_vector;
 	SCB_VTOR = (uint32_t)_VectorsRam;
 
+	// Configure clocks
+	// TODO: make sure all affected peripherals are turned off!
 	// PIT & GPT timers to run from 24 MHz clock (independent of CPU speed)
 	CCM_CSCMR1 = (CCM_CSCMR1 & ~CCM_CSCMR1_PERCLK_PODF(0x3F)) | CCM_CSCMR1_PERCLK_CLK_SEL;
+	// UARTs run from 24 MHz clock (works if PLL3 off or bypassed)
+	CCM_CSCDR1 = (CCM_CSCDR1 & ~CCM_CSCDR1_UART_CLK_PODF(0x3F)) | CCM_CSCDR1_UART_CLK_SEL;
 
 	// must enable PRINT_DEBUG_STUFF in debug/print.h
 	printf_init();