Workaround for USB memory bandwidth at very slow CPU speeds

teensy4/usb_desc.c

 #include "avr_functions.h"
 #include "avr/pgmspace.h"
 
+// At very slow CPU speeds, the OCRAM just isn't fast enough for
+// USB to work reliably.  But the precious/limited DTCM is.  So
+// as an ugly workaround, undefine DMAMEM so all buffers which
+// would normally be allocated in OCRAM are placed in DTCM.
+#if defined(F_CPU) && F_CPU < 30000000
+#undef DMAMEM
+#endif
+
 // USB Descriptors are binary data which the USB host reads to
 // automatically detect a USB device's capabilities.  The format
 // and meaning of every field is documented in numerous USB

teensy4/usb_serial.c

 #if defined(CDC_STATUS_INTERFACE) && defined(CDC_DATA_INTERFACE)
 //#if F_CPU >= 20000000
 
+// At very slow CPU speeds, the OCRAM just isn't fast enough for
+// USB to work reliably.  But the precious/limited DTCM is.  So
+// as an ugly workaround, undefine DMAMEM so all buffers which
+// would normally be allocated in OCRAM are placed in DTCM.
+#if defined(F_CPU) && F_CPU < 30000000
+#undef DMAMEM
+#define DMAMEM
+#endif
+
 uint32_t usb_cdc_line_coding[2];
 volatile uint32_t usb_cdc_line_rtsdtr_millis;
 volatile uint8_t usb_cdc_line_rtsdtr=0;