This error only exists for isochronous IN/OUT endpoints. It sets the Underflow Interrupt (USBHS_DEVEPTISRx.UNDERFI) bit, which triggers a PEP_x interrupt if the Underflow Interrupt Enable (USBHS_DEVEPTIMRx.UNDERFE) bit is one.
- An underflow can occur during the IN stage if the host attempts to read from an empty bank. A zero-length packet is then automatically sent by the USBHS.
- An underflow cannot occur during the OUT stage on a CPU action, since the user may only read if the bank is not empty (USBHS_DEVEPTISRx.RXOUTI = 1 or USBHS_DEVEPTISRx.RWALL = 1).
- An underflow can also occur during the OUT stage if the host sends a packet while the bank is already full. Typically, the CPU is not fast enough. The packet is lost.
- An underflow cannot occur during the IN stage on a CPU action, since the user may only write if the bank is not full (USBHS_DEVEPTISRx.TXINI = 1or USBHS_DEVEPTISRx.RWALL = 1).