Using USART3 (VCOM on the NUCLEO-H723ZG board) an IDLE IRQ is generated after 512 bytes even if the host is transmitting more than 512 bytes. The IRQ is generated since bit[4] of register ISR is 1 and bit[4] of register CR1 is 1. From where "512" comes?

[waclawek.jan]

IDLE means a gap between individual characters, so there was gap after 512 bytes arrived from the host. This may be caused by the STLink firmware, the software on the host, or the USB drivers stack on the host.

You should not anticipate any particular number of characters until IDLE, the IDLE (gap in transmission) may occur anywhere.

JW