Надо делать SEGGER_RTT_ConfigUpBuffer() void main(void) {
SEGGER_RTT_ConfigUpBuffer(0, NULL, NULL, 0, SEGGER_RTT_MODE_BLOCK_IF_FIFO_FULL);
SEGGER_RTT_WriteString(0, "SEGGER Real-Time-Terminal Sample\r\n\r\n");
}
// C:\Program Files\SEGGER\JLink\Samples\RTT\SEGGER_RTT_V752b.zip \Examples\Main_RTT_PrintfTest.c