Hi!I am new to FreeRTOS,and recently I run FreeRTOS on STM32F4Discovery board,but I found that it is hard to use serial port since there are too many uart pins on this board,so My questions is that how can I successfully connect the "usb to ttl" board to Discovery board(You know that B6/B7,A8/A9 all stands for USART1). IN addition, I change the value of "HSE_VALUE" to 8000000 since the frequency of external crystal is 8MHz.

Another questions is that whether the tickless idle mode is suitable for STM32F4discovery board. Any demos afford?

[Most of your question is not related to FreeRTOS, so would be better posted to an STM32 forum]

The default tickless idle mode can be used on any Cortex-M3/4/7 part, but the power saving it can achieve is limited by two things:

1) The SysTick clock is only 24-bits, and runs very quickly - and therefore overflows often. That limits the maximum tickless period that can be achieved.

2) You cannot enter a very low power state because the really low power modes with stop the SysTick.

Therefore for optimal power saving you will need to create a target specific implementation, such as the one found on the following link:

http://www.freertos.org/STM32L-discovery-low-power-tickless-RTOS-demo.html