Hello I am developing a evaluation board for STM32F427. I am connecting an external RTC IC (DS3231). Do i need to connect an other external oscillator for processor? Because in STM32F407 discovery board they have connected an external 8MHz crystal other than 32.768KHz which is used for RTC.
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As you are using an external RTC with embedded 32K oscillator, you don't need to connect a 32K crystal to the STM32F427. In this case, to get an accurate timestamp you will have to communicate with the external RTC.
However, as you are developing an evaluation board, your users might prefer getting their (accurate) timestamp from the STM32F427. In this case, you will need to connect an external 32K crystal to the STM32F427.
The STM32F427 has also an internal 32K oscillator. Your users could use it, however the accuracy will not be great.
As for the high speed crystal (8MHz), it depends on what you have on your board. If you are using only synchronous communication buses (I2C, SPI...), the internal crystal will be sufficient. But, if you have asynchronous communication buses (USB, UART,...), I strongly advise you to have an external crystal.
In conclusion, it really depends on what you want to achieve. But for evaluation purposes, it is good practice to put the crystals, even if you don't fit or use them.
Guillaume Michel
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If you are happy with the answer, could you please mark it as the "Answer" – Guillaume Michel Jul 20 '16 at 08:23