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STM32F103 RS-485 only with HSI and HSI calibration?

Question asked by Charles on Sep 8, 2015
Latest reply on Sep 9, 2015 by peacock.jack.003
It isa goodidea to usetheSTM32F103communicating overRS485(38400bps)onlyHSI clock? STM32F103 will works in temperature -10 + 50C.

I was looking atthe possibility ofuser calibrationHSI,toodo not understand.
ST say. High-speed internal oscillator of 8 MHz, typically). At 25 °C, the HSI has an accuracy of ±1% typically, this is factory trimed.
The accuracy value of the RC frequency increases to the maximum value of -2% - +2,5% over temperature. -10 +105C.
It can say, typically error is less than 80kHz.
Maximum error is -160kHz +200kHz
Why is it then user triming RCC_CR register step 40kHz and user trimming range +- 640kHz?