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EFM32: Another update to USART oversampling calculation. Clip to positive; update comments

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This commit is contained in:
Gregory Nutt 2014-10-20 13:54:43 -06:00
parent 1a77fbcaca
commit ebf9bce9d8
1 changed files with 20 additions and 13 deletions
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33
arch/arm/src/efm32/efm32_lowputc.c
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@ -125,21 +125,21 @@ static void efm32_setbaud(uintptr_t base, uint32_t baud)
*
* Or, equivalently:
*
* CLKDIV = 256 * fHFPERCLK/(oversample * baud) - 1
* CLKDIV = 256 * (fHFPERCLK / (oversample * baud) - 1)
* oversample = 256 * fHFPERCLK / (baud * (CLKDIV + 256))
*
* Example: fHPERCLK = 32MHz, baud=115200
* oversample = 8,192,000,000 / (115200 * (CLKDIV + 256))
* CLKDIV = 71111.1111 / oversample + 1
*
* Suppose we insist on a CLKDIV >= 24, then:
*
* MAXoversample = 256 * fHFPERCLK / (280 * baud))
*
* Example: fHPERCLK = 32MHz, baud=115200
* MAXoversample = 254.0 -> 16
* CLKDIV = 4445.4
* baud = 115,249.3
* Example 1: fHPERCLK = 32MHz, baud=115200
* MAXoversample = 254.0, Use oversample = 16
* CLKDIV = 4188.4
* baud = 115200.0
* Example 2: fHPERCLK = 32.768KHz, baud=2400
* MAXoversample = 12.5, Use oversample = 8
* CLKDIV = 180.90
* baud = 2400.0
*/
maxover = ((BOARD_HFPERCLK_FREQUENCY << 8) / 280) / baud;
@ -170,15 +170,22 @@ static void efm32_setbaud(uintptr_t base, uint32_t baud)
/* CLKDIV in asynchronous mode is given by:
*
* CLKDIV = 256 * (fHFPERCLK/(oversample * baud) - 1)
* CLKDIV = 256 * (fHFPERCLK / (oversample * baud) - 1)
*
* or
*
* CLKDIV = (256 * fHFPERCLK)/(oversample * baud) - 256
* CLKDIV = (256 * fHFPERCLK) / (oversample * baud) - 256
*/
clkdiv = ((uint64_t)BOARD_HFPERCLK_FREQUENCY << 8) / ((uint64_t)baud * oversample);
clkdiv -= 256;
clkdiv = ((uint64_t)BOARD_HFPERCLK_FREQUENCY << 8) / ((uint64_t)baud * oversample);
if (clkdiv > 256)
{
clkdiv -= 256;
}
else
{
clkdiv = 0;
}
DEBUGASSERT(clkdiv <= _USART_CLKDIV_MASK);