From f566c5ed53f0af2fe4427ce039a6e26fcb9b96e1 Mon Sep 17 00:00:00 2001
From: Gregory Nutt <gnutt@nuttx.org>
Date: Mon, 20 Oct 2014 13:54:43 -0600
Subject: [PATCH] EFM32: Another update to USART oversampling calculation. 
 Clip to positive; update comments

---
 arch/arm/src/efm32/efm32_lowputc.c | 33 ++++++++++++++++++------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/arch/arm/src/efm32/efm32_lowputc.c b/arch/arm/src/efm32/efm32_lowputc.c
index 77bcb7294a..9525bc8260 100644
--- a/arch/arm/src/efm32/efm32_lowputc.c
+++ b/arch/arm/src/efm32/efm32_lowputc.c
@@ -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);