diff --git a/src/platform/stm32/platform.c b/src/platform/stm32/platform.c
index dca9aa33..982035b3 100755
--- a/src/platform/stm32/platform.c
+++ b/src/platform/stm32/platform.c
@@ -490,13 +490,17 @@ static void spis_init()
   RCC_APB1PeriphClockCmd(RCC_APB1Periph_SPI2, ENABLE);
 }
 
-#define SPI_GET_BASE_CLK( id ) ( ( id ) == 2 ? ( HCLK / PCLK2_DIV ) : ( HCLK / PCLK1_DIV ) )
+#define SPI_GET_BASE_CLK( id ) ( ( id ) == 1 ? ( HCLK / PCLK2_DIV ) : ( HCLK / PCLK1_DIV ) )
 
 u32 platform_spi_setup( unsigned id, int mode, u32 clock, unsigned cpol, unsigned cpha, unsigned databits )
 {
   SPI_InitTypeDef SPI_InitStructure;
   GPIO_InitTypeDef GPIO_InitStructure;
-  u8 prescaler_idx = intlog2( ( unsigned ) ( SPI_GET_BASE_CLK( id ) / clock ) ) - 1;
+  u8 prescaler_idx = intlog2( ( unsigned ) ( SPI_GET_BASE_CLK( id ) / clock ) ) - 2;
+  if ( prescaler_idx < 0 )
+    prescaler_idx = 0;
+  if ( prescaler_idx > 7 )
+    prescaler_idx = 7;
   
   /* Configure SPI pins */
   GPIO_InitStructure.GPIO_Pin = spi_gpio_pins[ id ];