Unexpected cast during assignment

Question

There is piece of code in stm32 library that's behaving strangely. This is assignment made from initializing structure to timer auto-reload register:

/* Set the Autoreload value */
TIMx->ARR = TIM_TimeBaseInitStruct->TIM_Period ;

I've TIM_Period = 1999999, however after assignment TIMx->ARR = 33919. Smaller number usually points to overflow, so I checked: (1999999-33919) / 65536 = 30. This would mean the number overflowed 30 times on 16 bit data type, but both variables are 32 bit unsigned integers. Extracted from structure declarations:

For TIMx:

  __IO uint32_t ARR;         /*!< TIM auto-reload register,            Address offset: 0x2C */

For TIM_TimeBaseInitStruct:

uint32_t TIM_Period;            /*!< Specifies the period value to be loaded into the active
                                     Auto-Reload Register at the next update event.
                                     This parameter must be a number between 0x0000 and 0xFFFF.  */ 

Where __IO is defined as volatile.

This is disassembly of that assignment:

 296        TIMx->ARR = TIM_TimeBaseInitStruct->TIM_Period ;
0800c37c:   ldr r3, [r7, #0]
0800c37e:   ldr r2, [r3, #4]
0800c380:   ldr r3, [r7, #4]
0800c382:   str r2, [r3, #44]       ; 0x2c

What is happening here.. could it be something external causing the value to overflow? Note that I'm debugging on real hardware through ST-Link with no code optimization.

Answer 1

I'm going to guess that your chip has 16-bit timer registers. That is, it might still be a 32-bit register, but only have 16 useful bits in it.

Something like:

 31                16 15               0
+--------------------+------------------+
|      RESERVED      | Auto-reload value|
+--------------------+------------------+

Fact checking forthcoming (if you have a specific part number that would help me out).

Edit: By looking at some documentation [PDF link], my guess seems to be confirmed:

Edit 2: Since you mentioned which chip you were using, I found that documentation too [PDF link], which contains this handy diagram:

As you can see, some timers have 32-bit autoreload, and some don't. Which timer you've chosen will affect the behaviour you see.