ST STM32F2 User Manual page 32

Overview of HAL drivers
The peripheral handle is used for the following purposes:
Multi-instance support: each peripheral/module instance has its own handle. As a
result instance resources are independent.
Peripheral process intercommunication: the handle is used to manage shared data
resources between the process routines.
Example: global pointers, DMA handles, state machine.
Storage : this handle is used also to manage global variables within a given HAL
driver.
An example of peripheral structure is shown below:
typedef struct
{
USART_TypeDef *Instance; /* USART registers base address */
USART_InitTypeDef Init; /* Usart communication parameters */
uint8_t *pTxBuffPtr;/* Pointer to Usart Tx transfer Buffer */
uint16_t TxXferSize; /* Usart Tx Transfer size */
__IO uint16_t TxXferCount;/* Usart Tx Transfer Counter */
uint8_t *pRxBuffPtr;/* Pointer to Usart Rx transfer Buffer */
uint16_t RxXferSize; /* Usart Rx Transfer size */
__IO uint16_t RxXferCount; /* Usart Rx Transfer Counter */
DMA_HandleTypeDef *hdmatx; /* Usart Tx DMA Handle parameters */
DMA_HandleTypeDef *hdmarx; /* Usart Rx DMA Handle parameters */
HAL_LockTypeDef Lock; /* Locking object */
__IO HAL_USART_StateTypeDef State; /* Usart communication state */
__IO uint32_t ErrorCode;/* USART Error code */
}USART_HandleTypeDef;
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1) The multi-instance feature implies that all the APIs used in the application are
re-entrant and avoid using global variables because subroutines can fail to be re-
entrant if they rely on a global variable to remain unchanged but that variable is
modified when the subroutine is recursively invoked. For this reason, the following
rules are respected:
Re-entrant code does not hold any static (or global) non-constant data: re-
entrant functions can work with global data. For example, a re-entrant
interrupt service routine can grab a piece of hardware status to work with
(e.g. serial port read buffer) which is not only global, but volatile. Still, typical
use of static variables and global data is not advised, in the sense that only
atomic read-modify-write instructions should be used in these variables. It
should not be possible for an interrupt or signal to occur during the execution
of such an instruction.
Reentrant code does not modify its own code.
2) When a peripheral can manage several processes simultaneously using the
DMA (full duplex case), the DMA interface handle for each process is added in the
PPP_HandleTypeDef.
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