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AURIX™ MCU: Write Access to multiple bitfields of an SFR - KBA235506

AURIX™ MCU: Write Access to multiple bitfields of an SFR - KBA235506

BinduPriya_G
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AURIX™ MCU: Write Access to multiple bitfields of an SFR - KBA235506

Version: **

Accessing the Special Function Register (SFR) is not difficult, as it is memory-mapped. There is no difference from the programming point of view to access an SFR or a memory-mapped variable. These are laid out to contain multiple bitfields, where each have a different functionality to control or to report the status. This article focusses about the recommended method of write access to multiple bitfields of an SFR. The following conditions apply:

  • The access action ensures read or write to the physical location each time
  • The access action ensures protect read-modify-write operation from the race condition
  • The access action is efficient during runtime, with smaller code size, or both depending on necessity
  • The access action code is readable to show the purpose of access

Recommendation:

  1. Make sure that the SFR access is within a code block. This ensures the following:
    • Defines the C variable scope for the SFR local copy
    • Allows you to define the protection or critical section
    • Makes the code readable
  2. Open critical section (optional).
  3. Define a nonvolatile, local variable of SFR type because the SFR header file ensures this SFR type has no volatile member in it or that type itself is not volatile qualified.
  4. Read SFR from its physical location (optional). There is no need to read if:
    • All the bitfields are to be written
    • Other bitfields, which are not written and do not have a specific pattern. In such a scenario, initialize with such pattern
    • To avoid read-modify-write operation and related data handling scenarios
  5. Write to individual bitfields using the following assignment statement:

       <local SFR variable>.B.<bit-field>= <value>

  1. Now write back to the physical register.
  2. Close the critical section, if previously opened.
  3. Close the code block.

To illustrate the recommendation, let’s take an example from AURIX™ TC3xx.

Configure the marked bitfields of the SCU_CCUCON0 register to be written as shown in Figure 1:

BinduPriya_G_0-1657690225438.png

Figure 1    Marked bitfields

Following is the code snippet to show the recommendations.

#include "IfxScu_reg.h"

void MyExample_accessRegMultibit(void)
{

    Ifx_SCU_CCUCON0 l_ScuCcuCon0;
    l_ScuCcuCon0= MODULE_SCU.CCUCON0;         /* Read the register content    */

    /* Now write to individual register bit-fields as required                    */

    l_ScuCcuCon0.B.FSIDIV= 1;                   /* fFSI = fSRI                           */
    l_ScuCcuCon0.B.SPBDIV= 2;                   /* fSPB = fSource0 / 2          */
    l_ScuCcuCon0.B.LPDIV=  1;                     /* fSPB = fSource0 / 30        */
    l_ScuCcuCon0.B.STMDIV= 1;                  /* fFSI = fSource0                   */
    l_ScuCcuCon0.B.UP=     1;                         /* Update CCUCON0 and CCUCON5   */
    MODULE_SCU.CCUCON0.U = l_ScuCcuCon0.U;
}

The disassembly of generated code is as shown below.

Starting disassembly of section 82 (.text.Test.MyExample_accessRegMultibit):
 

                  MyExampl:            movh.a a15,#0xf003
                8000172e:            lea         a15,[a15]0x6030
                80001732:            ld.w        d15,[a15]
                80001734:            mov       d0,#0x1
                80001736:            insert     d15,d15,d0,#0x18,#0x2
                8000173a:            mov       d0,#0x2
                8000173c:             insert     d15,d15,d0,#0x10,#0x4
                80001740:            mov       d0,#0x1
                80001742:            insert     d15,d15,d0,#0xc,#0x3
                80001746:            mov       d0,#0x1
                80001748:            insert     d15,d15,d0,#0x0,#0x4
                8000174c:             mov       d0,#0x1
                8000174e:            insert     d15,d15,d0,#0x1e,#0x1
                80001752:            movh.a a15,#0xf003
                80001756:            st.w        [a15]0x6030,d15
                8000175a:            ret

Note that@80001732, the SFR is read from its physical location and @80001756, the SFR is written to its physical location. Individual bitfields are updated locally, improving the runtime behavior and data consistency.

For more details, see the "Special Function Register (SFR)” section in the TriCore™ TC1.6.2 core architecture manual (volume 1).

Note:   This KBA applies to the following series of AURIX™ MCUs:

  • AURIX™ TC2xx series
  • AURIX™ TC3xx series
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