SPRZ423K October   2014  – May 2024 TMS320F28075 , TMS320F28075-Q1 , TMS320F28076

 

  1.   1
  2.   Abstract
  3. 1Usage Notes and Advisories Matrices
    1. 1.1 Usage Notes Matrix
    2. 1.2 Advisories Matrix
  4. 2Nomenclature, Package Symbolization, and Revision Identification
    1. 2.1 Device and Development Support Tool Nomenclature
    2. 2.2 Devices Supported
    3. 2.3 Package Symbolization and Revision Identification
  5. 3Silicon Revision C Usage Notes and Advisories
    1. 3.1 Silicon Revision C Usage Notes
      1. 3.1.1 PIE: Spurious Nested Interrupt After Back-to-Back PIEACK Write and Manual CPU Interrupt Mask Clear
      2. 3.1.2 Caution While Using Nested Interrupts
      3. 3.1.3 SYS/BIOS: Version Implemented in Device ROM is not Maintained
      4. 3.1.4 SDFM: Use Caution While Using SDFM Under Noisy Conditions
      5. 3.1.5 McBSP: XRDY Bit can Hold the Not-Ready Status (0) if New Data is Written to the DX1 Register Without Verifying if the XRDY Bit is in its Ready State (1)
    2. 3.2 Silicon Revision C Advisories
      1.      Advisory
      2.      Advisory
      3.      Advisory
      4.      Advisory
      5.      Advisory
      6.      Advisory
      7.      Advisory
      8.      Advisory
      9.      Advisory
      10.      Advisory
      11.      Advisory
      12.      Advisory
      13.      Advisory
      14.      Advisory
      15.      Advisory
      16.      Advisory
      17.      Advisory
      18.      Advisory
      19.      Advisory
      20.      Advisory
      21.      Advisory
      22.      Advisory
      23.      Advisory
      24.      Advisory
      25.      Advisory
      26.      Advisory
      27.      Advisory
      28.      Advisory
      29.      Advisory
      30.      Advisory
      31.      Advisory
      32.      Advisory
      33.      Advisory
      34.      Advisory
      35.      Advisory
      36.      Advisory
  6. 4Silicon Revision B Usage Notes and Advisories
    1. 4.1 Silicon Revision B Usage Notes
    2. 4.2 Silicon Revision B Advisories
      1.      Advisory
      2.      Advisory
      3.      Advisory
      4.      Advisory
      5.      Advisory
      6.      Advisory
      7.      Advisory
      8.      Advisory
      9.      Advisory
  7. 5Documentation Support
  8. 6Trademarks
  9. 7Revision History

Advisory

FPU: FPU-to-CPU Register Move Operation Preceded by Any FPU 2p Operation

Revisions Affected

B, C

Details

This advisory applies when a multicycle (2p) FPU instruction is followed by a FPU-to-CPU register transfer. If the FPU-to-CPU read instruction source register is the same as the 2p instruction destination, then the read may be of the value of the FPU register before the 2p instruction completes. This occurs because the 2p instructions rely on data-forwarding of the result during the E3 phase of the pipeline. If a pipeline stall happens to occur in the E3 phase, the result does not get forwarded in time for the read instruction.

The 2p instructions impacted by this advisory are MPYF32, ADDF32, SUBF32, and MACF32. The destination of the FPU register read must be a CPU register (ACC, P, T, XAR0...XAR7). This advisory does not apply if the register read is a FPU-to-FPU register transfer.

In the example below, the 2p instruction, MPYF32, uses R6H as its destination. The FPU register read, MOV32, uses the same register, R6H, as its source, and a CPU register as the destination. If a stall occurs in the E3 pipeline phase, then MOV32 will read the value of R6H before the MPYF32 instruction completes.

Example of Problem:


   MPYF32 R6H, R5H, R0H  ; 2p FPU instruction that writes to R6H
|| MOV32 *XAR7++, R4H
   F32TOUI16R R3H, R4H   ; delay slot
   ADDF32 R2H, R2H, R0H
|| MOV32 *--SP, R2H      ; alignment cycle
   MOV32 @XAR3, R6H      ; FPU register read of R6H

Figure 3-1 shows the pipeline diagram of the issue when there are no stalls in the pipeline.

TMS320F28076 TMS320F28075 Pipeline Diagram of the Issue When
               There are no Stalls in the Pipeline Figure 3-1 Pipeline Diagram of the Issue When There are no Stalls in the Pipeline

Figure 3-2 shows the pipeline diagram of the issue if there is a stall in the E3 slot of the instruction I1.

TMS320F28076 TMS320F28075 Pipeline Diagram of the Issue if
               There is a Stall in the E3 Slot of the Instruction I1 Figure 3-2 Pipeline Diagram of the Issue if There is a Stall in the E3 Slot of the Instruction I1

Workarounds

Treat MPYF32, ADDF32, SUBF32, and MACF32 in this scenario as 3p-cycle instructions. Three NOPs or non-conflicting instructions must be placed in the delay slot of the instruction.

The C28x Code Generation Tools v.6.2.0 and later will both generate the correct instruction sequence and detect the error in assembly code. In previous versions, v6.0.5 (for the 6.0.x branch) and v.6.1.2 (for the 6.1.x branch), the compiler will generate the correct instruction sequence but the assembler will not detect the error in assembly code.

Example of Workaround:


   MPYF32 R6H, R5H, R0H
|| MOV32 *XAR7++, R4H     ; 3p FPU instruction that writes to R6H
   F32TOUI16R R3H, R4H    ; delay slot
   ADDF32 R2H, R2H, R0H
|| MOV32 *--SP, R2H       ; delay slot
   NOP                    ; alignment cycle
   MOV32 @XAR3, R6H       ; FPU register read of R6H

Figure 3-3 shows the pipeline diagram with the workaround in place.

TMS320F28076 TMS320F28075 Pipeline Diagram With Workaround
               in Place Figure 3-3 Pipeline Diagram With Workaround in Place