MAP electronics Cocerto-B Especificaciones descargar pdf (Pagina 25)

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Usage Notes and Known Design Exceptions to Functional Specifications

Advisory FPU: FPU Register Read Following MPYF32, ADDF32, SUBF32, or MACF32

Revision(s) Affected 0, A, B

Details This advisory applies when a multi-cycle (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 E2 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, XT,

XAR0…XAR7). This advisory does not apply if the register read is an FPU-to-FPU

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

read the value of RH6 before the MPYF32 completes.

Example of Problem:

MPYF32 R6H, R5H, R0H || MOV32 *XAR7++, R4H ; 2p FPU instruction that writes

; to R6H

F32TOUI16R R3H, R4H ; delay slot

ADDF32 R2H, R2H, R0H || MOV32 *--SP, R2H ; alignment cycle

MOV32 @XAR3, R6H ; FPU register read of R6H

Workaround(s) 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 V6.0.5 and later check for this scenario.

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

SPRZ357J–August 2011–Revised July 2014 F28M35x Concerto™ MCU Silicon Errata

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MAP electronics Cocerto-B Especificaciones Pagina 25