Cortex M4, FPU a ISR

[Jaroslav Buchta]

Nejak nechapu, kdy se ukladaji nebo neukladaji registry na zasobnik, to 
procesor nejak automaticky detekuje?
Zatim jsem vypozoroval, ze v zavislosti na tom, jestli v hlavnim 
programu nepouziju/pouziju FP aritmetiku (v ISR je vzdy) tak je v LR 
hodnota 0xFFFFFFFD/0xFFFFFFED (Return to Thread mode, exception return 
uses non-floating-point state
from the PSP and execution uses PSP after return./Return to Thread mode, 
exception return uses floating-point state from PSP and execution uses 
PSP after return.)
Co me mate trosku vic je, ze latence ISR je v obou pripadech cca 350 ns 
(AHB 72MHz)  Jednak mi to prijde trosku moc a pak bych cekal, ze tady 
bude docela rozdil...
Je to tak normalni nebo je nekde chyba?

Tetsuju na tomto kodu:
volatile float fff = 1;

void TIM4_IRQHandler()
{
     GPIOE->BSRR = GPIO_Pin_8;
     TIM4->SR = (uint16_t)~TIM_IT_CC1;
     fff = fff * 1.01F;
     if (fff > 10.0F) fff = 1.0F;
     GPIOE->BRR = GPIO_Pin_8;
}

v asm to vyjde:
08003558    push    {r7}
0800355A    add    r7, sp, #0
(131)      GPIOE->BSRR = GPIO_Pin_8;
0800355C    ldr    r3, [pc, #80]    ; (0x80035b0 <TIM4_IRQHandler+88>)
0800355E    mov.w    r2, #256    ; 0x100
08003562    str    r2, [r3, #24]
(136)      TIM4->SR = (uint16_t)~TIM_IT_CC1;
08003564    ldr    r3, [pc, #76]    ; (0x80035b4 <TIM4_IRQHandler+92>)
08003566    movw    r2, #65533    ; 0xfffd
0800356A    str    r2, [r3, #16]
(137)      fff = fff * 1.01F;
0800356C    ldr    r3, [pc, #72]    ; (0x80035b8 <TIM4_IRQHandler+96>)
0800356E    vldr    s15, [r3]
08003572    vldr    s14, [pc, #72]    ; 0x80035bc <TIM4_IRQHandler+100>
08003576    vmul.f32    s15, s15, s14
0800357A    ldr    r3, [pc, #60]    ; (0x80035b8 <TIM4_IRQHandler+96>)
0800357C    vstr    s15, [r3]
(138)      if (fff > 10.0F) fff = 1.0F;
08003580    ldr    r3, [pc, #52]    ; (0x80035b8 <TIM4_IRQHandler+96>)
08003582    vldr    s15, [r3]
08003586    vmov.f32    s14, #36    ; 0x24
0800358A    vcmpe.f32    s15, s14
0800358E    vmrs    APSR_nzcv, fpscr
08003592    ble.n    0x800359c <TIM4_IRQHandler+68>
08003594    ldr    r3, [pc, #32]    ; (0x80035b8 <TIM4_IRQHandler+96>)
08003596    mov.w    r2, #1065353216    ; 0x3f800000
0800359A    str    r2, [r3, #0]
(139)
(140)      GPIOE->BRR = GPIO_Pin_8;
0800359C    ldr    r3, [pc, #16]    ; (0x80035b0 <TIM4_IRQHandler+88>)
0800359E    mov.w    r2, #256    ; 0x100
080035A2    strh    r2, [r3, #40]    ; 0x28
(141)  }
080035A4    nop
080035A6    mov    sp, r7
080035A8    ldr.w    r7, [sp], #4
080035AC    bx    lr

A na osciloskopu stav v priloze: (zluta vystup PWM z TIM4, azurova PE8)

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