15.1.1997 Vers. M.Mierse When using timers reload mode, main intention
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Note calculating reload values FFMC8L Fujitsu Mikroelektronik GmbH
15.1.1997 Vers. M.Mierse When using timers reload mode, main intention generate circular interrupts with fixed reload time. when interrupt service routines called, certain overhead will produced compiler (context save This note shows calculate correct reload value taking account produced extra code.
this example, rectangular signal with frequency should produced reload value calculated obtain this period. positive width signal fixed much smaller than period itself. diagram below shows signal procedure When interrupt signaled processed immediately, which would assume certain priority other interrupt currently active, context will saved stack after ISR-subroutine called. This context save produced compiler. After that intended taks processed this case signal with definite width.
Interrupt
Interrupt
offset
Reload-value without correction
offset
period
Fig. Producing signal using 16-bit(reload)-timer interrupt produced code, offset (due calling context save) takes cycles (40hex which must included calculation Reload FFFFhex fint) 40hex Quartzfreq. Interruptfreq.
this example With int=1500Hz Reload=F9BDhex
Interrupt-Service Routione 16-bit-Timer
(C-Source-code printed bold before calling finish actual command save address stack cycles approx. 10-14
220: void TC16INT6() 221: C2BA: PUSHW C2BB: XCHW C2BC: PUSHW C2BD: MOVW A,EP C2BE: PUSHW C2BF: PUSHW C2C0: MOVW A,SP C2C1: MOVW IX,A C2C2: A,R0 C2C3: SWAP C2C4: A,R1 C2C5: PUSHW 222: TMCR 0x22; enable egain C2C6: 851822 18,#22 223: TCHR 0xF6; Reload value C2C9: 8519F6 19,#F6 224: TCLR 0x3B; C2CC: 851A3B 1A,#3B 225: start counter again C2CF: A818 SETB 18:00 226: PDR4_0 show pulse C2D1: A80F SETB 0F:00 227: wait(20); C2D3: E40014 MOVW A,#0014 C2D6: PUSHW C2D7: 31C04B CALL \wait C2DA: POPW 228: PDR4_0 C2DB: A00F CLRB 0F:00 229:} C2DD: POPW 2DE: R1,A 2DF: SWAP 2E0: R0,A 2E1: MOVW A,IX 2E2: MOVW SP,A 2E3: POPW 2E4: POPW 2E5: MOVW EP,A 2E6: POPW 2E7: XCHW 2E8: POPW
cycles
2E9: RETI
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