The timer_isr function implemented in C change the CPU flags before the assembly code save the "af" registers.
The timer_isr must be written in assembler to come around(for ez80L92):

;
.assume adl=1
.def _timer_isr
.ref _vTaskIncrementTick
.ref _vTaskSwitchContext
.ref _pxCurrentTCB

_timer_isr:

; portSAVE_CONTEXT();

PUSH IX
push af ; save flags
LD IX,0
ADD IX,SP
in0 a,(80h)
push bc
push de
push hl
push iy
ex af,af'
exx
push af
push bc
push de
push hl
ld ix,0
add ix,sp
ld hl,(_pxCurrentTCB)
ld (hl),ix

CALL _vTaskIncrementTick

CALL _vTaskSwitchContext

; portRESTORE_CONTEXT_ISR();

ld hl,(_pxCurrentTCB)
ld hl,(hl)
ld sp,hl
pop hl
pop de
pop bc
pop af
exx
ex af,af'
pop iy
pop hl
pop de
pop bc
pop af
pop ix
ei
reti

Best regards:
Ivar Jeppesen
Embedit A/S
Denmark