Hello List, I have asked this before on several mailing lists, but still no solution, so I am trying again here. I'm having trouble with using FIQ interrupts with the ARM timer on raspberypi zero, (BCM2835 chip). I created a kernel module, and the test version of this driver can be found in https://github.com/SietseAchterop/Batradio/blob/master/batradio_module/fiqte... I made sure that FIQ is not used by USB by adding dwc_otg.fiq_fsm_enable=0 dwc_otg.fiq_enable=0 dwc_otg.nak_holdoff=0 to cmdline.txt in /boot I first tried to use the regular sequence of functions to use. In the init function: claim_fiq, set_fiq_regs, enable_fiq In the exit function disable_fiq, release_fiq. But I cannot find which irq number to use, e.g. using irq_of_parse_and_map I tried several values. 0, 64, but nothing works. So instead of the enable/disable functions I directly set the timer interrupt via the IRQFIQ register. This works; a bit. The fiq_handler only resets the ARM timer, increments a timer and toggles a LED. I see the timer counting, the fiq being called. But the rpi fairly quickly crashes. In the, sometimes, 10 seconds that it runs I see the led flashing very irregular. What is wrong here? Has the irregularity something to do with suspend/resume? Please find below the relevant snippets from the init and exit function of the driver. And also the fiq_handler. Thanks in advance, Sietse ====== init_bat ..... // directly set ARM timer registers TIMCNTR = 0x0000000; // stop timer TIMLOAD = 100000-1; // load value TIMCINT = 0; // clear interrupt ret = claim_fiq(&bat_fh); if (ret) { printk("batradio: claim_fiq failed.\n"); return ret; } set_fiq_handler(&batradio_handler, &batradio_handler_end - &batradio_handler); regs.ARM_r8 = (long)gpiospi; regs.ARM_r9 = (long)irqtimer; regs.ARM_r10 = (long)0; set_fiq_regs(®s); TIMCNTR = 0x000000A2; // start timer with interrupt, 23 bit counter IRQFIQ = 0xC0; // timer interrupt to fiq directly via register //enable_fiq(64); .... ====== exit_bat .... IRQFIQ = 0x00; // stop fiq interrupts TIMCNTR = 0x003E0000; // stop ARM timer //disable_fiq(64); .... ====== batradio fiq handler .text .global batradio_handler .global batradio_handler_end batradio_handler: stmdb sp!, {r6-r7} mov r6, #0 str r6, [r9, #0x40C] // TIMCINT = 0 // clear interrupt mov r7, #0x2000 // 1 << CNVST add r10, r10, #1 ands r6, r10, #0x0001 // set toggle speed bne off str r7, [r8, #40] // led on ldmia sp!, {r6-r7} subs pc, lr, #4 off: str r7, [r8, #28] // led off ldmia sp!, {r6-r7} subs pc, lr, #4 batradio_handler_end: