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FreeRTOS v10.2.0, GCC 12.2.0 (with const)
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| 40006156 <vPortSetupTimerInterrupt>: | |
| 40006156: 0200c6b7 lui a3,0x200c | |
| 4000615a: ffc68713 addi a4,a3,-4 # 200bffc <__ram_length+0x1ffec0c> | |
| 4000615e: ff868793 addi a5,a3,-8 | |
| 40006162: 4310 lw a2,0(a4) | |
| 40006164: 439c lw a5,0(a5) | |
| 40006166: 4318 lw a4,0(a4) | |
| 40006168: fec719e3 bne a4,a2,4000615a <vPortSetupTimerInterrupt+0x4> | |
| 4000616c: 13278593 addi a1,a5,306 | |
| 40006170: 00f5b533 sltu a0,a1,a5 | |
| 40006174: 862e mv a2,a1 | |
| 40006176: 020045b7 lui a1,0x2004 | |
| 4000617a: c190 sw a2,0(a1) | |
| 4000617c: 26478613 addi a2,a5,612 | |
| 40006180: 00e506b3 add a3,a0,a4 | |
| 40006184: 00f637b3 sltu a5,a2,a5 | |
| 40006188: c1d4 sw a3,4(a1) | |
| 4000618a: 97ba add a5,a5,a4 | |
| 4000618c: 800006b7 lui a3,0x80000 | |
| 40006190: 1ec6a823 sw a2,496(a3) # 800001f0 <__freertos_irq_stack_top+0xffff2df0> | |
| 40006194: 1ef6aa23 sw a5,500(a3) | |
| 40006198: 8082 ret | |
| 40004c5e <handle_asynchronous>: | |
| 40004c5e: 800002b7 lui t0,0x80000 | |
| 40004c62: 00728313 addi t1,t0,7 # 80000007 <__freertos_irq_stack_top+0xffff2c07> | |
| 40004c66: 04651a63 bne a0,t1,40004cba <test_if_external_interrupt> | |
| 40004c6a: 3fffb297 auipc t0,0x3fffb | |
| 40004c6e: 48e2a283 lw t0,1166(t0) # 800000f8 <pullMachineTimerCompareRegister> | |
| 40004c72: 3fffb317 auipc t1,0x3fffb | |
| 40004c76: 4b632303 lw t1,1206(t1) # 80000128 <pullNextTime> | |
| 40004c7a: 00032383 lw t2,0(t1) | |
| 40004c7e: 00432e03 lw t3,4(t1) | |
| 40004c82: 0072a023 sw t2,0(t0) | |
| 40004c86: 01c2a223 sw t3,4(t0) | |
| 40004c8a: 3fffb297 auipc t0,0x3fffb | |
| 40004c8e: 49a2a283 lw t0,1178(t0) # 80000124 <ulTimerIncrementsForOneTick> | |
| 40004c92: 00728eb3 add t4,t0,t2 | |
| 40004c96: 007ebf33 sltu t5,t4,t2 | |
| 40004c9a: 01ee0fb3 add t6,t3,t5 | |
| 40004c9e: 01d32023 sw t4,0(t1) | |
| 40004ca2: 01f32223 sw t6,4(t1) | |
| 40004ca6: 3fffb117 auipc sp,0x3fffb | |
| 40004caa: 48612103 lw sp,1158(sp) # 8000012c <xISRStackTop> | |
| 40004cae: 3c0000ef jal ra,4000506e <xTaskIncrementTick> | |
| 40004cb2: cd15 beqz a0,40004cee <processed_source> | |
| 40004cb4: 4f8000ef jal ra,400051ac <vTaskSwitchContext> | |
| 40004cb8: a81d j 40004cee <processed_source> |
configRTC_CLOCK_HZ / configTICK_RATE_HZ = (122666666 / 100) / 4000 = 306
The compiler seems to have been smart enough to optimise this variable into a direct register access in vPortSetupTimerInterrupt():
4000616c: 13278593 addi a1,a5,306
However, vPortSetupTimerInterrupt() is only called when the FreeRTOS is started. The MTIMECMP register needs to be updated in portASM.S: lw t0,1178(t0) # 80000124 <ulTimerIncrementsForOneTick>. However, this memory address contains the value 0 which means the machine timer will generate an interrupt almost immediately.
Link to reciprocal gist: https://gist.github.com/marksat/12e6830e7d14247103b324f5c3ef9fa0
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From port.c:
const uint32_t ulTimerIncrementsForOneTick = ( uint32_t ) ( configRTC_CLOCK_HZ / configTICK_RATE_HZ ); /* Assumes increment won't go over 32-bits. */