commaai · savojovic · Mar 6, 2024
diff --git a/SConscript b/SConscript
@@ -167,7 +167,7 @@ Export('base_project_f4', 'base_project_h7', 'build_project')
 
 # Common autogenerated includes
 with open("board/obj/gitversion.h", "w") as f:
-  f.write(f'const uint8_t gitversion[] = "{get_version(BUILDER, BUILD_TYPE)}";\n')
+  f.write(f'const uint8_t gitversion[] = "{get_version(BUILDER, BUILD_TYPE)}"; // cppcheck-suppress misra-c2012-8.4\n')
 
 with open("board/obj/version", "w") as f:
   f.write(f'{get_version(BUILDER, BUILD_TYPE)}')

diff --git a/board/boards/black.h b/board/boards/black.h
@@ -148,7 +148,7 @@ void black_init_bootloader(void) {
   set_gpio_output(GPIOC, 12, 0);
 }
 
-const harness_configuration black_harness_config = {
+static const harness_configuration black_harness_config = {
   .has_harness = true,
   .GPIO_SBU1 = GPIOC,
   .GPIO_SBU2 = GPIOC,
@@ -162,6 +162,8 @@ const harness_configuration black_harness_config = {
   .adc_channel_SBU2 = 13
 };
 
+// Unable to use extern becaouse of a composite struct dos_harness_config
+// cppcheck-suppress misra-c2012-8.4
 const board board_black = {
   .set_bootkick = unused_set_bootkick,
   .harness_config = &black_harness_config,

diff --git a/board/boards/cuatro.h b/board/boards/cuatro.h
@@ -103,6 +103,9 @@ void cuatro_init(void) {
   clock_source_init();
 }
 
+// To be able to use extern and separate declaration
+// all of the functions should have separated declaration from implementation
+// cppcheck-suppress misra-c2012-8.4
 const board board_cuatro = {
   .harness_config = &red_chiplet_harness_config,
   .has_obd = true,

diff --git a/board/boards/dos.h b/board/boards/dos.h
@@ -173,7 +173,7 @@ void dos_init(void) {
   clock_source_init();
 }
 
-const harness_configuration dos_harness_config = {
+static const harness_configuration dos_harness_config = {
   .has_harness = true,
   .GPIO_SBU1 = GPIOC,
   .GPIO_SBU2 = GPIOC,
@@ -187,6 +187,8 @@ const harness_configuration dos_harness_config = {
   .adc_channel_SBU2 = 13
 };
 
+// Unable to use extern becaouse of a composite struct dos_harness_config
+// cppcheck-suppress misra-c2012-8.4
 const board board_dos = {
   .harness_config = &dos_harness_config,
   .has_obd = true,

diff --git a/board/boards/grey.h b/board/boards/grey.h
@@ -4,6 +4,8 @@
 
 // Most hardware functionality is similar to white panda
 
+// If used extern and struct implemented in board.h purpose of this file vanishes
+// cppcheck-suppress misra-c2012-8.4
 const board board_grey = {
   .set_bootkick = unused_set_bootkick,
   .harness_config = &white_harness_config,

diff --git a/board/boards/red.h b/board/boards/red.h
@@ -160,7 +160,7 @@ void red_init(void) {
   }
 }
 
-const harness_configuration red_harness_config = {
+static const harness_configuration red_harness_config = {
   .has_harness = true,
   .GPIO_SBU1 = GPIOC,
   .GPIO_SBU2 = GPIOA,
@@ -174,6 +174,9 @@ const harness_configuration red_harness_config = {
   .adc_channel_SBU2 = 17 //ADC1_INP17
 };
 
+// To be able to use extern and separate declaration
+// all of the functions should have separated declaration from implementation
+// cppcheck-suppress misra-c2012-8.4
 const board board_red = {
   .set_bootkick = unused_set_bootkick,
   .harness_config = &red_harness_config,

diff --git a/board/boards/red_chiplet.h b/board/boards/red_chiplet.h
@@ -138,7 +138,9 @@ void red_chiplet_init(void) {
     can_flip_buses(0, 2);
   }
 }
-
+// Extern should be used for declaration, but at the current moment there is no appropriate place for
+// the implementation
+// cppcheck-suppress misra-c2012-8.4
 const harness_configuration red_chiplet_harness_config = {
   .has_harness = true,
   .GPIO_SBU1 = GPIOC,

diff --git a/board/boards/tres.h b/board/boards/tres.h
@@ -2,8 +2,8 @@
 // Tres (STM32H7) + Harness //
 // ///////////////////////////
 
-bool tres_ir_enabled;
-bool tres_fan_enabled;
+static bool tres_ir_enabled;
+static bool tres_fan_enabled;
 void tres_update_fan_ir_power(void) {
   red_chiplet_set_fan_or_usb_load_switch(tres_ir_enabled || tres_fan_enabled);
 }
@@ -70,6 +70,9 @@ void tres_init(void) {
   clock_source_init();
 }
 
+// To be able to use extern and separate declaration
+// all of the functions should have separated declaration from implementation
+// cppcheck-suppress misra-c2012-8.4
 const board board_tres = {
   .harness_config = &red_chiplet_harness_config,
   .has_obd = true,

diff --git a/board/boards/uno.h b/board/boards/uno.h
@@ -184,7 +184,7 @@ void uno_init_bootloader(void) {
   set_gpio_output(GPIOC, 12, 0);
 }
 
-const harness_configuration uno_harness_config = {
+static const harness_configuration uno_harness_config = {
   .has_harness = true,
   .GPIO_SBU1 = GPIOC,
   .GPIO_SBU2 = GPIOC,
@@ -198,6 +198,8 @@ const harness_configuration uno_harness_config = {
   .adc_channel_SBU2 = 13
 };
 
+// Unable to use extern becaouse of a composite struct uno_harness_config
+// cppcheck-suppress misra-c2012-8.4
 const board board_uno = {
   .harness_config = &uno_harness_config,
   .has_obd = true,

diff --git a/board/boards/white.h b/board/boards/white.h
@@ -221,10 +221,12 @@ void white_grey_init_bootloader(void) {
   set_gpio_output(GPIOC, 14, 0);
 }
 
-const harness_configuration white_harness_config = {
+static const harness_configuration white_harness_config = {
   .has_harness = false
 };
 
+// Unable to use extern because of composite struct white_harness_config
+// cppcheck-suppress misra-c2012-8.4
 const board board_white = {
   .set_bootkick = unused_set_bootkick,
   .harness_config = &white_harness_config,

diff --git a/board/can_comms.h b/board/can_comms.h
@@ -18,7 +18,7 @@ typedef struct {
   uint8_t data[72];
 } asm_buffer;
 
-asm_buffer can_read_buffer = {.ptr = 0U, .tail_size = 0U};
+static asm_buffer can_read_buffer = {.ptr = 0U, .tail_size = 0U};
 
 int comms_can_read(uint8_t *data, uint32_t max_len) {
   uint32_t pos = 0U;
@@ -53,7 +53,7 @@ int comms_can_read(uint8_t *data, uint32_t max_len) {
   return pos;
 }
 
-asm_buffer can_write_buffer = {.ptr = 0U, .tail_size = 0U};
+static asm_buffer can_write_buffer = {.ptr = 0U, .tail_size = 0U};
 
 // send on CAN
 void comms_can_write(const uint8_t *data, uint32_t len) {

diff --git a/board/can_definitions.h b/board/can_definitions.h
@@ -1,6 +1,10 @@
 #pragma once
 
+// Unable to use extern, because of initialization
+// cppcheck-suppress misra-c2012-8.4
 const uint8_t PANDA_CAN_CNT = 3U;
+// Unable to use extern, because of initialization
+// cppcheck-suppress misra-c2012-8.4
 const uint8_t PANDA_BUS_CNT = 4U;
 
 // bump this when changing the CAN packet
@@ -27,6 +31,8 @@ typedef struct {
   unsigned char data[CANPACKET_DATA_SIZE_MAX];
 } __attribute__((packed, aligned(4))) CANPacket_t;
 
+// Unable to use extern, because of initialization
+// cppcheck-suppress misra-c2012-8.4
 const unsigned char dlc_to_len[] = {0U, 1U, 2U, 3U, 4U, 5U, 6U, 7U, 8U, 12U, 16U, 20U, 24U, 32U, 48U, 64U};
 
 #define GET_BUS(msg) ((msg)->bus)

diff --git a/board/critical.h b/board/critical.h
@@ -1,5 +1,5 @@
 // ********************* Critical section helpers *********************
-volatile bool interrupts_enabled = false;
+static volatile bool interrupts_enabled = false;
 
 void enable_interrupts(void) {
   interrupts_enabled = true;
@@ -11,6 +11,8 @@ void disable_interrupts(void) {
   __disable_irq();
 }
 
+// Used across multiple files, unable to use extern
+// cppcheck-suppress misra-c2012-8.4
 uint8_t global_critical_depth = 0U;
 #define ENTER_CRITICAL()                                      \
   __disable_irq();                                            \

diff --git a/board/drivers/bootkick.h b/board/drivers/bootkick.h
@@ -1,13 +1,14 @@
-bool bootkick_ign_prev = false;
-BootState boot_state = BOOT_BOOTKICK;
-uint8_t bootkick_harness_status_prev = HARNESS_STATUS_NC;
-
-uint8_t boot_reset_countdown = 0;
-uint8_t waiting_to_boot_countdown = 0;
+// Unable to use extern keyword at initialization
+// cppcheck-suppress misra-c2012-8.4
 bool bootkick_reset_triggered = false;
-uint16_t bootkick_last_serial_ptr = 0;
 
 void bootkick_tick(bool ignition, bool recent_heartbeat) {
+  static bool bootkick_ign_prev = false;
+  static uint8_t bootkick_harness_status_prev = HARNESS_STATUS_NC;
+  static BootState boot_state = BOOT_BOOTKICK;
+  static uint8_t boot_reset_countdown = 0;
+  static uint8_t waiting_to_boot_countdown = 0;
+  uint16_t bootkick_last_serial_ptr = 0;
   BootState boot_state_prev = boot_state;
   const bool harness_inserted = (harness.status != bootkick_harness_status_prev) && (harness.status != HARNESS_STATUS_NC);
 
@@ -56,6 +57,9 @@ void bootkick_tick(bool ignition, bool recent_heartbeat) {
   // update state
   bootkick_ign_prev = ignition;
   bootkick_harness_status_prev = harness.status;
+
+  // misra-c2012-8.4 exposed this assignment
+  // cppcheck-suppress unreadVariable
   bootkick_last_serial_ptr = uart_ring_som_debug.w_ptr_tx;
   if (waiting_to_boot_countdown > 0U) {
     waiting_to_boot_countdown--;

diff --git a/board/drivers/bxcan.h b/board/drivers/bxcan.h
@@ -2,12 +2,8 @@
 //       CAN2_TX, CAN2_RX0, CAN2_SCE
 //       CAN3_TX, CAN3_RX0, CAN3_SCE
 
+// cppcheck-suppress misra-c2012-8.4
 CAN_TypeDef *cans[] = {CAN1, CAN2, CAN3};
-uint8_t can_irq_number[3][3] = {
-  { CAN1_TX_IRQn, CAN1_RX0_IRQn, CAN1_SCE_IRQn },
-  { CAN2_TX_IRQn, CAN2_RX0_IRQn, CAN2_SCE_IRQn },
-  { CAN3_TX_IRQn, CAN3_RX0_IRQn, CAN3_SCE_IRQn },
-};
 
 bool can_set_speed(uint8_t can_number) {
   bool ret = true;
@@ -24,6 +20,11 @@ bool can_set_speed(uint8_t can_number) {
 }
 
 void update_can_health_pkt(uint8_t can_number, uint32_t ir_reg) {
+  uint8_t can_irq_number[3][3] = {
+    { CAN1_TX_IRQn, CAN1_RX0_IRQn, CAN1_SCE_IRQn },
+    { CAN2_TX_IRQn, CAN2_RX0_IRQn, CAN2_SCE_IRQn },
+    { CAN3_TX_IRQn, CAN3_RX0_IRQn, CAN3_SCE_IRQn },
+  };
   CAN_TypeDef *CANx = CANIF_FROM_CAN_NUM(can_number);
   uint32_t esr_reg = CANx->ESR;
 

diff --git a/board/drivers/can_common.h b/board/drivers/can_common.h
@@ -16,13 +16,17 @@ typedef struct {
   bool canfd_non_iso;
 } bus_config_t;
 
+// cppcheck-suppress-begin misra-c2012-8.4
 uint32_t safety_tx_blocked = 0;
 uint32_t safety_rx_invalid = 0;
 uint32_t tx_buffer_overflow = 0;
 uint32_t rx_buffer_overflow = 0;
 uint32_t gmlan_send_errs = 0;
-
 can_health_t can_health[] = {{0}, {0}, {0}};
+// Ignition detected from CAN meessages
+bool ignition_can = false;
+uint32_t ignition_can_cnt = 0U;
+// cppcheck-suppress-end misra-c2012-8.4
 
 extern int can_live;
 extern int pending_can_live;
@@ -31,10 +35,6 @@ extern int pending_can_live;
 extern int can_silent;
 extern bool can_loopback;
 
-// Ignition detected from CAN meessages
-bool ignition_can = false;
-uint32_t ignition_can_cnt = 0U;
-
 #define ALL_CAN_SILENT 0xFF
 #define ALL_CAN_LIVE 0
 
@@ -56,6 +56,8 @@ void process_can(uint8_t can_number);
 #define CAN_TX_BUFFER_SIZE 416U
 #define GMLAN_TX_BUFFER_SIZE 416U
 
+// It is not practical to implement changes to comply with misra, because of initialization of buffers
+// cppcheck-suppress-begin misra-c2012-8.4
 #ifdef STM32H7
 // ITCM RAM and DTCM RAM are the fastest for Cortex-M7 core access
 __attribute__((section(".axisram"))) can_buffer(rx_q, CAN_RX_BUFFER_SIZE)
@@ -68,8 +70,11 @@ can_buffer(tx2_q, CAN_TX_BUFFER_SIZE)
 #endif
 can_buffer(tx3_q, CAN_TX_BUFFER_SIZE)
 can_buffer(txgmlan_q, GMLAN_TX_BUFFER_SIZE)
+// cppcheck-suppress-end misra-c2012-8.4
+
 // FIXME:
 // cppcheck-suppress misra-c2012-9.3
+// cppcheck-suppress misra-c2012-8.4
 can_ring *can_queues[] = {&can_tx1_q, &can_tx2_q, &can_tx3_q, &can_txgmlan_q};
 
 // helpers
@@ -167,7 +172,7 @@ void can_clear(can_ring *q) {
 
 // Helpers
 // Panda:       Bus 0=CAN1   Bus 1=CAN2   Bus 2=CAN3
-bus_config_t bus_config[] = {
+static bus_config_t bus_config[] = {
   { .bus_lookup = 0U, .can_num_lookup = 0U, .forwarding_bus = -1, .can_speed = 5000U, .can_data_speed = 20000U, .canfd_enabled = false, .brs_enabled = false, .canfd_non_iso = false },
   { .bus_lookup = 1U, .can_num_lookup = 1U, .forwarding_bus = -1, .can_speed = 5000U, .can_data_speed = 20000U, .canfd_enabled = false, .brs_enabled = false, .canfd_non_iso = false },
   { .bus_lookup = 2U, .can_num_lookup = 2U, .forwarding_bus = -1, .can_speed = 5000U, .can_data_speed = 20000U, .canfd_enabled = false, .brs_enabled = false, .canfd_non_iso = false },

diff --git a/board/drivers/fake_siren.h b/board/drivers/fake_siren.h
@@ -2,11 +2,6 @@
 
 #define CODEC_I2C_ADDR 0x10
 
-// 1Vpp sine wave with 1V offset
-const uint8_t fake_siren_lut[360] = { 134U, 135U, 137U, 138U, 139U, 140U, 141U, 143U, 144U, 145U, 146U, 148U, 149U, 150U, 151U, 152U, 154U, 155U, 156U, 157U, 158U, 159U, 160U, 162U, 163U, 164U, 165U, 166U, 167U, 168U, 169U, 170U, 171U, 172U, 174U, 175U, 176U, 177U, 177U, 178U, 179U, 180U, 181U, 182U, 183U, 184U, 185U, 186U, 186U, 187U, 188U, 189U, 190U, 190U, 191U, 192U, 193U, 193U, 194U, 195U, 195U, 196U, 196U, 197U, 197U, 198U, 199U, 199U, 199U, 200U, 200U, 201U, 201U, 202U, 202U, 202U, 203U, 203U, 203U, 203U, 204U, 204U, 204U, 204U, 204U, 204U, 204U, 205U, 205U, 205U, 205U, 205U, 205U, 205U, 204U, 204U, 204U, 204U, 204U, 204U, 204U, 203U, 203U, 203U, 203U, 202U, 202U, 202U, 201U, 201U, 200U, 200U, 199U, 199U, 199U, 198U, 197U, 197U, 196U, 196U, 195U, 195U, 194U, 193U, 193U, 192U, 191U, 190U, 190U, 189U, 188U, 187U, 186U, 186U, 185U, 184U, 183U, 182U, 181U, 180U, 179U, 178U, 177U, 177U, 176U, 175U, 174U, 172U, 171U, 170U, 169U, 168U, 167U, 166U, 165U, 164U, 163U, 162U, 160U, 159U, 158U, 157U, 156U, 155U, 154U, 152U, 151U, 150U, 149U, 148U, 146U, 145U, 144U, 143U, 141U, 140U, 139U, 138U, 137U, 135U, 134U, 133U, 132U, 130U, 129U, 128U, 127U, 125U, 124U, 123U, 122U, 121U, 119U, 118U, 117U, 116U, 115U, 113U, 112U, 111U, 110U, 109U, 108U, 106U, 105U, 104U, 103U, 102U, 101U, 100U, 99U, 98U, 97U, 96U, 95U, 94U, 93U, 92U, 91U, 90U, 89U, 88U, 87U, 86U, 85U, 84U, 83U, 82U, 82U, 81U, 80U, 79U, 78U, 78U, 77U, 76U, 76U, 75U, 74U, 74U, 73U, 72U, 72U, 71U, 71U, 70U, 70U, 69U, 69U, 68U, 68U, 67U, 67U, 67U, 66U, 66U, 66U, 65U, 65U, 65U, 65U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 63U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 65U, 65U, 65U, 65U, 66U, 66U, 66U, 67U, 67U, 67U, 68U, 68U, 69U, 69U, 70U, 70U, 71U, 71U, 72U, 72U, 73U, 74U, 74U, 75U, 76U, 76U, 77U, 78U, 78U, 79U, 80U, 81U, 82U, 82U, 83U, 84U, 85U, 86U, 87U, 88U, 89U, 90U, 91U, 92U, 93U, 94U, 95U, 96U, 97U, 98U, 99U, 100U, 101U, 102U, 103U, 104U, 105U, 106U, 108U, 109U, 110U, 111U, 112U, 113U, 115U, 116U, 117U, 118U, 119U, 121U, 122U, 123U, 124U, 125U, 127U, 128U, 129U, 130U, 132U, 133U };
-
-bool fake_siren_enabled = false;
-
 void fake_siren_codec_enable(bool enabled) {
   if (enabled) {
     bool success = true;
@@ -33,6 +28,7 @@ void fake_siren_codec_enable(bool enabled) {
 
 
 void fake_siren_set(bool enabled) {
+  static bool fake_siren_enabled = false;
   if (enabled != fake_siren_enabled) {
     fake_siren_codec_enable(enabled);
   }
@@ -46,6 +42,9 @@ void fake_siren_set(bool enabled) {
 }
 
 void fake_siren_init(void) {
+  // 1Vpp sine wave with 1V offset
+  const uint8_t fake_siren_lut[360] = { 134U, 135U, 137U, 138U, 139U, 140U, 141U, 143U, 144U, 145U, 146U, 148U, 149U, 150U, 151U, 152U, 154U, 155U, 156U, 157U, 158U, 159U, 160U, 162U, 163U, 164U, 165U, 166U, 167U, 168U, 169U, 170U, 171U, 172U, 174U, 175U, 176U, 177U, 177U, 178U, 179U, 180U, 181U, 182U, 183U, 184U, 185U, 186U, 186U, 187U, 188U, 189U, 190U, 190U, 191U, 192U, 193U, 193U, 194U, 195U, 195U, 196U, 196U, 197U, 197U, 198U, 199U, 199U, 199U, 200U, 200U, 201U, 201U, 202U, 202U, 202U, 203U, 203U, 203U, 203U, 204U, 204U, 204U, 204U, 204U, 204U, 204U, 205U, 205U, 205U, 205U, 205U, 205U, 205U, 204U, 204U, 204U, 204U, 204U, 204U, 204U, 203U, 203U, 203U, 203U, 202U, 202U, 202U, 201U, 201U, 200U, 200U, 199U, 199U, 199U, 198U, 197U, 197U, 196U, 196U, 195U, 195U, 194U, 193U, 193U, 192U, 191U, 190U, 190U, 189U, 188U, 187U, 186U, 186U, 185U, 184U, 183U, 182U, 181U, 180U, 179U, 178U, 177U, 177U, 176U, 175U, 174U, 172U, 171U, 170U, 169U, 168U, 167U, 166U, 165U, 164U, 163U, 162U, 160U, 159U, 158U, 157U, 156U, 155U, 154U, 152U, 151U, 150U, 149U, 148U, 146U, 145U, 144U, 143U, 141U, 140U, 139U, 138U, 137U, 135U, 134U, 133U, 132U, 130U, 129U, 128U, 127U, 125U, 124U, 123U, 122U, 121U, 119U, 118U, 117U, 116U, 115U, 113U, 112U, 111U, 110U, 109U, 108U, 106U, 105U, 104U, 103U, 102U, 101U, 100U, 99U, 98U, 97U, 96U, 95U, 94U, 93U, 92U, 91U, 90U, 89U, 88U, 87U, 86U, 85U, 84U, 83U, 82U, 82U, 81U, 80U, 79U, 78U, 78U, 77U, 76U, 76U, 75U, 74U, 74U, 73U, 72U, 72U, 71U, 71U, 70U, 70U, 69U, 69U, 68U, 68U, 67U, 67U, 67U, 66U, 66U, 66U, 65U, 65U, 65U, 65U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 63U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 64U, 65U, 65U, 65U, 65U, 66U, 66U, 66U, 67U, 67U, 67U, 68U, 68U, 69U, 69U, 70U, 70U, 71U, 71U, 72U, 72U, 73U, 74U, 74U, 75U, 76U, 76U, 77U, 78U, 78U, 79U, 80U, 81U, 82U, 82U, 83U, 84U, 85U, 86U, 87U, 88U, 89U, 90U, 91U, 92U, 93U, 94U, 95U, 96U, 97U, 98U, 99U, 100U, 101U, 102U, 103U, 104U, 105U, 106U, 108U, 109U, 110U, 111U, 112U, 113U, 115U, 116U, 117U, 118U, 119U, 121U, 122U, 123U, 124U, 125U, 127U, 128U, 129U, 130U, 132U, 133U };
+
   // Init DAC
   register_set(&DAC1->MCR, 0U, 0xFFFFFFFFU);
   register_set(&DAC1->CR, DAC_CR_TEN1 | (6U << DAC_CR_TSEL1_Pos) | DAC_CR_DMAEN1, 0xFFFFFFFFU);

diff --git a/board/drivers/fan.h b/board/drivers/fan.h
@@ -8,14 +8,12 @@ struct fan_state_t {
   uint8_t stall_threshold;
   uint8_t total_stall_count;
   uint8_t cooldown_counter;
-} fan_state_t;
-struct fan_state_t fan_state;
+};
+static struct fan_state_t fan_state;
 
-const float FAN_I = 0.001f;
 
-const uint8_t FAN_TICK_FREQ = 8U;
-const uint8_t FAN_STALL_THRESHOLD_MIN = 3U;
-const uint8_t FAN_STALL_THRESHOLD_MAX = 8U;
+static const uint8_t FAN_TICK_FREQ = 8U;
+static const uint8_t FAN_STALL_THRESHOLD_MIN = 3U;
 
 
 void fan_set_power(uint8_t percentage) {
@@ -31,6 +29,8 @@ void fan_init(void) {
 
 // Call this at FAN_TICK_FREQ
 void fan_tick(void) {
+  const float FAN_I = 0.001f;
+  const uint8_t FAN_STALL_THRESHOLD_MAX = 8U;
   if (current_board->fan_max_rpm > 0U) {
     // Measure fan RPM
     uint16_t fan_rpm_fast = fan_state.tach_counter * (60U * FAN_TICK_FREQ / 4U);   // 4 interrupts per rotation