Voltage Manager

Overview

Task loop:

1. Measure voltage using ADC
2. Take mutex
3. Write to global voltage variable
4. Send a task notification to the Status LED task
5. Give the mutex
6. Update task health flag to feed the watchdog

Tasks

Name	Priority	Stack Size	Periodicity/Trigger	Description
static TaskHandle_t xTaskSensor	Low	128 words	100 ms	Monitor voltage using ADC
staticTaskHandle_t xTaskAlarm	High	128 words	Event-driven (sensor update)	Control LEDs
static TaskHandle_t xTaskUART	Medium	128 words	Event-driven (sensor update)	Transmit data

• Static: limit variable’s scope to main.c file for encapsulation

Concurrency and Synchronization

Component	Description
SemaphoreHandle_t xMutex	Manage shared access for fBatteryVoltage
SemaphoreHandle_t xBinSemaphore	Wake up TaskAlarm only when there is new data
QueueHandle_t xUARTQueue	Queue to send messages to TaskUART

• Why does TaskSensor gives the semaphore before giving the mutex?
  • Prevent tasks from fighting over the mutex before they are “synced” by the semaphore
  • Give the semaphore first to let the scheduler organize the ready tasks
• Why statically allocate tasks, mutexes, and semaphores?
  • Ensures determinism, no risk of runtime memory allocation failure

Variables

Variable	Description
volatile float fBatteryVoltage	Battery voltage read by ADC
const float fThresholdVoltage	Threshold for lighting up green/red LED
volatile uint32_t currentTime	Button debouncing
volatile uint32_t prevTime	Button debouncing

• Volatile: ensure compiler doesn’t optimize read/writes

Messages

typedef enum {
    MSG_TYPE_VOLTAGE,
    MSG_TYPE_BUTTON
} MsgType_t;
 
typedef struct {
    MsgType_t type;    
    float value;       
} UARTMsg_t;

ADC - Ring Buffer

Implemented a ring buffer that saves the last 8 readings. Passes the average value to the alarm task to make it more consistent

Task Notification

• Clear the value after reading it

Checking a task notification

BaseType_t xTaskNotifyWait(
    uint32_t ulBitsToClearOnEntry, 
    uint32_t ulBitsToClearOnExit, 
    uint32_t *pulNotificationValue,
    TickType_t xTicksToWait
);

1. Enter the function xTaskNotifyWait
2. Clear the bits in the mask ulBitsToClearOnEntry
3. Check the value of the notification
   • Each task has a 32-bit value
     • Value was sent from another task and received by the task that calls xTaskNotifyWait
   • If there is a notification present (check the flag that indicates if there is a notif)
     • Read the result and store it in the pulNotificationValue
     • Clear bits in ulBitsToClearOnExit
     • Return pdTRUE
   • If there is no notification present
     • Wait for xTicksToWait until a notification arrives
     • When it arrives, go to step 3
     • If it doesn’t arrive, return pdFALSE