Harden standalone mode watchdog and SD-card mount lifecycle

src/main.c

@@ -1,11 +1,12 @@
 #include "registers/system/system.h"
 #include "states.h"
+#include "sdcard/sdcard.h"
 
 int main(void) {
 
     SYSTEM_Initialize();
 
-    state_t current_state = STATE_STANDBY;
+    state_t current_state = SDCARD_standalone_check() == SUCCESS ? STATE_STANDALONE : STATE_STANDBY;
 
     while (1) {
         current_state = STATES_RunState(current_state);

src/sdcard/sdcard.c

@@ -1,6 +1,7 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <string.h>
 
 #include "../bus/uart/uart.h"
 #include "../commands.h"
@@ -9,13 +10,19 @@
 #include "fatfs/ff.h"
 #include "fatfs/ffconf.h"
 #include "sd_spi.h"
+#include "sdcard.h"
 
 #define SFN_MAX 8
 #define SFN_SUFFIX_LEN 4 // Period + at most three chars.
 #define FILENAME_SIZE (SFN_MAX + SFN_SUFFIX_LEN + 1)
 #define BUF_MAX FF_MIN_SS
 #define SDCARD_DRIVE "0:"
+#define CONFIG_FILENAME "PSLAB.CFG"
+#define STANDALONE_MAGIC_LEN 5
 
+// Standalone-mode mount state and volume object.
+static bool s_mounted = false;
+static FATFS s_standalone_drive;
 static TCHAR s_sector_buf[BUF_MAX];
 _Static_assert(
     sizeof(s_sector_buf) == 512,
@@ -141,3 +148,50 @@ response_t SDCARD_get_file_info(void)
 
     return SUCCESS;
 }
+
+/**
+ * @brief Read the configuration file from the SD card.
+ * @return true if the config file was read successfully, false otherwise or if the config structure is invalid.
+ */
+static bool read_config_file(void) {
+    FIL file;
+    if (f_open(&file, CONFIG_FILENAME, FA_READ) != FR_OK) {
+        return false;
+    }
+
+    uint8_t buf[STANDALONE_MAGIC_LEN] = {0};
+    UINT bytes_read = 0;
+    if (f_read(&file, buf, STANDALONE_MAGIC_LEN, &bytes_read) != FR_OK
+        || bytes_read != STANDALONE_MAGIC_LEN) {
+        f_close(&file);
+        return false;
+    }
+    f_close(&file);
+
+    return memcmp(buf, "PSLAB", STANDALONE_MAGIC_LEN) == 0;
+}
+
+response_t SDCARD_standalone_check(void) {
+    if (!SD_SPI_IsMediaPresent()) {
+        return FAILED;
+    }
+
+    if (f_mount(&s_standalone_drive, SDCARD_DRIVE, 1) != FR_OK) {
+        return FAILED;
+    }
+    s_mounted = true;
+
+    if (!read_config_file()) {
+        SDCARD_standalone_unmount();
+        return FAILED;
+    }
+
+    return SUCCESS;
+}
+
+void SDCARD_standalone_unmount(void) {
+    if (s_mounted) {
+        f_mount(NULL, SDCARD_DRIVE, 0);
+        s_mounted = false;
+    }
+}

src/sdcard/sdcard.h

@@ -97,4 +97,20 @@ response_t SDCARD_read_file(void);
  */
 response_t SDCARD_get_file_info(void);
 
+/**
+ * @brief Check if SD-card is present and can be mounted.
+ * 
+ * @details Checks if the SD-card can be mounted and if the configuration file is present and valid.
+ *          The configuration file is expected to be named "PSLAB.CFG" and
+ *          contain the 5-byte magic header "PSLAB".
+ * 
+ * @return SUCCESS or FAILED
+ */
+response_t SDCARD_standalone_check(void);
+
+/**
+ * @brief Unmount standalone SD-card volume if mounted.
+ */
+void SDCARD_standalone_unmount(void);
+
 #endif // _SDCARD_H

src/states.c

@@ -36,9 +36,23 @@ state_t RunCommand(void) {
     return STATE_STANDBY;
 }
 
+/**
+ * @brief Run in standalone mode until serial traffic is detected.
+ * @return STATE_RUNCOMMAND if serial traffic is detected, otherwise STATE_STANDALONE.
+ */
+state_t Standalone(void) {
+    if (UART_IsRxReady(U1SELECT)) {
+        return STATE_RUNCOMMAND;
+    }
+
+    WATCHDOG_TimerClear();
+    return STATE_STANDALONE;
+}
+
 state_func_t* const state_table[NUM_STATES] = {
     Standby,
     RunCommand,
+    Standalone,
 };
 
 state_t STATES_RunState(state_t current_state) {

src/states.h

@@ -16,6 +16,7 @@ extern "C" {
     typedef enum {
         STATE_STANDBY,
         STATE_RUNCOMMAND,
+        STATE_STANDALONE,
         NUM_STATES
     } state_t;