This tutorial explains how to create a Flash-optimized key-value store using the Flash memory of the Portenta H7. It builds on top of the Flash In-Application Programming tutorial.
In this tutorial you will learn how to use the Mbed OS TDBStore API to create a Key value store in the free space of the microcontroller's internal Flash.
The core software of Portenta H7 is based on the Mbed OS operating system, allowing developers to integrate the Arduino API with the APIs exposed by Mbed OS.
Mbed OS has a rich API for managing storage on different mediums, ranging from the small internal Flash memory of a microcontroller to external SecureDigital cards with large data storage space.
In this tutorial, you are going to save a value persistently inside the Flash memory. That allows to access that value even after a reset of the microcontroller. You will retrieve some information from a Flash block by using the FlashIAPBlockDevice and the TDBStore APIs. You will use the
FlashIAPBlockDevice
class to create a block device on the free space of the Flash and you will create a Key-Value Store in it using the TDBStore
API.Important: The TBStore API optimizes for access speed, reduce wearing of the flash and minimize storage overhead. TBStore is also resilient to power failures. If you want to use the Flash memory of the microcontroller, always prefer the TDBStore approach over a direct access to the FlashIAP block device.
Begin by plugging in your Portenta board to the computer using a USB-CĀ® cable and open the Arduino IDE. If this is your first time running Arduino sketch files on the board, we suggest you check out how to set up the Portenta H7 for Arduino before you proceed.
Let's program the Portenta with a sketch. You will also define a few helper functions in a supporting header file.
FlashKeyValue.ino
FlashIAPLimits.h
to store the helper functions in a reusable file.Note: Finished sketch its inside the tutorials library wrapper at:
Examples > Arduino_Pro_Tutorials > Creating a Flash-Optimized Key-Value Store > FlashKeyValueStore
First let's add the helper functions to the
FlashIAPLimits.h
header. This will determine the available Flash limits to allocate the custom data.1/**2Helper functions for calculating FlashIAP block device limits3**/4
5// Ensures that this file is only included once6#pragma once 7
8#include <Arduino.h>9#include <FlashIAP.h>10#include <FlashIAPBlockDevice.h>11
12using namespace mbed;13
14// A helper struct for FlashIAP limits15struct FlashIAPLimits {16 size_t flash_size;17 uint32_t start_address;18 uint32_t available_size;19};20
21// Get the actual start address and available size for the FlashIAP Block Device22// considering the space already occupied by the sketch (firmware).23FlashIAPLimits getFlashIAPLimits()24{25 // Alignment lambdas26 auto align_down = [](uint64_t val, uint64_t size) {27 return (((val) / size)) * size;28 };29 auto align_up = [](uint32_t val, uint32_t size) {30 return (((val - 1) / size) + 1) * size;31 };32
33 size_t flash_size;34 uint32_t flash_start_address;35 uint32_t start_address;36 FlashIAP flash;37
38 auto result = flash.init();39 if (result != 0)40 return { };41
42 // Find the start of first sector after text area43 int sector_size = flash.get_sector_size(FLASHIAP_APP_ROM_END_ADDR);44 start_address = align_up(FLASHIAP_APP_ROM_END_ADDR, sector_size);45 flash_start_address = flash.get_flash_start();46 flash_size = flash.get_flash_size();47
48 result = flash.deinit();49
50 int available_size = flash_start_address + flash_size - start_address;51 if (available_size % (sector_size * 2)) {52 available_size = align_down(available_size, sector_size * 2);53 }54
55 return { flash_size, start_address, available_size };56}
Go to
FlashKeyValue.ino
and include the libraries that you need from MBED and your header helper (FlashIAPLimits.h
). The getFlashIAPLimits()
function which is defined in the FlashIAPLimits.h
header takes care of not overwriting data already stored on the Flash and aligns the start and stop addresses with the size of the Flash sector. You can use those calculated limits to create a block device and a TDBStore
on top of them.1#include <FlashIAPBlockDevice.h>2#include <TDBStore.h>3
4using namespace mbed;5
6// Get limits of the In Application Program (IAP) flash, ie. the internal MCU flash.7#include "FlashIAPLimits.h"8auto iapLimits { getFlashIAPLimits() };9
10// Create a block device on the available space of the FlashIAP11FlashIAPBlockDevice blockDevice(iapLimits.start_address, iapLimits.available_size);12
13// Create a key-value store on the Flash IAP block device14TDBStore store(&blockDevice);15
16// Dummy sketch stats data for demonstration purposes17struct SketchStats {18 uint32_t startupTime;19 uint32_t randomValue;20 uint32_t runCount;21};
In the
setup()
function at the beginning you will have to wait until the Serial port is ready and then print some info about the FlashIAP block device (blockDevice
).1void setup()2{3 Serial.begin(115200);4 while (!Serial);5
6 // Wait for terminal to come up7 delay(1000);8
9 Serial.println("FlashIAPBlockDevice + TDBStore Test");10
11 // Feed the random number generator for later content generation12 srand(micros());13
14 // Initialize the flash IAP block device and print the memory layout15 blockDevice.init(); 16 Serial.print("FlashIAP block device size: ");17 Serial.println(blockDevice.size());18 Serial.print("FlashIAP block device read size: ");19 Serial.println(blockDevice.get_read_size());20 Serial.print("FlashIAP block device program size: ");21 Serial.println(blockDevice.get_program_size());22 Serial.print("FlashIAP block device erase size: ");23 Serial.println(blockDevice.get_erase_size());24 // Deinitialize the device25 blockDevice.deinit();
After that, initialize the TDBstore (our storage space), set the key for the store data (
stats
), initialize the value that you will save runCount
and declare an object to fetch the previous values (previousStats
).1// Initialize the key-value store2 Serial.print("Initializing TDBStore: ");3 auto result = store.init();4 Serial.println(result == MBED_SUCCESS ? "OK" : "Failed");5 if (result != MBED_SUCCESS)6 while (true); // Stop the sketch if an error occurs7
8 // An example key name for the stats on the store9 const char statsKey[] { "stats" };10
11 // Keep track of the number of sketch executions12 uint32_t runCount { 0 };13
14 // Previous stats15 SketchStats previousStats;
Now that you have everything ready, let's retrieve the previous values from the store and update the store with the new values.
1// Get previous run stats from the key-value store2 Serial.println("Retrieving Sketch Stats");3 result = getSketchStats(statsKey, &previousStats);4
5 if (result == MBED_SUCCESS) {6 Serial.println("Previous Stats");7 Serial.print("\tStartup Time: ");8 Serial.println(previousStats.startupTime);9 Serial.print("\tRandom Value: ");10 Serial.println(previousStats.randomValue);11 Serial.print("\tRun Count: ");12
13 Serial.println(previousStats.runCount);14
15 runCount = previousStats.runCount;16
17 } else if (result == MBED_ERROR_ITEM_NOT_FOUND) {18 Serial.println("No previous data was found.");19 } else {20 Serial.println("Error reading from key-value store.");21 while (true);22 }23
24 // Update the stats and save them to the store25 SketchStats currentStats { millis(), rand(), ++runCount }; 26 result = setSketchStats(statsKey, currentStats);27 28 if (result == MBED_SUCCESS) {29 Serial.println("Sketch Stats updated");30 Serial.println("Current Stats");31 Serial.print("\tStartup Time: ");32 Serial.println(currentStats.startupTime);33 Serial.print("\tRandom Value: ");34 Serial.println(currentStats.randomValue);35 Serial.print("\tRun Count: ");36
37 Serial.println(currentStats.runCount);38 } else {39 Serial.println("Error while saving to key-value store");40 while (true);41 }42}
To finish the sketch, create
getSketchStats
and setSketchStats
functions at the bottom of the sketch (after the setup()
and loop()
). The
getSketchStats
function tries to retrieve the stats values stored in the Flash using the key key
and returns them via the stats
pointer parameter. Our SketchStats
data struct is very simple and has a fixed size. You can therefore deserialize the buffer with a simple memcpy
.The
setSketchStats
function stores the stats
data and assigns the key key
to it. The key will be created in the key-value store if it does not exist yet.1// Retrieve SketchStats from the key-value store2int getSketchStats(const char* key, SketchStats* stats)3{4 // Retrieve key-value info5 TDBStore::info_t info;6 auto result = store.get_info(key, &info);7
8 if (result == MBED_ERROR_ITEM_NOT_FOUND)9 return result;10
11 // Allocate space for the value12 uint8_t buffer[info.size] {};13 size_t actual_size;14
15 // Get the value16 result = store.get(key, buffer, sizeof(buffer), &actual_size);17 if (result != MBED_SUCCESS)18 return result;19
20 memcpy(stats, buffer, sizeof(SketchStats));21 return result;22}23
24// Store a SketchStats to the the k/v store25int setSketchStats(const char* key, SketchStats stats)26{27 return store.set(key, reinterpret_cast<uint8_t*>(&stats), sizeof(SketchStats), 0); 28}
Upload the sketch, the output should be similar to the following:
1FlashIAPBlockDevice + TDBStore Test2FlashIAP block device size: 15728643FlashIAP block device read size: 14FlashIAP block device program size: 325FlashIAP block device erase size: 1310726Initializing TDBStore: OK7Retrieving Sketch Stats8Previous Stats9 Startup Time: 1272710 Random Value: 151480162911 Run Count: 1312Sketch Stats updated13Current Stats14 Startup Time: 428515 Random Value: 213317002516 Run Count: 14
Note that the Flash memory will be erased when a new sketch is uploaded.
Push the reset button to restart the sketch. The values of the stats have been updated.
Previous Stats
which is retrieved from the key-value store now contains values from the previous execution.You have learned how to use the available space in the Flash memory of the microcontroller to create a key-value store and use it to retrieve and store data.
It is not recommended to use the Flash of the microcontroller as the primary storage for data-intensive applications. It is best suited for read/write operations that are performed only once in a while such as storing and retrieving application configurations or persistent parameters.
iterator_open
and iterator_next
set_start
, set_add_data
and set_finalize