An In-Depth Look Into Microcontrollers

Integrated Computer Solutions Inc. www.ics.com 1

Integrated Computer Solutions Inc. www.ics.com
About ICS
● Founded in 1987
● Largest source of independent Qt expertise in North America
● Trusted Qt Service Partner since 2002
● Exclusive Open Enrollment Training Partner in North America
● Provides integrated custom software development and user experience (UX) design
● Embedded, touchscreen, mobile and desktop applications
● HQ in Waltham, MA with offices in California, Canada, Europe
Boston UX
● Part of the ICS family, focusing on UX design
● Designs intuitive touchscreen interfaces for high-impact embedded and
connected medical, industrial and consumer devices

What we will cover
3
Overview of MCUs and their features
● Current scope of MCU-based applications
● Convergence of microcontrollers and microprocessors
● Peripherals available and protocols used in today´s projects
Discussion on programming approaches
● Pros and cons of programming bare-metal vs RTOS
● Running Python scripts on MCUs
Challenges in firmware implementation (with example code)
● Application fragments implemented with the ubiquitous ESP32 MCU
● Network-enabled demo that connects to a Mender server and retrieves over-the-air
updates

Current scope of MCU-based applications
4
● Reduce the cost of the project
○ Hardware costs lower than MPU-based (less peripherals)
○ Firmware development costs equal or higher (more tools available on
embedded Linux, for example Qt, imply less time for implementation)
○ => Bottom line will depend on the expected size of production.
● Solve specific tasks within a system containing other controllers
○ Split the application into smaller subtasks handled by several MCUs
○ Tough timing requirements (high speed protocols)

Convergence of MCU and MPU ﬁelds
5
● Traditional approach
○ MPUs = external storage and runtime memory
○ MCUs = single chip processors
○ MCUs ran customized firmware on devices, MPUs ran OS on desktops with
general-purpose applications
● Today: blurring boundaries
○ Improvements in levels of integration
○ Widespread growth of Linux for devices, i.e. ‘non-computers’
○ Reduced MCU development time.
■ Versatile hardware configuration options
■ Small OS available (mostly FreeRTOS)
■ Hardware abstraction layers (STM32´s CubeMx, for example)
■ Large ecosystems of developers bring many open source code

STM32CubeMX: MCU setup with auto-code generation

Today´s peripherals
7
● MCUs embed as many peripherals as possible inside the device, in line with the
targeted market: automotive, consumer goods with low power needs, etc.
○ Timers, counters, Pulse Width Modulation (PWM)
○ Serial ports (USART, SPI, I2
C, CAN, Ethernet, …)
○ ADCs & DACs (analog/digital converters)
○ Radiofrequency transceivers (WiFi, Bluetooth, …)
○ Cryptographic accelerators
● External peripherals:
○ Inertial Measurement Units: accelerometers, gyroscopes, magnetometers
○ Weather sensors: temperature, humidity, pressure
○ Other RF transceivers (Zigbee, 5G, LoRa…)
○ …

SPI I2
C
8
Serial Peripheral Interface
● Full-duplex
● Originally was 1:1, then extended
to 1:N
Inter-Integrated Circuit
● Bus topology
● Addressing mechanism needed,
peripheral specific

CAN
9
Controller Area Network
● Bus topology
● No device ID, but message ID
● Frames
○ Data
○ Remote
○ Error
○ Overload
● Bitwise arbitration
● Widespread usage in automobile and
industrial automation
● Higher layers (CANOpen)

Bare-metal development
10
● Firmware duties:
○ interaction with peripherals/hardware resources (lower level)
○ business logic (higher level)
○ The glue needed to accomplish with the business logic also needs to
provide concurrency
● On bare-metal programming, the operating system is “us” (developers)
○ Concurrency implemented as a round-robin of “never-blocking” functions..

Bare-metal example
11
void main_loop() {
// ….
while (1) {
// …
slow_status = aSlowTask ( slow_params,
slow_result);
fast_status = aFastTask ( fast_params,
fast_result);
do_business_logic ( slow_status,
slow_params,
slow_result,
fast_status,
fast_params,
fast_result );
}
}

Example (continued)
12
char neverBLockingTask (char *params, char *result) {
switch (stateMachine) {
//…
case WAIT_ANSWER:
if ( timedOut(&timer) ) {
stateMachine = HANDLE_TIMEOUT;
result = ANSWER_TIMEOUT;
} else {
receiveData( rxBuff + buffLen, *buffLen);
if (*buff_len==0) {
result = ANSWER_PENDING;
} else {
if ( ! fullAnswerReceived( *rxBuff, uint16_t *buffLen) ) {
result = ANSWER_PENDING;
} else {
stateMachine = IDLE;
result = ANSWER_RECEIVED;
}
}
}
break;
// …
} // switch
return result;
}

Running Python scripts on MCUs
13
● Traditionally, MCUs languages are C and C++
○ compilers, debuggers
● Python popularity
○ Huge community
○ Sintax simplicity
○ Excellent libraries
○ Good for data science/machine learning
○ Zoomers (Generation Z) prefer .py rather than .c
● Micropython: a small Python 3 interpreter and runtime for MCUs (bare-metal)
○ Open Source, MIT License
○ Footprint: 256 kb flash, 16 kb RAM
○ Not as performant as C (Python runtime needs to parse the script)

Micropython example
14
from machine import SPI, Pin
spi = SPI(0, baudrate=400000)
cs = Pin(4, mode=Pin.OUT, value=1)
txdata = b"12345678"
rxdata = bytearray(len(txdata))
try:
cs(0)
spi.write_readinto(txdata, rxdata)
finally:
cs(1)

FreeRTOS
15
● Small kernel, it provides concurrency, with a small computing overhead
○ Open source and lightweight: footprint is around 4Kbytes
○ Concurrency implemented through threads.
○ Inter-task communication
■ Queues
■ Semaphores
■ Mutexes
https://www.freertos.org/Inter-Task-Communication.html

FreeRTOS example
16
Two tasks communicating through queues
#include <stdio.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "freertos/queue.h"
QueueHandle_t q=NULL;
void app_main()
{
q=xQueueCreate(20,sizeof(unsigned long));
if (q != NULL) {
vTaskDelay(1000/portTICK_PERIOD_MS); // one second delay
xTaskCreate(&producer_task,"producer_task",2048,NULL,5,NULL);
xTaskCreate(&consumer_task,"consumer_task",2048,NULL,5,NULL);
} else {
printf("Queue creation failed");
}
}

FreeRTOS example (cont)
17
void producer_task(void *pvParameter)
{
unsigned long counter=1;
BaseType_t resultQueue;
while(1) {
resultQueue = xQueueSend( q,
&counter,
(TickType_t )0);
if (pdTrue == resultQueue) {
printf("sent to the queue: %lun",counter);
counter++;
}
vTaskDelay(1000/portTICK_PERIOD_MS);
}
}
void consumer_task(void *pvParameter)
{
unsigned long counter;
BaseType_t resultQueue;
while(1) {
resultQueue = xQueueReceive( q,
&counter,
(TickType_t ) 0);
if (pdTrue == resultQueue) {
printf("received from the queue: %lu n",counter);
}
vTaskDelay(20/portTICK_PERIOD_MS);
}
}

About ESP32
18

ESP32 ﬂash management
19
● Bootloader
● Flexible application partition options:
○ flat
○ ota_0/ota_1 (a.k.a. A/B partition)
● Optional user data, called ‘storage’ (can be FAT, or plain)
# Name, Type, SubType, Offset, Size, Flags
nvs, data, nvs, 0x9000, 0x4000,
otadata, data, ota, 0xd000, 0x2000
phy_init, data, phy, 0xf000, 0x1000,
ota_0, app, ota_0, 0x10000, 0x140000,
ota_1, app, ota_1, , 0x140000,
storage, data, fat, , 0x140000,

A microcontroller challenge…
20
Could a third-party Over-The-Air updates solution like Mender
be implemented on a ESP32 module?
● Mender originally targeted for Linux cards, not MCUs…
● Enough ﬂash space?

Updating capabilities
21

Web management console
22

ESP32: ﬂash partitioning trade-offs…
23
● with the storage partition
○ flash space divided by 3
○ full download before updating
○ uncompress after download feasible
# Name, Type, SubType, Offset, Size,
nvs, data, nvs, 0x9000, 0x4000,
ota_0, app, ota_0, 0x10000, 0x140000,
ota_1, app, ota_1, , 0x140000,
storage, data, fat, , 0x140000,
● without the storage partition
○ flash space divided by 2
○ partial downloads, update by chunks
○ needs uncompressed artifacts
# Name, Type, SubType, Offset, Size,
nvs, data, nvs, 0x9000, 0x4000,
ota_0, app, ota_0, 0x10000, 0x1F8000,
ota_1, app, ota_1, , 0x1F8000,

Understanding a 3rd party ﬁle format
24
● Based on tar format
○ Header
○ One or more files
packed, including
version, payload type,
checksum.
● Mender utility for artifact preparation
● Compression options (gzip, lzma or none)

Moving from challenge into action
25
● #1 receive firmware by chunks
○ firmware footprint around 2 Mbytes
○ Receive buffer size tradeoffs
● #2 parse a tar file on the fly
○ At proper offset, obtain the firmware file size
○ At proper offset, get fragments of the firmware file as they
arrive.

Receive ﬁrmware by chunks
26
● Use HTTP Range header
● Tested with cURL first, then implemented on the ESP32 HTTP
API
GET /api/devices/v1/deployments/device/deployments/next?artifact_name=fw_v1_0.mender&device_type=ESP32 HTTP/1.1
Host: docker.mender.io
Range: bytes=0-1023
HTTP/1.1 206 Partial Content
Content-Range: bytes 0-1023/1246515
Content-Length: 1024
...
(binary content)

Receive ﬁrmware by chunks (cont.)
27
if (body_length>0)
{
offset=0;
esp_http_client_handle_t client = esp_http_client_init(&config);
to=-1;
while (1)
{
from=to+1;
if (from>=body_length-1)
{ /* Reached the end of GET answer, run latest OTA API call , for latest bootloader setup */
esp_ota_end(update_handle);
esp_ota_set_boot_partition(update_partition);
result=MENDER_DOWNLOAD_OK;
break;
}
to=from+MENDER_RESPONSE_BUFFER-1;
if (to>body_length-1)
{
to=body_length-1;
}
sprintf (range_string,"bytes=%d-%d", from, to);
esp_http_client_set_header(client, "Range", range_string);
esp_http_client_perform(client);
content_length = esp_http_client_get_content_length(client);
artifact_processBlock( client->response_buffer, ...);
// …
}
}

Parse a tarball by chunks
28
● Understand the tarball definition
● Understand artifacts structure (tar inside a tar)
● Implement the stream parser.

Parse a tarball by chunks
29
int artifact_processBlock ( char *inbuff, size_t inbuff_length, size_t *offset, char *outbuff, size_t *outbuff_length, size_t outbuff_size)
{
//….
if (*offset + inbuff_length <= FILESIZE_OFFSET + FILESIZE_LENGTH) { // start of file size reached, end of file size not reached
if (*offset< FILESIZE_OFFSET) { // start of buffer before start of file size
size_t amount = inbuff_length-(FILESIZE_OFFSET-*offset);
memcpy (filesizeBuffer+filesizeBufferLength, inbuff+(FILESIZE_OFFSET-*offset), amount);
filesizeBufferLength += amount;
} else { // start of buffer after start of file size
size_t amount = inbuff_length;
memcpy (filesizeBuffer+filesizeBufferLength, inbuff, amount);
filesizeBufferLength += amount;
}
if (FILESIZE_LENGTH == filesizeBufferLength) {
convertBufferToFileSize (filesizeBuffer, &fileSize);
fileSizeValid = true;
}
*offset += inbuff_length;
result = 0;
goto exit_func;
}
// …..

Security aspects
30
● Open source version:
○ Transport Secure Layer protocol, TLS
○ Self signed or Certificate Authority
○ Provisioning @ factory plant
● Paid plans
○ Same as above, plus mutual TLS authentication

Conclusions
● Big MCU/MPUs ecosystems raise the bar for more complex projects
● Modules integrating MCU and radio chips provide quick yet powerful resource
for IoT.
● Cybersecurity concerns for IoT devices.
● Peripherals integrated to the MCU processor. Serial links (SPI, I2C, CAN,
USART) for off-chip peripherals,
● FreeRTOS and similar small footprint simplifies implementation on MCUs

All images were taken from STM32, Mender’s online documentation.
Questions?
gstola@ics.com

An In-Depth Look Into Microcontrollers

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