Telemetry protocol

[Mefl]

Telemetry protocol

[ThomasL01]

Hello, I have started a PR for this topic #141
Feel free to comment and propose solution and idea.

[Mefl]

The data snapshot (sent each 1/100s)

Data is composed with 9 fields, each field is written in binary to compress the data flow (as these exchange will not be used to be human readable).

• D: to start the line

• The MCU serial ID, to identify the machine (96bits)
  it can be retrieved at *(uint32_t*)(UID_BASE) :
  Serial.println("UID [HEX] : "+String(*(uint32_t*)(UID_BASE), HEX)+" "+String(*(uint32_t*)(UID_BASE+0x04), HEX)+" "+String(*(uint32_t*)(UID_BASE+0x08), HEX));

• the Systick of the STM32 (64bits)

• centile number (16bits)

• current pressure (16bits)

• phase and sub-phase number encoded as it :

• Inspiration - inspi = 17
• Inspiration - hold = 18
• Expiration - expi = 68
• Expiration - hold = 72
  So the phase is coded on the 4 high bits, and the subphase is coded on least 4 bits

• blower valve position (8 bits)

• patient valva position (8 bits)

• blower rpm (8 bits)

• battery level (8 bits)

all theses fields are separed by a tab char.

[Mefl]

The machine state snapshot, sent at the beginning of a cycle.
This message summarize the machine state and the respiration metric of the last cycle (called previous cycle)

• S: to start the line

• The MCU serial ID, to identify the machine (96bits)

• the cycle number (32 bits)

• consigne plateau (8 bits)

• consigne Peep (8 bits)

• consigne Breath Rate per minute (8 bits)

• consigne Patient weight (or volume) (8 bits)

• previous peak pressure (8 bits)

• previous plateau pressure (8 bits)

• previous peep pressure (8 bits)

• current alarms (the list of the alarms triggered during the previous cycle)

• previous alarms (the list of the alarms wich were existing at the beginning of the previous cycle)

• EOL mark (since the alarms list can be empty, this mark is used to define the real end of the message)

[Mefl]

alarm Trap

• T: to start the line

• The MCU serial ID, to identify the machine (96bits)

• the Systick of the STM32 (64bits)

• centile number (16bits)

• current pressure (16bits)

• phase and sub-phase number

• the cycle number (32 bits)

• the alarm code

• the alarm level

• triggered / untriggered toggle encoded as it :

if triggered => the 4 high bits are HIGH and the 4 least are LOW (11110000)
if suppression msg => the 4 high bits are LOW and the 4 least are HIGH (00001111)

• wanted value

• measured value

• duration of the alarm in cycle count (0 for trigger message)

[fpistm]

• The MCU serial ID, to identify the machine (96bits)
  it can be retrieved at *(uint32_t*)(UID_BASE) :
  Serial.println("UID [HEX] : "+String(*(uint32_t*)(UID_BASE), HEX)+" "+String(*(uint32_t*)(UID_BASE+0x04), HEX)+" "+String(*(uint32_t*)(UID_BASE+0x08), HEX));

FYI, one other way could be to use LL:

#include "stm32yyxx_ll_utils.h"
...
  Serial.printf("%x%x%x\n", LL_GetUID_Word0(), LL_GetUID_Word1(), LL_GetUID_Word2());

[Mefl]

all the pressure values are in mmH2O, so the typical plateau is around 300, and the peep around 80

[pi-r-p]

A quick look at UART performance, with HardwareSerial lib :

• There is no DMA implemented
• There is no hardware FIFO (because the CPU is DMA capable...)
• There is a 64 bytes software FIFO. It means an interrupt is triggered after each byte confirmed sent to push the next byte into TX register. Todo : look at this interrupt priority...

At 115200 baud:

• Transmission of 1 to 62 bytes take 50µs CPU time when calling Serial6.print (constant time)
• If you want to send X bytes with X greater than 62 bytes, calling Serial6.print takes 50µs+(X-62)*95µs (mean, see measures)

Measures at 115200 bauds:

byte count	blocking time	time on tx line
1	50 µs
62	50µs
128	6.1ms	12.1 ms
256	18.2ms	24.2ms

Effective max bandwith with the interrupt overhead is 10.5 ko/s (without any other preemptive interrupts)

[fpistm]

Buffer size can be increased:
https://github.com/stm32duino/Arduino_Core_STM32/blob/61b14ecb4ce640bbb37b733791b51aa28d906e56/cores/arduino/HardwareSerial.h#L43-L48

IRQ prio is 1 so not advised to increase to 0 as 0 is used for the systick :
https://github.com/stm32duino/Arduino_Core_STM32/blob/61b14ecb4ce640bbb37b733791b51aa28d906e56/cores/arduino/stm32/uart.h#L53

[pi-r-p]

Not easily... I tried #define SERIAL_TX_BUFFER_SIZE 200 before including anything else, but no way to compile in the order I want.

[fpistm]

Use this:
https://github.com/stm32duino/wiki/wiki/Customize-build-options-using-build_opt.h

[dsferruzza]

@fpistm How does it work with Arduino CLI?

It seems I already have build.xSerial=-DHAL_UART_MODULE_ENABLED in my Arduino15/packages/STM32/hardware/stm32/1.8.0/platform.txt file. If I want to add another option like -DSERIAL_RX_BUFFER_SIZE, where should I put it?

[fpistm]

Add a file named build_opt.h in your 'sketch' folder, which is (if I'm not wrong) : src/software/firmware/srcs

Then in this file add -DSERIAL_TX_BUFFER_SIZE=256
Note that I used a "power of 2" buffer size which is recommended for ease modulo operation.

[fpistm]

I've tested and redefinition works as expected.
I've made a mistake in my previous post to the content, I've forgot an '=', I've edited my comment.

If you need more support I'm on slack, do not hesitate 😉

[pi-r-p]

Tested, it works. Nice.
That's kind of weird to have a header file in the srcs directory... But it is working. Don't forget to rm -rf /tmp/arduino-* before compiling again.

[fpistm]

I know .h is not a good extension but Arduino IDE could only create file with .h, .hpp, .c, .cpp and .ino extension. That's why .h is used.
For the clean you could use arduino-cli cache clean before the first build with a build_opt.h file

[valeriansaliou]

As a note discussed w/ @dsferruzza this morning: we should go for a pure-binary protocol, where the receiver parses the packets by simple binary array dereferencing. The first 8 bits should contain the protocol version (eg. V1 as 00000000, V2 as 00000001, etc.). This way the receiver can handle version changes smoothly and support multiple firmware / board versions.

[nimamoslemy]

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