SENSIRION SFC6 Mass Flow Controller User Manual

: June 14, 2024
: SENSIRION

Table of Contents

SENSIRION SFC6 Mass Flow Controller
Document Summary
Hardware
Frame Definition
Protocol Definition
Protocol Timings
Data Types and Representation
Commands
Appendix
Revision History
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SENSIRION SFC6 Mass Flow Controller

SENSIRION-SFC6-Mass-Flow-Controller-product

Document Summary

This document describes the SHDLC RS485 communication interface of the Sensirion SFC6xxx mass flow controller and SFM6xxx mass flow meter. The intention of defining the SHDLC protocol was to have a single protocol for different Sensirion devices such as mass flow controllers, meters, and other microcontroller devices. The protocol is based on serial communication and is easy to implement. It allows communication between a single master and several connected slave devices.

In the following, there are some of the SHDLC protocol key features:

Based on an byte orientated hardware (like UART,. . . )
Half-duplex system, which allows to use half-duplex hardware
Single Master/Multiple Slaves protocol without the need for bus arbitration
Allows up to 255 bytes payload data for read or write transfers
Broadcasting allows to safe bus load and synchronize devices

Hardware

RS485/RS422

SENSIRION-SFC6-Mass-Flow-Controller-fig-1

As shown in the picture above, the RS485/422 setup combined with the SHLDC addressing allows to connect several SHDLC devices on the same bus. Due to the differential signals, the communication is robust against disturbances and allows long distance data transmission. For long distance data lines a termination resistor of 120 Ohm is recommended on both ends of the bus. The logic behind the protocol is based on a UART hardware, using the following settings:

Baudrate as defined in datasheet (default: 115’200 baud)
8 Data bits (LSb first)
No Parity
1 Stop Bit

Frame Definition

Every transfer in SHDLC is based on MOSI (Master Out Slave In) and MISO (Master In Slave Out) frames. In the following chapter, the composition of the frames and the meaning of the different frame parts is shown.

Note

In the following, the order of the bytes in frames is defined. The order of the bits inside a byte (MSb/LSb) is defined by the used hardware.

Type of Frames

MOSI Frame

SENSIRION-SFC6-Mass-Flow-Controller-fig-2

The MOSI frame is the message sent from master to slave as a request. Communication on the SHDLC bus is always initiated by the master. Therefore, this frame is always the begin of communication on the bus:

The begin and end of the frame is marked by start and stop bytes, which are unique bytes (0x7E) in the frame (guaran- teed by byte suffing, see Section 5.2). After the start byte, the address field defines the receiver of the frame, followed by the command which tells the slave what to do with the data. Before the payload data is sent, there is a length information (L) and after the data a checksum to guarantee the validity of the data.

MISO Frame

The MISO frame is the response from a slave device after reception of the MOSI frame. The SHDLC protocol guarantees an answer on any master request (except broadcast commands). This allows the master to ensure his request was correctly transferred and processed. The MISO frame looks as follows:

SENSIRION-SFC6-Mass-Flow-Controller-fig-3

It is similar to the MOSI frame, except the additional state byte. In this field, the slave reports any problem during command execution and additional general health information. In the address field, the slave fills in his own address and in the command field the command which was received and executed.

Start and Stop

Since SHDLC does not use hardware handshaking, we are using special bytes in the data stream to mark the begin and end of a frame:

Start: 0x7E (01111110b)
Stop: 0x7E (01111110b)

To make this work properly, we have to avoid this byte (0x7E) inside the frame. For this reason, byte stuffing is used, which means we are replacing 0x7E anywhere inside the frame with a sequence of two other bytes. This will also be done for Escape (0x7D), XON (0x11) and XOFF (0x13) bytes. The following table shows how the original data bytes are replaced in the sent data. All other data bytes are sent 1:1.

Original data byte	Transferred data bytes
0x7E	0x7D, 0x5E
0x7D	0x7D, 0x5D
0x11	0x7D, 0x31
0x13	0x7D, 0x33

Address Field

The address field in the MOSI frame (1 Byte) defines the receiver of the frame (slave device address). The address range is divided as follows:

0. . . 254: Slave addresses (All device addresses are initially set to 0x0, and can be changed with the get/set slave address command)
255: Broadcast address

In a MISO frame the address field contains the slave address (address of the slave which has sent the MISO frame).

Command

Typically (in a MOSI frame), this field contains the application command which defines for the specific application what to do with the given data. There are some reserved commands which are used for special frame transfers. In a MISO frame the slave will return the received and processed command in the CMD field. The following table shows the organization of the available command space:

Command ID

(Hex)

| Size| Usage
---|---|---
0x00 .. 0x7F| 128| Individual, device specific commands. The meaning and payload data of these commands depend on the type of device.
0x80 .. 0xEF| 112| Common commands. The meaning and payload data of these commands are identical for all device types. However, not all commands are supported by each device type, i.e. the commands are optional.
0xF0 .. 0xFF| 16| Special frame identifiers.

The size of the command is 1 byte.

State

The MISO frame contains a state byte, which allows the master to detect errors on slave side. There are two kinds of errors signalized in the state byte:

Execution Errors
Device Error Flag

The following table shows the composition of the state byte:

b7	b6	b5	b4	b3	b2	b1	b0
Device Error Flag

Execution error code

Length

The length byte defines the number of transferred bytes in the data field (Rx or Tx). It is the length of the data field before byte stuffing, not the number of bytes which are transferred (because they contain stuffed bytes).

Example: The sender wants to transmit data [0xA7, 0xB4, 0x7E, 0x24]. Because of byte stuffing, it needs to transmit the stream [0xA7, 0xB4, 0x7D, 0x5E, 0x24]. The transmitted length information in this case is 0x04.

Execution Error Code

The execution error code signalizes all errors which occured while processing the frame or executing the command. The errors are binary encoded in 7 bits with the following meaning:

Error Code	Meaning
0x00	No Error. Processing and execution of the command was successful.

0x01 . . .

0x1F

| Common error codes, which have the same meaning on every SHDLC device.
0x20 . . .

0x7F

| Error codes which are specific for every device. See appendix for definition.

Device Error Code

Device Error Flag is not used in SFx6xxx, this flag is always zero.

Data

The data has a usable size of [0. . . 255] bytes (original data, before byte stuffing). The meaning of the data content is defined in the command section.

Checksum

The checksum is built before byte stuffing and checked after removing stuffed bytes from the frame. It is calculated as follows:

Sum all bytes between start and stop (without start and stop bytes)
Take the LSB of the result and invert it. This will be the checksum.

Length

Consider this MOSI frame (without start/stop and without byte stuffing):

SENSIRION-SFC6-Mass-Flow-Controller-fig-4

The checksum calculates as follows:

ADR	0x02
CMD	0x43
L	0x04
Data 0	0x64
Data 1	0xA0
Data 2	0x22
Data 3	0xFC
Sum of all Bytes	0x26B
LSB of the Sum	0x6B
Inverted (=Checksum)	0x94

Protocol Definition

This chapter describes the frame communication protocol in a SHDLC system. There are some basic rules:

On every master request (MOSI frame), the slave will respond with a slave response (MISO frame). There are two exclusions where the slave will not send a response:
- If the checksum of a MOSI frame did not match
- If the MOSI frame was a broadcast
Between receiving a MOSI frame and sending slave response, the slave will not accept any other frame from master. In case of a broadcast, the master must wait for the specified command execution time.

Transfer Types

By default, the master sends a standard frame which contains up to 255 bytes Tx data. This is called a standard frame transfer. Additionally, there are some special frame transfers defined. They are marked with a special frame identifier in the CMD field of the frame. The following chapters describe the different transfer types.

Standard Transfer

In this transfer, the master initiates a transfer with a MOSI frame containing command ID and up to 255 bytes of Tx data. After executing the command, the slave will respond with a MISO frame containing state and up to 255 bytes of Rx data. The command field of the frame is used for an application command. The transfer looks as follows:

SENSIRION-SFC6-Mass-Flow-Controller-fig-5

Broadcast Transfer

After sending a broadcast command (ADR = 0xFF), the slave executes the command but does not send the generated response. The “Get Broadcast Response” frame allows the master to get the slave response on a previously sent broadcast command. The following shows an example with two slaves:

SENSIRION-SFC6-Mass-Flow-Controller-fig-6

If the broadcast transfer is followed by a “Get Broadcast Response” frame, the slave will send the buffered answer. If any other frame is sent, the buffered response is discarded. The frame to get the broadcast response (MOSI) looks as follows and has to be addressed to a specific slave device:

SENSIRION-SFC6-Mass-Flow-Controller-fig-7

The slave answers with the same response as on an addressed command.

Error Response

In case of a command execution error, the device will return an error response. This response is transmitted without data (L=0). That means that a simple error response looks alike for any transfer type:

SENSIRION-SFC6-Mass-Flow-Controller-fig-8

Protocol Timings

Interbyte Timeout

The interbyte time defines the time between two bytes in the same frame. After reception of a frame byte, the receiver waits for the next frame byte. This time is limited by the interbyte timeout. See the following timing diagram which defines the interbyte time:

SENSIRION-SFC6-Mass-Flow-Controller-fig-9

The interbyte timeout is set to 200ms. If a timeout occurs on slave side, the received data is discarded, and no response frame will be sent to the master.

Slave Response Timeout

SENSIRION-SFC6-Mass-Flow-Controller-fig-10

The slave response time is the time between the MOSI frame has left the master port and the begin of the reception of the MISO frame. This time is defined in the command reference.
The timeout should be at least twice the specified maximum response time of the corresponding command and in non real time systems it should not be smaller than 200ms due to possible system side delays.

Data Types and Representation

The data in the frames is transmitted in big-endian order (MSB first).

Integer

Integers can be transmitted as signed or unsigned integers. If signed, use the two’s complement. The following types of integers are known:

Integer

Type

| Size| Range
---|---|---
u8t| 1 Byte| 0 . . . 28-1
u16t| 2 Byte| 0 . . . 216-1
u32t| 4 Byte| 0 . . . 232-1
u64t| 8 Byte| 0 . . . 264-1
i8t| 1 Byte| -27 . . . 27-1
i16t| 2 Byte| -215 . . .

215-1

i32t| 4 Byte| -231 . . .

231-1

i64t| 8 Byte| -263 . . .

263-1

Boolean

A boolean is represented by 1 byte:

False = 0
True = 1. . . 255

Float (32 bit single precision)

For floating-point representation, the IEEE 754 format is used which has the following structure:

SENSIRION-SFC6-Mass-Flow-Controller-fig-11

Use the following coding to signal invalid float, positive or negative infinity:

Value	Coding (hex)

invalid float

(NaN)

| 0xFFFFFFFF
+ infinity| 0x7F800000
– infinity| 0xFF800000

String

Strings are transferred as C-strings. This means in ASCII coding, one byte per character and terminated with a final null-character (0x00). The first letter will be sent first. To make string handling as safe and robust as possible, it’s highly recommended to follow these rules when implementing an SHDLC master:

When sending strings to the slave (MOSI data), always terminate the string with exactly one null-character (0x00). In case a fixed data length is specified for a particular string (e.g. if more MOSI parameters are following that string), fill all unused data bytes after the null-termination with 0x00 as well. But usually, string parameters are specified with a variable data length, i.e. no such padding is needed.
When receiving strings from the slave (MISO data), parse the string up to the first null-character (0x00) in the received data and discard all the following data bytes (if any). In case no null-character is contained in the whole frame data, take the received data length into account to determine the string length, i.e. abort string parsing after the last data byte to accept strings which are not null-terminated.

Commands

[0x00] Setpoint

Get/Set Setpoint

Get Setpoint

Must be set to 0x01.

MISO Data| Byte #| Description
0 . . .

3

| Setpoint: float

The current setpoint.

Set Setpoint

Description| Set the flow setpoint as a physical value which is used by the flow controller as reference input.

**Note:** the setpoint is set to 0 if calibration is changed.

Must be set to 0x01.

1 . . .

4

| Setpoint: float

The new setpoint

MISO Data| None

[0x08] Read Measured Value

Read Measured Value

The command returns the latest measured flow as physical value.

Read Measured Value

Must be set to 0x01.

MISO Data| Byte #| Description
0 . . .

3

| Measured Value: float

The latest measured flow.

Read Averaged Measured Value

The command returns the average of given numbers of flow measurement as a physical value.
A single measurement has a duration of 1ms, so the command response time depends on the given number of measurements to average.

Read Averaged Measured Value

Must be set to 0x11.

1| Measurements: uint8

number of measurements to average(1..100)

MISO Data| Byte #| Description
0 . . .

3

| Averaged Measured Value: float

The averaged flow measurement.

[0x03] Set Setpoint and Read Measured Value

This command is a combination of the two commands “Set Setpoint (0x00)” and “Read Measured Value (0x08)”. It is intended for process data exchange (setpoint and flow) and saves a lot of protocol overhead compared to separate command usage.

Set Setpoint And Read Measured Value

Must be set to 0x01.

1 . . .

4

| Setpoint: float

The new setpoint as physical value.

MISO Data| Byte #| Description
0 . . .

3

| Measured Value: float

The latest measured flow as physical value.

[0x22] Controller Configuration

Get and set user settings of the flow controller.

Get/Set User Controller Gain

Get User Controller Gain

Must be set to 0x00.

Get User Controller Gain

MISO Data| Byte #| Description
0 . . . 3| Gain: float

The current user controller gain.

Set User Controller Gain

Description| Set the user controller gain.

Note

This configuration is not stored in non-volatile memory of the device and this not persists after a device reset.

Must be set to 0x00.

1 . . .

4

| Gain: float

The user controller gain to set.

MISO Data| None

Get/Set User Init Ste

Get User Init Step

Must be set to 0x03.

MISO Data| Byte #| Description
0 . . .

3

| Init Step: float

The current user init step.

Set User Init Step

Description| Set the init step of flow controller

Note

This configuration is not stored in non-volatile memory of the device, and this does not persist after a device reset.

Must be set to 0x03.

1 . . .

4

| Init Step: float

the user init step to set.

MISO Data| None

[0x30] Advanced Measurements

Advanced measurements for get more information about gas characteristics

Measure Raw Flow

Return the measured raw flow ticks from the sensor.

Measure Raw Flow

Must be set to 0x00.

MISO Data| Byte #| Description
0 . . .

1

| Flow: uint16

Measured raw flow in ticks.

Measure Raw Thermal Conductivity With Closed Valve

Perform a thermal conductivity measurement and return the measured raw tick value.
The valve is automatically closed during the measurement.

Measure Raw Thermal Conductivity With Closed Valve

Must be set to 0x02.

MISO Data| Byte #| Description
0 . . .

1

| Thermal Conductivity: uint16

Measured raw thermal conductivity in ticks.

Measure Temperature

Return the temperature of flow sensor.

Measure Temperature

Must be set to 0x10.

MISO Data| Byte #| Description
0 . . .

3

| Temperature: float

Measured temperature [°C].

[0x40] Get Calibration Information

This command allows to read information about all available gas calibrations on the device.

Get Number of Calibrations

Get the number of calibrations, i.e. how many calibration the device memory is able to hold.

Note: Not all calibrations actually contain a valid gas calibration. Use the command Get Calibration Validity to check which calibrations are valid and thus can be activated.

Get Number Of Calibrations

Must be set to 0x00.

MISO Data| Byte #| Description
0 . . .

3

| Number Of Calibrations: uint32

Number of calibrations.

Get Calibration Validity

Check whether there exists a valid calibration at a specific index or not.

Get Calibration Validity

Must be set to 0x10.

1 . . .

4

| Index: uint32

The index to check whether there is a valid calibration or not.

MISO Data| Byte #| Description
0| Validity: bool

Whether there exists a valid calibration at the specified index or

not.

Get Calibration Gas Id

Get the gas ID of a specific calibration index.

Get Calibration Gas Id

Must be set to 0x12.

1 . . .

4

| Index: uint32

The calibration index to read the requested information from.

MISO Data| Byte #| Description
0 . . .

3

| Gas Id: uint32

The read gas ID.

Get Calibration Gas Unit

Get the gas unit of a specific calibration index.

Get Calibration Gas Unit

Must be set to 0x13.

1 . . .

4

| Index: uint32

The calibration index to read the requested information from.

MISO Data| Byte #| Description
0| Prefix: int8

Medium unit prefix, see appendix for encoding.

1| Unit: uint8

Medium unit, see appendix for encoding.

2| Timebase: uint8

Timebase, see appendix for encoding.

Get Calibration Full scale

Get the full scale flow of a specific calibration index.

Get Calibration Fullscale

Must be set to 0x14.

1 . . .

4

| Index: uint32

The calibration index to read the requested information from.

MISO Data| Byte #| Description
0 . . .

3

| Fullscale: float

The read fullscale flow in the unit of the corresponding

calibration.

[0x44] Get Current Calibration Information

Returns calibration information of the current (active) calibration.

Get Current Gas Id

Get the gas ID of the currently active calibration.

Get Current Gas Id

Must be set to 0x12.

MISO Data| Byte #| Description
0 . . . 3| Gas Id: uint32 The read gas ID.

Get Current Gas Unit

Get the gas unit of the currently active calibration.

Get Current Gas Unit

Must be set to 0x13.

MISO Data| Byte #| Description
0| Prefix: int8

Medium unit prefix, see appendix for encoding.

1| Unit: uint8

Medium unit, see appendix for encoding.

2| Timebase: uint8

Timebase, see appendix for encoding.

Get Current Full scale

Get the full scale flow of the currently active calibration.

Get Current Full scale

Must be set to 0x14.

MISO Data| Byte #| Description
0 . . .

3

| Full scale: float

The read full scale flow in the unit of the corresponding

calibration.

[0x45] Get/Set Calibration

Get Calibration

Description| This command returns the actual set calibration number of Flow

controller.

The current activated calibration number.

Set Calibration

Description| This command stops the controller, changes the used calibration of the flow controller and starts the controller. The selected calibration is stored and also used after a power-on or reset.

Note: Because of the limited write cycles of the flash, this command should not be called periodically with new calibration (max. 50’000 times). If calibration is already selected the function will not do a write cycle to flash.

The calibration number to activate.

MISO Data| None

[0x46] Set Calibration Volatile

This command stops the controller, changes the used calibration of the flow controller and starts the controller.
The selected calibration is not stored to non volatile memory.

Set Calibration Volatile

The calibration number to activate.

MISO Data| None

[0x90] Get/Set Slave Address

This command changes the device address.

Get Slave Address

The current slave address of the device.

Set Slave Address

Description| Sets the SHDLC slave address of the device.

Note: The slave address is stored in non-volatile memory of the device and thus persists after a device reset. So the next time connecting to the device, you have to use the new address. When changing the address of a slave, make sure there isn’t already a slave with that address on the same bus! In that case you would get communication issues which can only be fixed by disconnecting one of the slaves.

The new slave address to set.

Allowed range: 0 . . . 254

MISO Data| None

[0x91] Get/Set Baudrate

Get Baudrate

Current baudrate in bit/s.

Set Baudrate

Description| Sets the SHDLC baudrate of the device.

Note: The baudrate is stored in non-volatile memory of the device and thus persists after a device reset. The next time you connect to the device, you have to use the new baudrate.

The new baudrate in bit/s. Allowed values are 9600, 19200,

38400, 57600 and 115200 (default)

MISO Data| None

[0xD0] Device Information

Get Product Type

Gets the product type from the device.

Get Product Type

Must be set to 0x00.

MISO Data| Byte #| Description
0 . . .

n

| Product Type: string

String containing the product type.

Get Product Name

Get Product Name

Must be set to 0x01.

MISO Data| Byte #| Description
0 . . .

n

| Product Name: string

String containing the product name.

Get Article Code

Get Article Code

Must be set to 0x02.

MISO Data| Byte #| Description
0 . . .

n

| Article Code: string

The article code as an ASCII string.

Get Serial Number

Get Serial Number

Must be set to 0x03.

MISO Data| Byte #| Description
0 . . .n| Serial Number: string

String containing the serial number of the SFC6xxx sensor.

[0xD1] Get Version

Gets the version information for the hardware, firmware and SHDLC protocol.

Get Version

Firmware major version number.

1| Firmware Minor: uint8

Firmware minor version number.

2| Firmware Debug: bool

Firmware debug state. If the debug state is set, the firmware is

in development.

3| Hardware Major: uint8

Hardware major version number.

4| Hardware Minor: uint8

Hardware minor version number.

5| Protocol Major: uint8

Protocol major version number.

6| Protocol Minor: uint8

Protocol minor version number.

[0xD3] Device Reset

Executes a reset on the device. This has the same effect as a power cycle.

Note: The device will reply before executing the reset.

Device Reset

Command ID| 0xD3
Max. Response Time| 100 ms

Post Processing Time: 300 ms
MOSI Data: None
MISO Data: None

Appendix

Gas unit decoding

The following tables show the encoding of the gas unit:

Prefix

Prefix Code

(i8t)

| Prefix| Symbol| 10ˆn
---|---|---|---
-24| yocto| y| 10-24
-21| zepto| z| 10-21
-18| atto| a| 10-18
-15| femto| f| 10-15
-12| pico| p| 10-12
-9| nano| n| 10-9
-6| micro| u| 10-6
-3| milli| m| 10-3
-2| centi| c| 10-2
-1| deci| d| 10-1
0| | | 100
1| deca| da| 101
2| hecto| h| 102
3| kilo| k| 103
6| mega| M| 106
9| giga| G| 109
12| tera| T| 1012
15| peta| P| 1015
18| exa| E| 1018
21| zetta| Z| 1021
24| yotta| Y| 1024
127| undefined| –| –

Unit

Unit Code (u8t)	Description	Symbol
0	norm liter (0°C, 1013 hPa)	l
1	standard Liter (20°C, 1013

hPa)

| l
8| liter (liqui)| l
9| gram| g
16| pascal| Pa
17| bar| bar
18| meter H2O| mH2O
19| inch H2O| iH2O
255| undefined| –

Time Base

Time Base Code

(u8t)

| Description| Symbol
---|---|---
0| no time base|
1| per microsecond| /us
2| per millisecond| /ms
3| per second| /s
4| per minute| /min
5| per hour| /h
6| per day| /day
255| undefined| –

State Response Error Codes

The state response error codes describe the meaning of the lower 7 bits of the ’state’ byte which is returned in each MISO frame. See Section 5.6.1 for details.

Code| Nam

e

| Description
---|---|---
0| 0x00| Success| Command successfully executed (no error occurred).
1| 0x01| Data Size Error| Illegal data size of the MOSI frame. Either an invalid frame was sent or the firmware on the device does not support the requested feature.
2| 0x02| Unknown Command Error| The device does not know this command. Either an invalid command ID was sent or the firmware on the device doesn’t support it.
4| 0x04| Parameter Error| A sent parameter is out of range. Check if you sent the correct command parameters and if the firmware on the device supports them.
41| 0x29| I2c Nack Error| NACK received from I2C device.
42| 0x2A| I2c Master Hold Error| Master hold not released in I2C.
43| 0x2B| I2c Crc Error| I2C CRC mismatch.
44| 0x2C| Sensor Data Write Error| Sensor data read back differs from written value.
45| 0x2D| Sensor Measure Loop Not

Running Error

| Sensor measure loop is not running or runs on wrong gas number.
51| 0x33| Invalid Calibration Index

Error

Current State

| Command is not allowed in the current state.
127| 0x7F| Fatal Error| An error without specific error code occurred.