Bluefruit-LE-Shield-SDEP

SDEP (SPI Data Transport)

In order to facilitate switching between UART and SPI based Bluefruit LE modules, the Bluefruit LE SPI Friend and Shield uses the same AT command set at the UART modules (ATI, AT+HELP, etc.).

These text-based AT commands are encoded as binary messages using a simple binary protocol we've named SDEP (Simple Data Exhange Protocol).

Présentation de SDEP

SDEP was designed as a bus neutral protocol to handle binary commands and responses -- including error responses -- in a standard, easy to extend manner. 'Bus neutral' means that we can use SDEP regardless of the transport mechanism (USB HID, SPI, I2C, Wireless data over the air, etc.).

All SDEP messages have a four byte header, and in the case of the Bluefruit LE modules up to a 16 byte payloads. Larger messages are broken up into several 4+16 bytes message chunks which are rebuilt at either end of the transport bus. The 20 byte limit (4 byte header + 16 byte payload) was chosen to take into account the maximum packet size in Bluetooth Low Energy 4.0 (20 bytes per packet).

Configurer SPI

While SDEP is bus neutral, in the case of the Bluefruit LE SPI Friend or Shield, an SPI transport is used with the following constraints and assumptions, largely to take into account the HW limitations of the nRF51822 system on chip:

Conditions requises du SPI matérielles

• The SPI clock should run <=4MHz
• A 100us delay should be added between the moment that the CS line is asserted, and before any data is transmitted on the SPI bus
• The CS line must remain activée (asserted) for the entire packet, rather than toggling CS every byte
• The CS line can however be deasserted and then reasserted between individual SDEP packets (of up to 20 bytes each).* The SPI commands must be setup to transmit MSB (most significant bit) first (not LSB first)

Broche IRQ

The IRQ line is asserted by the Bluefruit LE SPI Friend/Shield as long as an entire SDEP packet is available in the buffer on the nRF51822, at which point you should read the packet, keeping the CS line asserted for the entire transaction (as detailed above).

The IRQ line will remain asserted as long as one or more packets are available, so the line may stay high after reading a packet, meaning that more packets are still available in the FIFO on the SPI slave side.

Paquets SDEP et identifaction des erreurs SPI

Once CS has been asserted and the mandatory 100us delay has passed, a single byte should be read from the SPI bus which will indicate the type of payload available on the nRF51822 (see Message Type Indicator below for more information on SDEP message types). Keep CS asserted after this byte has been read in case you need to continue reading the rest of the frame.

If a standard SDEP message type indicator (0x10, 0x20, 0x40 or 0x80) is encountered, keep reading as normal. There are two other indicators that should be taken into account, though, which indicate a problem on the nRF51822 SPI slave side:

• 0xFE : Slave device not ready (wait a bit and try again)
• 0xFF : Slave device read overflow indicator (you've read more data than is available)

This means there are six possible response bytes reading the message type indicator (the first byte read after an SDEP command is sent): 0x10, 0x20, 0x40, 0x80, which indicate a valid message type, or 0xFE, 0xFF which indicate an error condition.

Sample Transaction

The following image shows a sample SDEP response that is spread over two packets (since the response is > 20 bytes in size). Notice that the IRQ line stays asserted between the packets since more than one packet was available in the FIFO on the Bluefruit LE SPI side:

Crédit: AdaFruit Industries www.adafruit.com

SDEP - protocole d'échange de donnée simplifié

SDEP est l'acronyme de "Simple Data Exchange Protocol".

The Simple Data Exchange Protocol (SDEP) can be used to send and receive binary messages between two connected devices using any binary serial bus (USB HID, USB Bulk, SPI, I2C, Wireless, etc.), exchanging data using one of four distinct message types (Command, Response, Alert and Error messages).

The protocol is designed to be flexible and extensible, with the only requirement being that individual messages are 20 bytes or smaller, and that the first byte of every message is a one byte (U8) identifier that indicates the message type, which defines the format for the remainder of the payload.

Endianness

Endianness (wikipedia) est l'ordre dans lequel sont organiser les octets en mémoire et dans les communications lorsqu'ils transportent une informations qui doit être stocké une taille supérieure à 1 octet. C'est le cas des entiers qui se codent sur 16 ou 32 bits.

All values larger than 8-bits are encoded in little endian format. Any deviation from this rule should be clearly documented.

Indicateur du type de message

The first byte of every message is an 8-bit identifier called the Message Type Indicator en anglais. This value indicates the type of message being sent, and allows us to determine the format for the remainder of the message.

Message Type	ID (U8)
Command	0x10
Response	0x20
Alert	0x40
Error	0x80

Transactions de données SDEP

Either connected device can initiate SDEP transactions, though certain transport protocols imposes restrictions on who can initiate a transfer. The master device, for example, always initiates transactions with Bluetooth Low Energy or USB, meaning that slave devices can only reply to incoming commands.

Every device that receives a Command Message must reply with a Response Message, Error Message or Alert message.

• https://learn.adafruit.com/adafruit-bluefruit-le-shield/sdep-spi-data-transport#sdep-data-transactions

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Basé sur "Bluefruit LE Shield" d'Adafruit Industries, écrit par Kevin Townsend - Traduit en Français par shop.mchobby.be CC-BY-SA pour la traduction
Toute copie doit contenir ce crédit, lien vers cette page et la section "crédit de traduction".

Based on "Bluefruit LE Shield" from Adafruit Industries, written by Kevin Townsend - Translated to French by shop.mchobby.be CC-BY-SA for the translation
Copies must includes this credit, link to this page and the section "crédit de traduction" (translation credit).

Traduit avec l'autorisation d'AdaFruit Industries - Translated with the permission from Adafruit Industries - www.adafruit.com