UM0541

Architecture

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1.2

Communication mechanism

1.2.1

Introduction

This section describes the transmission mechanism and the reception mechanism between

CPU commander and a target device.

1.2.2

Transmission

To send data, the sender must build a DCP frame and insert information in each field.

The sender has to perform these steps:

1.

Insert the CommandID.

2.

Set the correct flags for the functionality requested (SYS, ACK, CRC).

3.

Insert the Length of the bytes of the data.

4.

Insert the data.

5.

Calculate the CRC if the CRC flag is set and insert the value at the end of the frame.

6.

Call the function SendFrame(frame).

1.2.3

Reception

The reception is interrupt implemented.

Using UART communication, the receiver receives one byte at a time, so the number of the

byte received depends from the properties of each frame.

If the acknowledge management is activated, the first seven bytes (FrameID, CommandID,

Flags, Length) are mandatory. If the acknowledge management is not activated the first six

bytes are always present (CommandID, Flags, Length). After the reception of the Length, it

is possible to know how many bytes of data the receiver has to receive. After the receiver

has received all data, it waits for CRC flag if the CRC flag is set, otherwise the reception is

complete.

When the frame is complete, the receiver can pass the frame to the DCP protocol for the

parsing calling DCP_ProcessFrame() function.

If the frame is a system (SYS) frame, DCP protocol executes the task programmed for the

specific SYS. If the frame is an acknowledge frame (ACK), the protocol checks if the

FrameID of the frame received corresponds to the expected FrameID. If the FrameID does

not match, an error occurs.

If the CommandID of the frame is a user CommandID, the management of this frame is

passed to the CallBack function managed by the user. In fact, the user has to insert the

managed frame into the Callback function with a CommandID which is specific to the

application.