It is time to make a debriefing about last week achievement.
Colour gradient and intensity
Benjamin worked on the Led driver in order to display a picture and has fixed some problems with luminosity. He balanced green, blue and red and we are now able to display a white colour on our blocks (although a blue tint remains on videos). From now on, we also have a linear gradient of intensity for each primary colour. In order to obtain this gradient, we first tried to put a linear variation of intensity in registers, but it appeared linearity didn’t worked and he had to adjusted individually each step.
Electronic visual display:
An interruption is raised with a frequency of 10kHz and each time a new line is displayed. As our Led matrix has 8 lines, we have a electronic visual display at 1,25kHz. Each time we want to display a new line we connect to the Led driver through SPI sending it thanks to a vector of 288bits matching a 8 pixel line with three colour and 12 bit of gray control. Actually we only use 4 bit for each colour.
Yesterday I implemented acknowledgement system with IrDA transmission. First I needed to divide our sending queue into 2 queues: one for quick packets which don’t need ack such as PING, PONG and synchronizing messages sent at high frequency and allowing some losses; the other one is for important packet needing a safe transmission. These packet are sent to a task which could be seen as a buffer sending a packet (every 40ms) until it receives an ACK and then picking up the next one. In order to identify a packet an ACK transmit the ID of the sender and the ID of the packet it acknowledge and when an ACK packet is received, a message containing these information is send to the task managing output packets. In order to avoid loop waiting for an ACK , we use a counter and if a packet is not acknowledged after 10 tries we consider it as a lost packet (for ping messages, if we don’t receive one PONG for 10 of our PING, we consider the neighbour as gone). Once a neighbour is gone we empty the sending queues for this UART.
This system is not optimal, being slow, but at higher speed we got a lot corruption during transmission and more packet are lost.
We decided to rethink our algorithm after the reactions of yesterday’s post. We changed the algorithm when some block is leaving the network. If someone is missing, the block which notice that says it to the network. Every other block answer by « I’m still here ». If the leader doesn’t answer (probably he’s gone), there is an other election; those who don’t answer are considered to be out of the network. In this algorithm, the blocks keep in memory their position and orientation and we don’t have to start an election every time some block quits.
A little video showing up some features as the use of LED driver, election and direction:
NOTE: Now once a direction is decided, it is kept when the connection is lost.
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