10th April, 2023

An Interrupt-based communication system between a custom hardware device and a custom processor.

The Device

We are designing a drone that houses sensors to periodically capture arial data and send to our custom designed CPU for processing. Some of the data is needed as and when it is captured and some is sent to the base for analysis.

For the safety of the drone, the most important data that we need to capture and process above all, is the data from the proximity sensor. The CPU needs this data to calculate the distance of the drone from the closest obstacle in real time. The result of this computation is then used to steer the drone away from crashing with the obstacle.

The CPU

We are designing a power efficient tiny CPU that can fit well into the drone chassis. There is the need for it to be well optimised to draw the least power usable and also to not heat up during computation. It should be resilient in diverse weather conditions by having a robust fault tolerance.

We need a communication system between the Sensors and the CPU. This communication system will serve as the means by which the sensors can send the data captured to the CPU for processing. We will design an interrupt-based communication system, allowing the sensors to communicate with the CPU through interrupts.

The PIC

The PIC(Programmable Interrupt Controller) is a microchip that enables our designed CPU to handle hardware interrupts. Hardware interrupts are the way by which our sensors communicate with our CPU.

Our PIC chip can be connected with 8 sensors at a time. 1 PIC can be connected to another if there is the need for us to use more sensors than 8. Below in a diagram showing our sensors connected to the PIC which connects to the CPU.

Taking a high level overview of how this system works, Sensors capture data and send to the CPU for processing to reorient the drone during flight.

The reason for using this interrupt-based communication system is to allow the CPU to do tasks and only attend to the sensors when they capture data and want it to process. This gives the illusion of the CPU multitasking which saves us a lot of processing power and energy as well.

When a sensor wants to send data to the CPU it first sends it to the PIC through a data line that connects it to the PIC. The PIC could be one or two in this case we use two. We are allowing the PIC to be connectable to each other to allow the Drone to use more sensors later without a lot of overhead getting it up and running. Let’s call the PIC that connects to the CPU as the Primary PIC and the one that connects to the Primary PIC as the Secondary PIC. If a sensor connects to the PPIC, it sends data to the PPIC which then sends the data directly to the CPU. If a sensor is connected to the SPIC, the data is relayed by the SPIC to the CPU through the PPIC.

On a normal day the sensors send data to the CPU for processing through a PICs. On some days where the weather is not so good we can have real problems. One of such problems is when interrupts happen so often that it becomes impossible to handle them all at the same time.

On a rainy day, there happens to be a lot of clouds, a lot of change in weather and temperature and a lot of turbulence with a lot of specks flying around due to the wind. The CPU could easily be overwhelmed.

Buffers

When the CPU gets overwhelmed with interrupts it sends a stop signal to the PIC. The PIC then stops sending new data to the CPU until it receives a send signal. While it waits, it will temporarily store all new data in a global fixed-size buffer.

When the PIC's buffer fills up, it in turn sends a stop signal to the sensors. How sensors respond to this signal is described below.

We are also going to fit the sensors with a fixed-size buffer too, similar to the PIC's. After receiving a stop signal, this is where new data will be temporarily stored until the send signal is received.

Prioritisation

Inside the PICs, we are going to assign priorities to the sensor data in the global buffer. We will be assigning the highest priority to the proximity sensor. When the CPU is ready to handle interrupts again, the data to first send must be that of the proximity sensor’s.

Handling Obsolete Data

Let’s say by the time the CPU is ready to handle interrupts, the global buffer is holding several different data from the proximity sensor. We already know what data we are getting from the proximity sensor and that is the distance. If we have let’s say 7 different data points, the first 6 would be obsolete since we need the latest data point to work with. We discard the first 6 and only send the 7th (last) data point to the CPU for processing.

Handling buffer overflows

We need to use a kind of buffer called a Ring/Circular buffer. A Ring buffer is a data structure that uses a fixed-sized buffer as if it were connected end-to-end. It starts out as an empty buffer. When it gets filled with data, the first data that was entered is evicted to allow space for the incoming data. This is a property known as First-in First-Out(FIFO). It should be noted how this looks similar to the implementation above.

Redundancy

As a designer of this community protocol it is my hope that the system never have to use to the maximum all the buffers and prioritisation in the PICs with Sensors at the same time but even if that happens, the implementation of the buffers, the prioritisation in the PICs and the buffers in the sensors, will ensure that the data from the proximity sensor is always received and processed by the CPU to help guide the drone to safety.