Computer architecture

Why this matters

A drone carries at least one computer that must read sensors hundreds of times a second and command the motors on time. Pick the wrong board, wire a device to the wrong bus, or use a memory card that cannot write fast enough, and video stutters or logs are lost. At worst, the control system reacts too late to keep the aircraft stable. Understanding computer architecture lets you read board specifications and design a system with enough resources.

The von Neumann computer

Almost every computer, from the microcontroller in a flight controller to a cloud server, has the same main parts:

• The central processing unit (CPU), made of a control unit that fetches and decodes instructions, an arithmetic logic unit (ALU) built from the adders and gates of module 1, and registers, tiny memories that are the fastest of all
• Memory, which holds both program instructions and data
• Input/output (I/O) devices such as sensors, motor drivers and radios
• Buses, the signal lines linking everything, split into address, data and control buses

The CPU runs an endless cycle: fetch an instruction from memory, decode it, then execute it. The clock sets the pace. A 480 MHz clock ticks 480 million times a second.

The width of the address bus sets how many memory locations it can point to. A 32-bit system addresses $2^{32}$ locations; at one byte each, that is 4 GiB.

clock_hz = 480e6
print(f"clock period {1e9 / clock_hz:.3f} ns")
print(2**32, "addresses =", 2**32 / 2**30, "GiB")

clock period 2.083 ns
4294967296 addresses = 4.0 GiB

The memory hierarchy and interrupts

Fast memory is expensive and small, so memory is arranged in layers: registers inside the CPU (fastest, smallest) → cache → RAM, the main memory that is lost when power goes off → persistent storage such as flash, SD cards or SSDs, which is large but slowest. Fast programs keep the data they need in the upper layers as much as possible.

A flight controller must react immediately, for instance when the IMU has a new sample ready. Having the CPU keep asking the device (polling) wastes cycles, so devices raise an interrupt: a signal that makes the CPU pause its current work to run a short urgent task first. A real-time operating system such as NuttX, which PX4 uses on flight controllers, schedules tasks by priority so that flight control is never blocked by other work.

The two computers on a drone

Modern drones often split work between two kinds of computer.

The flight controller has only 1 MB of RAM, which is enough for its specialised job. The Raspberry Pi 5 is many times faster, but Linux does not guarantee response times, so it should not drive the motors directly. The two computers talk MAVLink over UART or Ethernet.

On-board buses

8N1 UART sends one byte as 1 start bit, 8 data bits and 1 stop bit, 10 bits in total. At 57600 baud it can carry at most 5760 bytes per second.

baud = 57600
bytes_per_s = baud / 10
frame_bytes = 28 + 12
print(bytes_per_s, "byte/s")
print(bytes_per_s / frame_bytes, "GLOBAL_POSITION_INT frames/s at most")

5760.0 byte/s
144.0 GLOBAL_POSITION_INT frames/s at most

Storage and write speed

A survey drone records both flight logs and video. You must check both capacity (how long it can record) and write speed (whether it keeps up with incoming data). The SD Association defines the Video Speed Class, where the number on the symbol gives the minimum write speed; V30 means 30 MB/s.

log_rate_bytes = 50_000
print(log_rate_bytes * 3600 / 1e6, "MB of log per hour")

video_mbps = 150
video_mb_per_s = video_mbps / 8
print(video_mb_per_s, "MB/s needed, fits V30:", video_mb_per_s <= 30)

card_gb = 128
minutes = card_gb * 1e9 * 8 / (video_mbps * 1e6) / 60
print(f"{minutes:.1f} min of video on {card_gb} GB")

180.0 MB of log per hour
18.75 MB/s needed, fits V30: True
113.8 min of video on 128 GB

Module lab

Common mistakes

Summary

• A von Neumann computer has a CPU, memory, I/O and buses; the CPU repeats fetch, decode and execute at the pace of the clock
• Memory is layered from small and fast to large and slow, and interrupts let the system react on time
• The flight controller focuses on timing guarantees, the companion computer on computing power; they talk MAVLink
• Match the bus to the device, and check the capacity and speed of both buses and storage before flight

Check your understanding

1. What is the clock period of a 200 MHz CPU in nanoseconds?
2. How many memory locations can a 16-bit address bus point to?
3. What is the maximum byte rate of 115200 baud 8N1 UART?
4. Why does a flight controller use a real-time OS rather than Linux?
5. A camera records video at 100 Mbit/s. How many minutes fit on a 64 GB card?