Raspberry Pi 3 vs Pi 4 Architecture Differences

The Raspberry Pi 3 and Raspberry Pi 4 represent two distinct generations of single-board computers, with the Pi 4 introducing massive architectural upgrades. While the Raspberry Pi 3 relies on an older processing core and shares bandwidth across its USB and Ethernet controllers, the Raspberry Pi 4 features a completely overhauled system-on-chip (SoC) that separates these buses, vastly increasing throughput. This article breaks down the primary structural changes between these two boards, focusing on processor upgrades, memory capabilities, input/output (I/O) throughput, and power management.

Processor and Core Architecture

At the heart of the architectural shift is the upgrade to the System on Chip (SoC). The Raspberry Pi 3 (specifically the 3B+) utilizes the Broadcom BCM2837B0 SoC, which features a quad-core ARM Cortex-A53 processor running at 1.4 GHz. The Raspberry Pi 4 transitions to the Broadcom BCM2711 SoC, housing a quad-core ARM Cortex-A72 processor clocked at 1.5 GHz (or 1.8 GHz in later revisions). The Cortex-A72 is an out-of-order execution core, making it significantly more efficient per clock cycle than the in-order Cortex-A53, resulting in a substantial leap in raw computational performance.

Memory Capacity and Bus Technology

The memory architecture underwent a complete redesign for the Raspberry Pi 4:

• Memory Type: The Pi 3 uses older LPDDR2 RAM, whereas the Pi 4 upgrades to faster, more energy-efficient LPDDR4 memory.
• Capacity Limits: The Pi 3 architecture is hard-capped at 1 GB of RAM. The Pi 4 breaks this limitation by utilizing a 32-bit addressing extension (LPAE), allowing it to offer configurations of 1 GB, 2 GB, 4 GB, and 8 GB of RAM.
• Bandwidth: The shift to LPDDR4 significantly widens the memory bus bandwidth, preventing the RAM from becoming a bottleneck during heavy multitasking or 4K video playback.

I/O and Peripheral Bus Throughput

Perhaps the most critical architectural bottleneck cleared in the Raspberry Pi 4 is the I/O subsystem. On the Raspberry Pi 3, the Gigabit Ethernet port and all four USB 2.0 ports are routed through a single internal USB 2.0 hub chip. This means the entire network and USB traffic share a maximum theoretical bandwidth of just 480 Mbps.

The Raspberry Pi 4 eliminates this bottleneck by using a native PCIe (PCI Express) lane to connect a dedicated VIA VL805 USB 3.0 controller directly to the SoC. This provides two true USB 3.0 ports alongside two USB 2.0 ports. Furthermore, the Gigabit Ethernet controller on the Pi 4 connects directly to the SoC via an RGMII interface, allowing for full, unthrottled 1 Gbps network speeds without interfering with peripheral performance.

Multimedia and Display Output

The graphics and display architecture also received major updates to support modern media standards:

• GPU Upgrade: The Pi 3 features the older VideoCore IV GPU, capable of 1080p video decoding and supporting OpenGL ES 2.0. The Pi 4 introduces the VideoCore VI GPU, which scales up to OpenGL ES 3.1 and Vulkan support.
• Display Interface: The Pi 3 uses a single, full-sized HDMI port. The Pi 4 replaces this with two micro-HDMI ports, driven by independent display pipelines that allow the board to power dual 4K resolution displays simultaneously.
• Hardware Decoding: The Pi 4 includes hardware acceleration for H.265 (4K at 60fps) decoding, a feature entirely absent on the Pi 3.

Power Delivery and Thermal Design

Because of the high-performance architecture of the BCM2711 chip, the Raspberry Pi 4 requires a more robust power infrastructure. The Pi 3 uses a Micro-USB port for power, rated for roughly 2.5 Amps. The Pi 4 switches to a USB Type-C connector to accommodate a 3.0 Amp power supply, ensuring the board receives enough current to power the upgraded CPU, dual displays, and high-draw USB 3.0 devices without dropping voltage.