Difference Between Raspberry Pi and Arm

Raspberry Pi 4 craft computer in the test

That was a surprise: Almost out of nowhere, the Raspberry Pi Foundation presented the Raspberry Pi 4 with a faster main processor, much more RAM and more powerful interfaces such as USB 3.0 and Gigabit Ethernet. This means that Raspi projects are now possible that previously failed due to insufficient memory, too lame USB 2.0 connection or a missing decoder for H.265 (HEVC) video.

Thanks to clever decisions, the price for the 1 GByte Raspi remains almost the same, only the variants with 2 or 4 GByte RAM are more expensive - but still cheaper than most competing single-board computers. But one after the other.

The most important Raspi chip also comes from Broadcom: The BCM2711 contains four ARM Cortex-A72 CPU cores and a VideoCore-VI (VC6) GPU. With roughly the same power consumption as its predecessor BCM2387B0, the BCM2711 delivers noticeably more computing power because the CPU cores clock a little higher (up to 1.5 instead of 1.4 GHz) and can handle out-of-order execution. In the EEMBC CoreMark, the newcomer from the 28-nanometer production achieved a little more than double the score of its predecessor, both with only one core and with all cores at the same time. With continuous load on all four cores, the newcomer throttles significantly later, so it delivers much higher performance for longer. When a single core was loaded, there was no throttling in our tests, even without active cooling, unlike the BCM2837B0. Careful overclocking to 1750 MHz works without loss of warranty, but not worth it.

With VideoCore VI, the focus is less on the stronger 3D acceleration than on the improved video decoder and, above all, the memory controller integrated here, which now connects up to 4 GB of LPDDR4-SDRAM. The latter also transfers data much faster than the previously used LPDDR2 memory, but is still soldered on and cannot be upgraded.

A Media Access Controller (MAC) from Broadcom for Gigabit Ethernet (GE) is also permanently installed in the BCM2711, which controls the RJ45 socket via the so-called PHY chip BCM54213PE. The Raspi 4 finally manages full GE speed of around 100 MB / s, while the Raspi 3B + with its GE chip connected via USB 2.0 delivered a maximum of about a third of that. And because a USB 3.0 controller is also connected to the BCM2711 via PCI Express (PCIe 2.0), namely the VIA Labs VL805, the Raspberry Pi Model 4 B (RPi4B) is also much better than small network storage (NAS): From one USB 3.0 hard disk or SSD on the Raspi can read other devices in the LAN with up to 80 MB / s of data. If the SSD is encrypted with LUKS, there are still 60 MB / s. We will provide detailed benchmarks for this in one of the next issues.

USB Type C and double HDMI

For the power supply with 5 volts, the RPi4B now has a USB-C socket instead of a MicroUSB. You also need a power supply unit with a USB-C connector or a simple adapter. The Raspberry Pi Foundation sells a suitable USB-C power supply that can deliver 15 watts (5 V / 3 A). If there are no power-hungry USB devices such as fast mass storage devices attached to the USB sockets, the RPi4B should still get by with a 2.5 ampere power supply unit.

The two micro-HDMI sockets are also new instead of just one in a normal-sized HDMI design. Here, too, you can get further with adapters, but we cannot recommend that: No two such adapters fit next to each other and the adapters physically load the small socket much more than a cable with a micro-HDMI connector. Attention: There is also mini-HDMI, for example on the Raspi Zero - so don't buy the wrong adapter or cable! The RPi4B does not fit into the housing for its predecessor because of the two micro HDMI sockets, and the RJ45 network socket has been relocated. The new "official" case is unfortunately not yet available. The old original case can only be used if you file off a bridge and leave out a side panel.

The two micro HDMI sockets supply HDMI 2.0 signals for displays with resolutions up to 4K, aka Ultra HD (UHD), i.e. with 3840 × 2160 pixels. The Raspi can control such a screen with 60 Hz, two at the same time only with 30 Hz each. That is sufficient for 4K videos with 24 or 30 fps, but not for ergonomic work on the Linux desktop. On full HD displays there are also two 60 Hz simultaneously. The VideoCore can also decode and output 10-bit signals. However, the software is still missing for one of the various flavors of HDR.

4K video

In the test with a preliminary version of LibreELEC 10, HEVC (H.265) encoded 4K videos could be decoded smoothly at 60 frames per second (2160p60) and played on a 4K display - but so far only at 30 fps. The hardware decoder for H.264 (MPEG-4 AVC) only works up to Full HD (1080p60). There is no special hardware for the VP9 codec used in YouTube videos - you have to reduce the resolution or use a browser extension to persuade YouTube to deliver an H.264 stream.

The Chromium browser under Raspbian cannot yet use the video decoder in the BCM2711; here it jerks at high resolutions. It looks the same with the VLC video player. Both of them could take a while to use the HEVC hardware, says Eben Upton of the Raspberry Pi Foundation. The way of integration is different from the decoder and encoder for H.264, a lot of code has to be rewritten.

As with previous model changes, the Raspberry Pi Foundation pays attention to the best possible compatibility with its predecessors so that existing software can continue to be used. Therefore, Raspbian initially remains 32-bit. This version cannot exploit the potential advantages of the ARMv8-compatible Cortex-A72 cores; it addresses them as ARMv7 cores. The RPi4B remains a long way from the computing power of simple x86 processors such as Intel's “Atom-Celeron” N4100, but it costs only a fraction. The Foundation is working on a 64-bit kernel, but it is uncertain when it will come. Other distributions have had 64-bit kernels for the Raspi for a long time, but they are not yet running on the RPi4B.

Whether existing plug-on modules, the so-called HATs, continue to work depends primarily on their shape. Nothing has physically changed on the 40 GPIO pins, nothing at all on the internal connections for the camera and display.

Power consumption

The power consumption of the Raspberry Pi 4 Model B is absolutely low, it needs 3.7 watts when idling, including a Gigabit Ethernet connection and dongle for a wireless keyboard. But that is a whole watt more than its predecessor. To put the additional requirement in relation: Many USB 3.0 sticks already need more than 1 watt, even if they are only plugged into the USB socket. And in soft-off mode, the RPi4B gets by with just 0.3 watts, the predecessor used three times as much.

Under load, but without a monitor, the newcomer consumes a maximum of 6.1 watts, not significantly more than the 3B +, which converts 5.8 watts after the update to Raspbian Buster. Under continuous maximum load, for example with OpenSSL encryption on all CPU cores simultaneously, the latter heat up to over 80 degrees Celsius within a few minutes, then the chip gradually throttles its performance. Until then, it takes much longer than with the Raspi 3B +.

Mini server and WiFi

With Gigabit Ethernet and two USB 3.0 sockets, the RPi4B is much more suitable as a mini server than its predecessor. GE delivers the full transfer rate of around 925 Mbit / s with the RPi4B and is integrated into Linux or Raspbian via a Broadcom standard driver. The RPi4B is also prepared for power supply via Ethernet (PoE), but an additional board is required for this.

More about Raspberry Pi & Co.

As a NAS replacement, the Raspi is of course only suitable for certain purposes, because it is easy to score own goals: USB disks and additional hubs consume slightly more power than the Raspi itself. This is especially true for external hard drives in 3.5-inch format some of which consume more than 10 watts. Anyone who needs several terabytes of fast network storage is better served with a "real" NAS.

The WLAN chip of the RPi4B is like its predecessor the Cypress CYW43455 with 802.11ac. It is still connected to the BCM2711 via SDIO, but has since learned Bluetooth 5.0. SDIO is a variant of the interface for SD cards, but for I / O modules - and is particularly economical.

With its only antenna integrated on the board (1 stream, i.e. 1 × 1), the CYW43455 is not a sprinter, but it still manages up to 85 MBit / s, which is around 8 MByte / s net. That is enough for many handicraft projects for data transmission and also for video streaming up to full HD resolution. In the 5 GHz band, the performance drops significantly over the 20 meter distance, depending on the orientation of the antenna, which depends on the position of the entire Raspi.

Conclusion

The Raspberry Pi 4 Model B is a great surprise, it can do practically everything better than its equally expensive predecessor and expands the area of ​​application of the Raspi into many areas for which previously more expensive single-board computers with less sophisticated software infrastructure had to be used. However, the developer community still has to polish and polish Raspbian because a lot is not yet working: booting via USB and network, 64-bit kernel and applications, H.265 videos and the integration of the video decoder in Chromium and other software. Two-screen operation is also not without its pitfalls.

Whether the RPi4B is also suitable as a desktop PC alternative, as Eben Upton repeatedly emphasizes, depends heavily on personal expectations. If you want to use the RPi4B as a desktop PC, you should definitely use at least the 2 GB version. The 4 GB version is useful for more complex projects. There is already speculation about an 8 GB version, but if it comes at all, it will probably only be after the introduction of a 64-bit Raspbian - and that can still take a while. Raspi friends need patience anyway, because so far the RPi4B has only been sold in small numbers and the Foundation's very cheap USB-C power supply will probably only be available from August.

But even if almost every more expensive Raspi competitor can do something faster or better: The Raspi 4 is again the best all-rounder in one fell swoop. (ciw)


Awesome upgrade

The Raspberry Pi Foundation and Raspi mastermind Eben Upton show how to get the most out of limited resources: Raspi is not about top performance in individual disciplines, but about an efficient, affordable and manageable complete package. For the same price and with a similar power consumption, the Raspi 4 delivers considerably more performance and features. The Raspi makers achieve this by specifically improving as few parts as possible, paying attention to the broad impact of each individual measure and also keeping a close eye on side effects and costs. It's not just about hardware prices, but also about the effort involved in development, software support and delivery options. Overall, the Raspi implements the KISS principle in an ideal way: Keep it simple, Stupid!
But the idea of ​​manufacturing the Raspi in as few variants as possible for as many areas of application as possible and also attracting commercial buyers from the fields of industrial control, embedded systems and digital signage is ingenious - the RPi4B will be manufactured until 2026, so it is also suitable for longer-running projects and will probably get a successor to whom it is comparatively easy to switch.
Eben Upton cleverly uses his close ties to Broadcom, where he is still on the payroll as a chip architect. This makes things possible that other projects do not manage, such as open Linux drivers for the VideoCore VI and the development of the special BCM2711 chip, which is closely related to Broadcom SoCs for set-top boxes. And even if Broadcom has sold the WLAN division to Cypress (which should soon be part of Infineon), the contacts are apparently good. I am very curious to see what else is to come! (Christof Windeck)


This article comes from c't 15/2019