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Raspberry Pi Pico 2 Review by Tim Blythman T he new Raspberry Pi Pico 2 microcontroller board was released in August this year. We have reviewed the new Raspberry Pi 5 single-board computer (SBC) in the July 2024 issue (siliconchip.au/Article/16323). The original Pico was released in 2021, followed by the WiFi and Bluetooth equipped Pico W in 2022. Both these boards are based on the RP2040 microcontroller, the first microcontroller designed by the Raspberry Pi Foundation. The Raspberry Pi 5 introduced the RP1 microcontroller, acting as an I/O controller. Like the Raspberry Pi SBCs, the Pico was designed to be low cost and easy to use, with a target price of US$4 (about $6). By the time we reviewed The last 12 months saw the release of the Raspberry Pi 5 single-board computer (SBC) and Raspberry Pi Ltd being listed on the London Stock Exchange. Most interesting for us was the recent release of the Raspberry Pi Pico 2 microcontroller board with the new RP2350 microcontroller. it, it could be programmed in the C language, with the Arduino IDE and MicroPython; PicoMite BASIC was released soon afterwards (December 2021; siliconchip.au/Article/15125). About a year later, the Pico W was released. It shares the same form factor and processor as the Pico but includes an Infineon CYW43439 radio module, adding WiFi and Bluetooth support. The bare RP2040 microcontroller later became available for purchase at around one dollar, from the likes of DigiKey and Mouser. That led to its incorporation into many thirdparty boards. At Silicon Chip, we created the Pico BackPack, which adds features like an LCD touchscreen, microSD card socket and audio output to a Pico or Pico W. That was detailed in the March 2022 issue (siliconchip.au/ Article/15236), with the Pico W BackPack introduced in January 2023 (siliconchip.au/ Article/15616). We have used the Pico and Pico W in various projects, including the VGA PicoMite, WebMite, Pico Audio Analyser and Pico Gamer. So we were very interested to see what the Pico 2 has to offer. There are a lot of similarities; it has the same layout and footprint as the Pico & Pico W. Apart from the silkscreen being marked as a Pico 2, you might not even know it was a different board! It appears the Pico 2 is backwards compatible with the Pico; we shall investigate that later. The Pico 2 is aimed to be available for US$5, and we purchased our test boards for about $8 (excluding delivery), which is much the same price at the time of writing. The RP2350 The new RP2350 microcontroller is actually a series of four new parts; it is the RP2350A variant that is fitted to the Pico 2. Table 1 shows a comparison between the RP2040 and the members of the RP2350 family. Like their respective microcontroller boards, there is a lot of similarity between the RP2040 and the RP2350. The two important differences are in the processor and the inbuilt RAM; these explain the differences between the part numbers. The Pico 2 (left) looks very similar to the Pico (right). The notable differences are in the silkscreen and that the Pico 2 uses smaller passives. The different core power supply is visible in the components above and to the right of the RP2350. The larger component in that area is an inductor that’s used in the switching mode of the RP2350 core supply. 62 Silicon Chip Australia's electronics magazine siliconchip.com.au The RP2350 has a dual ARM Cortex M33 processor compared to the RP2040’s dual ARM Cortex M0+ (hence the ‘3’ in RP2350), while the ‘5’ indicates that it has twice as much RAM (see Fig.1). Its data sheet can be found at siliconchip.au/link/ac1u The QSPI controller (which is used to communicate with an external flash memory chip) has been provided with a second interface. This can be used to connect a second flash chip or a PSRAM (pseudo-static random access memory), to expand the memory available to the system. 8MiB (64Mbit) PSRAM chips are available for a few dollars. That is a phenomenal amount of RAM for a microcontroller, but note that the Pico 2 board does not have provision for a PSRAM chip to be fitted. The RP2350 also has a dual Hazard3 RISC-V (pronounced ‘risk five’) processor that can be selected at boot time. RISC-V is an open RISC (reduced instruction set computer) architecture that is gaining traction as an alternative to other proprietary architectures. In theory, one core can be a RISC-V processor and the other, an ARM processor. The new M33 ARM processor has native floating-point instructions that the M0+ processor in the RP2040 lacks; floating-point support for the RP2040 is provided by software routines in ROM. That means a big uplift in performance when performing floating-point calculations. The M33 also includes Arm TrustZone and secure boot, using an OTP (one-time programmable) memory to store an encryption key. The M33 processor also performs better (at the same processor clock speed) than the M0+ in tests such as the Dhrystone benchmarks. The security features are not available when the RISC-V cores are used. The RP2350 has a nominal maximum clock speed of 150MHz, although we have already read reports that it can be overclocked (much like the RP2040). There are reports of operation up to 300MHz. Such overclocking is also subject to the limits of the flash memory chip. Two of the RP2350 variants boast a larger chip with more I/O pins; those have the ‘B’ suffix. These have 48 general-­purpose I/O pins, compared to just 30 on the RP2040 and ‘A’ variants. Then there are the RP2354 variants, siliconchip.com.au Table 1 – RP2040 and RP2350 family comparison RP2040 RP2350A RP2350B Dual ARM Cortex M0+ Dual ARM Cortex M33 and Hazard3 RISC-V External only RP2354A RP2354B 2MiB internal Processor (CPU) Flash memory 264kiB 520kiB plus external PSRAM RAM 133MHz 150MHz Clock 56 60 80 60 80 Pins 30 30 48 30 48 GPIO 2 UART 2 SPI 2 I2C 16 24 4 4 PWM 8 4 8 ADC channels Full-speed host or device USB 8 12 PIO state machines – HSTX peripheral, secure boot with OTP storage, hardware random number generator Other which bond a 2MiB (16Mbit) Winbond W25Q16JVWI QSPI NOR flash memory chip to the RP2350 processor die; this die is otherwise identical to a bare RP2350A or RP2350B chip. Thus, the four variants of the RP2350 are the 60-pin ‘A’ versions and 80-pin ‘B’ versions, either with (RP2354) or without (RP2350) an attached flash memory chip. Having only four ADC (analog-­todigital converter) channels on the RP2040 saw the Pico falling short compared to many other microcontrollers’ analog abilities. The larger RP2350B variants now have eight ADC channels, which means that the Pico 2 is still stuck with only four channels. During our development of the Pico Audio Analyser (November 2023 issue; siliconchip.au/Article/16011), we looked closely at some errors that had been identified in the ADC silicon Fig.1: the part naming of the RP2350 (and RP2040) is based on this scheme. The RP2354 parts have 2MiB (24 × 128kB) of non-volatile storage in the form of a flash memory chip bonded to the processor die. Australia's electronics magazine hardware of the RP2040. The RP2350 data sheet indicates that those have been fixed in the newer chip. The novel PIO (programmable input output) peripheral saw a lot of attention, and has been put to good use in emulating all sorts of peripheral functions. That includes SPI, USB and even the protocol that is used to control WS2812 programmable LEDs. We used the PIO to generate digital video in the Pico Digital Video Terminal (March & April 2024; siliconchip. au/Series/413). The RP2350 provides 12 PIO state machines, up from the RP2040’s eight. There are also some minor updates to the PIO peripheral itself. The RP2350 also has a new HSTX peripheral; this stands for ‘high-speed serial transmit’. It can stream data out on eight I/O pins at up to 300MHz (using double-data-rate output registers). There is example code to use the HSTX to generate DVI-­compatible video. The RP2350 data sheet notes that each processor core implements a TMDS (transition minimised differential signalling) encoding algorithm. TMDS is an encoding used with HDMI and DVI video, so clearly there is an intention for the RP2350 to be able to directly produce video output. Power management on the RP2350 has been improved by splitting the power domains and allowing some December 2024  63 1 2 39 USB BOOTSEL LED Fig.3: an easy way to tell the Pico from the Pico 2 is the drive volume label displayed by the bootloader. The RP2350 label indicates that it’s a Pico 2. A Pico or other RP2040-based board would show this as RPI-RP2. DEBUG parts to be selectively powered off, thus potentially using less power than the RP2040 in sleep mode. The Pico 2 Unsurprisingly, the biggest difference between the Pico and Pico 2 is the new processor chip. As well as doubling the RAM, the Pico 2 has double the available flash memory, with a 4MiB (32Mbit) flash memory chip onboard. The data sheet for the Pico 2 can be downloaded from siliconchip. au/link/ac1v That’s about the extent of the changes between the two boards. The same RT6150 buck/boost regulator allows the Pico 2 to operate from anywhere between 1.8V and 5.5V. Similar to the Pico, the Pico 2 also has a diode between the VBUS and VSYS pins. The Pico 2 appears to use smaller passive components, and there is some extra circuitry related to the RP2350’s core 1.1V power supply, which has a regulator that can operate in both linear and switching modes, allowing it to achieve better efficiency. The rear of the Pico 2 has test points in the same place as the Pico, with the addition of an extra test point in the area of the switching regulator’s circuitry. Otherwise, a 2024 copyright notice is the most prominent difference. From what we can see, there isn’t even a new pinout diagram for the Pico 2; the Pico diagram has simply been annotated to include the Pico 2. So it appears that there are no electrical or mechanical reasons that rule out using a Pico 2 in place of a Pico. 64 Silicon Chip Fig.2: the Pico and Pico 2 share this pinout diagram, meaning that I/O and peripheral mappings are identical. The new HSTX peripheral is not shown; it uses the GP12-GP19 pins of the Pico 2. Source: www.raspberrypi.com/ documentation/microcontrollers/ pico-series.html Fig.2 shows the pinout. It does not note the HSTX-capable pins, presumably to retain the consistency between the Pico and Pico 2 diagrams. The HSTX pins are fixed to GPIOs 12-19. A fault in the silicon While the Pico 2 may appear to be better in all ways than the Pico, there is already a severe erratum that can probably only be fixed by a revision of the RP2350 silicon. The data sheet notes this as erratum RP2350-E9, and it applies to stepping A2; this is the marking on our Pico 2. An excessive leakage current is sourced from a digital input pin if its voltage is in the undefined input voltage region, between valid high and low levels. When connected to a high impedance source, this could result in erroneous readings. It is especially a problem if the internal pull-down is active, since the weak pull-down cannot overcome the leakage and the pin remains stuck in the undefined input voltage region (around 2.2V for a 3.3V supply). Software fixes can be applied to some but not all situations. The general advice is to use an external pulldown resistor of no more than 8.2kW instead of the internal pull-down when required. The good news is that the bug that caused poor ADC performance in the RP2040 is fixed in the RP2350. Security We aren’t surprised that security was a low priority for the Raspberry Pi Foundation in creating a cheap and Australia's electronics magazine easy to use board in the Pico. The Arm TrustZone and secure boot features of the RP2350 intend to address one of the claimed weaknesses of the RP2040: a lack of security for the program flash memory. For example, reading or modifying the program in the flash chip (on the original Pico) would be as easy as accessing the flash chip and performing read or write commands. The security on the RP2350 depends on the flash memory contents being encrypted and signed. The encryption means that the data stored on the chip is meaningless until the processor decrypts it. The signing process is a way to tell if the data has been modified, and generally involves creating a hash or checksum of the data that can indicate if it has been changed. The signing is necessary as the encryption only means that the data cannot be easily read. It would still be possible, for example, to write random data to the flash chip in the hope of provoking insecure behaviour. The signing prevents any modified data from being run. The OTP (one-time-programmable) memory of the RP2350 can be used to store the keys needed to decrypt and check flash data, among other things. The OTP can be locked and hidden by programming specific bits. To test the security, the Raspberry Pi Foundation launched a competition with a $20,000 prize to see if anyone can break into the locked OTP memory. The competition is available at https://github.com/raspberrypi/ rp2350_hacking_challenge siliconchip.com.au Photos 1-4 (left-to-right): » the Seeed Technology XIAO RP2350 is one of the smaller RP2350 boards and has a USB-C socket. It appears that it will not cost much more than a Pico 2. » Pimoroni’s PGA2350 RP2350B is a compact but comprehensive breakout board for the 80-pin RP2350B. It includes 16MiB of flash memory and an 8MiB PSRAM chip. » the Pimoroni Tiny 2350 appears to be pin-compatible with their Tiny 2040. We noted the Tiny 2040 in our original review of the Pico; it was one of the early RP2040 boards. » Sparkfun’s Pro Micro RP2350 has a USB-C socket and incorporates a PSRAM chip, giving access to over 8MiB of random access memory. It also has a 16MiB flash memory chip. One of the great features of the RP2040 on the original Pico is the ROM bootloader, which makes it almost impossible to ‘brick’. The OTP provides a means to permanently modify the RP2350’s behaviour, so it’s possible that a wrong OTP operation could brick the RP2350. However, we understand that has been deliberately made difficult to do. Hands-on testing We are in the process of doing some detailed testing of the Pico 2 with our previous Pico projects, including the Pico Audio Analyser, which should hopefully improve its performance. To summarise what we’ve found, the Pico 2 works just about seamlessly in all cases where we had previously used a Pico! Of course, the differing architectures mean that code recompilation is required, but we generally have not had to make any changes to the code itself. For example, we fitted a Pico 2 to the prototype for our Pico Computer project (see page 66) and compiled the exact same Arduino sketch files (without any changes whatsoever) and the Pico 2 worked exactly as expected. Similarly, the example MicroPython program and libraries that we created for the BackPack worked without any changes on the Pico 2. The process for setting up the Pico-series C SDK (software development kit) on a Windows machine has changed substantially. Still, apart from that, we had little trouble in compiling the exact same code as we used with a Pico. siliconchip.com.au With just one click, we were able to create a separate project to use the RISC-V processor (instead of the ARM processor) and that too compiled flawlessly and worked identically. Curiously, the compiled RISC-V code is about half the size of the ARM code. We’ve also seen early versions of PicoMite BASIC for the RP2350. Downloads and a discussion can be found on TheBackShed Forum, see siliconchip.au/link/ac1w It looks like we will soon see new features in PicoMite BASIC. There is an HDMI video version, using the HSTX peripheral (in addition to VGA), and PicoMite BASIC has been bumped to version 6.0.0. For more background on setting up the Pico-series C SDK, trying out the various PicoMite BASIC RP2350 versions and porting our various projects to use the Pico 2. We plan to publish another article in the near future. What about a Pico 2 W? The launch announcement of the Pico 2 (siliconchip.au/link/ac1i) offered some hints on the availability of a WiFi version, as well as bare RP2350 chips. At this stage, it appears the Pico 2 W will feature the same Infineon CYW43439 radio module and should be available before the end of 2024. Bare RP2350 chips in all four variants are also expected to be available by the end of the year. DigiKey and Mouser already stock the RP2040 chip at just over $1, so we would not be surprised to see them carrying the RP2350 Australia's electronics magazine variants in the near future, presumably at a slightly higher price. Other RP2350 boards Other companies have already announced RP2350-based products. It appears some firms have had access to the RP2350 for some time before the launch, allowing them to develop a range of products, test out the chips and their software. It was the makers of the Bus Pirate (https://buspirate.com) who identified the erratum mentioned earlier. Bus Pirate is an open-source digital tool for working with microcontrollers and other digital ICs. Photos 1-4 show some of the new boards that have been announced. At the time of writing, we have not seen any of these boards available to purchase. Conclusion The Pico 2 looks to be just about better than the Pico in every way, as long you can avoid the leakage current problem. The extra RAM and improved ADC would definitely have been beneficial for our Pico Audio Analyser project had the Pico 2 been available when we were designing it. While it might appear that the Pico 2 could easily obsolete the Pico, there is a note on the Pico’s product page that it will be available until January 2036. The Pico 2 is similarly noted as being available until January 2040. Subject to stock levels and demand, the Pico 2 is available from Altronics, DigiKey and Mouser. SC December 2024  65