Fullscreen HTML5Med Res (??MB)HTML4

Raspberry Pi Pic W Review by Tim Blythman The Raspberry Pi Foundation has a habit of making surprise announcements of new hardware. Although created with education in mind, their low pricing has made the various single-board computers and microcontroller boards extremely popular in all sorts of applications. Adding WiFi support can only increase that popularity! A fter nearly ten years of their range of single-board computers, the Raspberry Pi Foundation surprised everyone last year with the release of its first microcontroller, the RP2040, and its own microcontroller board, the Pico. Now the Pico W is available, building on the already very handy Pico board, adding all-important WiFi capability. Background The RP2040 is a dual-core ARM Cortex M0+ processor with 264kiB of RAM, a 16kiB boot ROM that provides a USB bootloader, plus some other handy functions. Like many such 32-bit devices, it runs from 3.3V. The Pico is a fairly minimal board implementation that adds a 4MB flash memory chip and a 3.3V voltage regulator. It breaks out 26 of the RP2040’s I/O pins into a compact board suitable for breadboarding or even surface-­ mounting on another PCB. Its target launch price was $6, and incredibly it is still in stock from multiple vendors at about that price. While the board is minimal, it is very capable (especially considering the price) and could be programmed in C or MicroPython from launch. Arduino support appeared very quickly after its debut. Raspberry Pi is a trademark of the Raspberry Pi Foundation It’s the same size as the Pico and costs only a few dollars more. We can see the Pico W challenging other WiFi boards like those based on the ESP32 and ESP8266 due to its low price and excellent support. 56 Silicon Chip Australia's electronics magazine The RP2040 microcontroller was also available in good numbers and at a good price (under $2 in single quantities). It was quickly taken up by other companies who created their own spin on the Pico, including variants such as the Arduino Nano RP2040 Connect, one of the many boards that includes WiFi. We reviewed the Pico in December 2021 (siliconchip.au/Article/15125) and found that it is a well-designed board with a nimble microcontroller that has a very useful set of features. With the work of Geoff Graham and Peter Mather, it was soon possible to program the Pico in BASIC. This is the so-called PicoMite (January 2022; siliconchip.au/Article/15177). The PicoMite has much in common with the Micromite, including the ability to drive an LCD touchscreen. That led us to create the Pico BackPack in March 2022 (siliconchip. au/Article/15236). The BackPack is designed to connect to an LCD touchscreen and includes an RTC (realtime clock) chip, IR receiver, stereo line-level audio output and a micro SD card slot. We also showed how to program the Pico BackPack in four different languages. The very popular VGA PicoMite followed that in July 2022 (siliconchip. au/Article/15382). The addition of a VGA output and a keyboard input means that the VGA PicoMite is more siliconchip.com.au Table 1: GPIO differences between Pico and Pico W Function Pico Pico W Notes PSU PS pin GPIO23 WL_GPIO1 Has 100kΩ pulldown, low is PFM mode, high is PWM mode USB Vbus sense GPIO24 WL_GPIO2 Digital input with pulldown LED GPIO25 WL_GPIO0 Also broken out to TP5 downstream of resistor Vsys/3 GPIO29 GPIO29 For Pico W, only when enabled by GPIO25 being high WiFi CLK – GPIO29 WiFi CS – GPIO25 Also connected to Mosfet on Pico W to enable ADC readings on GPIO29 WiFi MISO/MOSI – GPIO24 Connects to four SDIO pins of CYW43439 WiFi EN – GPIO23 Connects to both WL_REG_ON and BT_REG_ON of CYW43439 like a Maximite than a Micromite and is reminiscent of the computers of yore, like the Commodore 64. Enter the Pico W The Pico W, as you might guess, is an RP2040-based microcontroller board to which a WiFi module has been added. Its target price is $9. It’s the same size and has much the same pinout as a Pico, but with a small metal can at the end opposite the micro USB socket. That can contains the WiFi chip. The three-pin SWD (serial wire debug) header has been moved to make way for the WiFi antenna. The Pico W is not the first RP2040 board with WiFi, but given the Raspberry Pi Foundation’s aggressive pricing, it is among the cheapest. It’s similar in price to ‘off-brand’ boards with ESP8266 or ESP32 microcontrollers, which also have a similar set of features. The WiFi module is based on an Infineon CYW43439 chip, which boasts 802.11n (2.4GHz) WiFi support and Bluetooth 5.2. However, there doesn’t appear to be any software support for Bluetooth on the Pico W at this stage. The CYW43439 chip has separate WiFi and Bluetooth subsystems which each have their own ARM microcontroller core. There are several ways to communicate with the WiFi Chip but, importantly, the common SPI interface has been used on the Pico W. That means that Bluetooth could be supported on the Pico W in the future. Design evolution Having another chip interfaced to the RP2040 inevitably leads to the question of what has been sacrificed, since some resources will at least need to be shared if not given up entirely. The first clue is that all the exposed siliconchip.com.au pins are marked the same as the original Pico. The WiFi interface happens through four GPIO pins that are not broken out on either board. These are GPIO23, GPIO24, GPIO25 and GPIO29. On the original Pico, these all had various internal functions. The Pico W now uses these to communicate with the WiFi chip. Some of the original functions are kept, while others are delegated to GPIO pins on the WiFi chip. Table 1 summarises these differences. There are other subtle differences. The original Pico uses an RT6150 buck/boost regulator to provide the 3.3V rail. This is rated to 800mA and can work with an input between 1.8V and 5.5V. The Pico W uses an RT6154, which has a similar input voltage range but can deliver over 2A. Some extra current will be necessary to drive the WiFi chip; this extra capacity will surely come in handy elsewhere. Apart from the pin changes noted in Table 1, the interfaces to the power supply are much the same despite the pin changes. One I/O pin monitors Vbus to detect the presence of USB power. At the same time, another is used to select between the more efficient PFM (pulse frequency modulation) and lower ripple PWM (pulse width modulation) modes. In practice, the biggest difference is how the onboard LED is driven, since the Pico W uses one of the CYW43439’s I/O pins to drive it. On paper, the Pico W appears to be a mostly compatible and painless upgrade to the Pico. Some functions have changed locations, but that is to be expected. Hands-on testing The Pico W was unavailable in Australia until August this year due to Australia's electronics magazine delays in the Raspberry Pi Foundation getting RCM (Regulatory Compliance Mark) approval. As soon as we could get one, we tried it out. We built a Pico BackPack and fitted it with female headers, allowing us to change out the two different boards (fitted with male headers) quickly for testing. This arrangement also helps keep the Pico W’s WiFi antenna clear of the solid copper pours on the BackPack PCB and LCD module. We used stacked spacers for testing the WiFi capabilities of the Pico W. We loaded each of the four UF2 test files provided for the original Pico BackPack article onto both the Pico and Pico W in turn and checked their operation. The only difference we noted was that the Pico W does not flash its LED when using the PicoMite example firmware, which was expected due to pin 25 being used for the WiFi module instead of the LED. Otherwise, all the test programs worked identically between the two boards. So if you can afford the premium of an upgrade from the Pico to a Pico W, there aren’t really any downsides. Next, we decided to see what could be done with the Pico W’s WiFi. Using WiFi The Raspberry Pi Foundation does a great job of making it easy to get started; perhaps that is not surprising, given their educational focus. They have published online numerous data sheets, user guides and other resources such as the full circuit diagrams for both boards. Among these documents is a guide on connecting to the internet with the Pico W. You can find it at siliconchip. au/link/abgv We made good use of that information during our testing. Although the November 2022  57 WiFi chip on the Pico W uses SPI, it is in half-duplex mode, using the same data pin for communication in both directions. According to the official C SDK (software development kit) for the Pico and Pico W, the SPI peripheral is actually implemented in one of the PIO blocks. As we mentioned in our original article on the Pico, PIO blocks are I/O-orientated state machines that can run a small program and thus emulate communication peripherals like SPI or UART. There are even DVI implementations that can generate signals that work on HDMI displays. This means that existing designs that use the PIO state machines may need to be modified to work with the Pico W but presumably, only if the WiFi features are actually used. Unfortunately, it looks like the PicoMite (MMBasic) firmware will not be updated to include support for the WiFi chip, as explained at www. thebackshed.com/forum/ViewTopic. php?TID=14977 That’s because the WiFi chip on the Pico W needs a firmware image to be loaded at runtime, and that alone consumes around 300kB of flash memory, plus RAM at runtime. It would also require a framework and commands to provide a way to control the WiFi module through the BASIC language. We can see that this would be a lot of work and perhaps isn’t justified, especially as it is so easy to interface to an ESP8266 loaded with an AT command firmware. Regardless of the language used, any project that is updated to use the WiFi feature will undergo considerable changes anyway. MicroPython support There are two ways of programming the Pico W (and the Pico) provided by the Raspberry Pi Foundation: the C SDK and MicroPython, a port of the Python language optimised for microcontrollers. The Pico W’s MicroPython port supports WiFi out of the box. It is not the first microcontroller with WiFi to work with MicroPython; MicroPython has long had support for WiFi on the ESP8266 and ESP32. There is a prebuilt UF2 image of MicroPython for the Pico W available. Once uploaded, the following commands initialise the WiFi chip and print out a scan of nearby WiFi networks: import network wlan = network. WLAN(network.STA_IF) wlan.active(True) print(wlan.scan()) The Connecting to the Internet guide mentioned earlier has chapters on using MicroPython to connect to networks, make HTTP requests and build HTTP servers, including controlling Besides the added WiFi (the metal can), the PCB trace WiFi antenna has necessitated the SWD (serial wire debug) header being moved to a different location. Otherwise, the pins are in much the same locations as the original Pico. The back of the Pico W has pin markings that are identical to those on the Pico. The six test points all remain in the same locations. Like the Pico, the Pico W comes without headers; the H and WH variants cost more but have presoldered headers. the onboard LED from a web page. The guide includes source code for performing those tasks. C SDK Similarly, the Connecting to the Internet guide has a chapter on using the WiFi features under the C SDK, although these depend heavily on downloadable example projects. We had previously installed the C SDK, so we simply had to perform an update at the command prompt to get access to the new libraries and examples. New installs of the C SDK should already include the most up-todate files. There are a few command line switches that need to be used in the “cmake” command for WiFi support: -DPICO_BOARD=pico_w -DWIFI_SSID=“Network Name” -DWIFI_PASSWORD=“Password” The first of these sets the board to be a Pico W; we had not needed to use a switch previously as the Pico was the default. If you wish to eliminate confusion, the switch argument for a Pico is simply “pico”. The other switches set the SSID and password parameters for a WiFi network. Doing it this way is much easier than manually updating many source files. It also avoids having to store these credentials in a source file that might need to be distributed to others. The release notes for version 1.4.0 of the C SDK describe the updates that coincided with the release of the Pico W. You can find them at https://github. com/raspberrypi/pico-sdk/releases As for the Pico, the Raspberry Pi Foundation’s guides assume that you are using a Raspberry Pi computer to compile for the Pico W. While not the quickest way to compile code, the instructions are clear and work well in that case. There is also Pico Setup for Windows, which installs all the programs needed to compile for the Pico and Pico W using the C SDK. It can be downloaded from https://github.com/ ndabas/pico-setup-windows/releases Although the most recent version of that software predates the Pico W, the installer can download more recent SDK files. Using the Arduino IDE We tried two different board profiles for the Pico in our review from last 58 Silicon Chip Australia's electronics magazine siliconchip.com.au year. There is an ‘official’ board profile that uses Mbed OS and another profile (Arduino-Pico) that uses the C SDK. The first is included as one of the default options that can be installed by the Arduino Boards Manager, while the other needs a link to be added to the Additional Board Manager URL field under Preferences. We found that the latter was actually the first to support the Pico W, although this is perhaps not surprising as it uses the C SDK from the Raspberry Pi Foundation itself. At the time of writing, we have not seen an official Arduino core for the Pico W. So all our tests with the Arduino IDE for the Pico W have been with the Arduino-Pico core: https://github.com/ earlephilhower/arduino-pico That link includes instructions on how to install this core into the IDE. Using the Arduino IDE for the Pico W was as simple as it gets. After updating or downloading the Board Profile, we set the board to “Raspberry Pi Pico W” and uploaded the “ScanNetworks” example sketch. It then produced a list of nearby WiFi networks on the Serial Monitor (see Screen 1). As a test, we tried the Pico W sandwiched between the BackPack and LCD PCBs, as well as sitting loose on our workbench. Most networks showed a decrease of around 10dB in the indicated RSSI level compared to when the Pico W was loose and unshielded by other boards. That’s probably not critical for most applications. Still, it’s worth noting that using the BackPack and accompanying LCD with the Pico W attached directly will likely reduce the range or performance of the WiFi chip. Comparing WiFi boards The release of the WiFi-capable ESP8266 almost a decade ago and support for it being added to the Arduino IDE suddenly made hobbyist WiFi projects both cheap and straightforward. At that time, many boards under $10 appeared on the market. The Pico W is in the same league as the ESP8266. The newer ESP32 is better-equipped and offers the promise of more I/O pins. We made a comparison with these in our original Pico review. The Arduino IDE does a great job of using consistent language across different hardware, making it easy to try other devices if you are already siliconchip.com.au Screen 1: With excellent support for the Pico W in the Arduino IDE via the Arduino-Pico core, the example sketch to scan for nearby WiFi networks was easy to compile and run, producing the expected results. familiar with the ESP8266 or ESP32. Those using MicroPython should have little trouble moving across, too. In the short term, we expect the Pico W’s documentation to be its strength compared to other WiFi-equipped boards, perhaps drawing in a greater share of novices. The continuing parts shortages may give the upper hand to any board which is simply available (as long as pricing remains reasonable), and the Raspberry Pi Foundation appears to have ample supplies of the RP2040 chips; however, the early supply of the Pico W is patchy. In the long term, we think the different boards will find their own niches. For example, the RP2040’s PIO appears to have a lot of untapped potential. The RP2040 also has two cores, although some devices in the ESP32 family also do. We will also keep a close eye on when Bluetooth becomes usable on the Pico W. Conclusion It’s still relatively early days, and we expect that there will be some refinement to the various libraries and other software for the Pico W. There are hints on some forums that the Bluetooth Australia's electronics magazine capabilities of the CYW43439 chip will be put to use. Devices like the ESP32 (which is similarly supported by MicroPython and the Arduino IDE) already offer various Bluetooth capabilities, such as serial and audio over Bluetooth and also Bluetooth Low Energy (BLE). Hopefully, the Pico W will gain similar features. We didn’t run into any problems using the Pico W on our BackPack with an LCD touch panel, although the other PCBs in proximity did seem to attenuate the WiFi signals slightly. With MicroPython and the Arduino IDE long having had support for WiFi and Bluetooth, it won’t be long before we see projects using the Pico W in place of other boards using ESP8266 and ESP32 chips, especially at current prices. At the time of writing, the Pico W was available from: ∎ Digi-Key Electronics (SC0918) siliconchip.au/link/abgw ∎ Core Electronics (CSE08703) siliconchip.au/link/abgx Other retailers we expect might stock the Pico W when it becomes available in volume include Altronics, element14, Mouser and Little Bird Electronics. SC November 2022  59