Contactless Data

In the first use case, the smartphone was the host and the Raspberry Pi was the device. Now, the small-board computer (SBC) is going to assume the host role. (SBC will always refer to the Raspberry Pi, not the Pi Pico.) A typical use case is data logging over Bluetooth. For this purpose, the microcontroller writes its output to the serial port in the usual way, but instead of a cable, an HC-05 serial Bluetooth breakout module is connected (Figure 4). This device implements universal asynchronous receiver/transmitter (UART) communication over Bluetooth. The big advantage is that you don't have to change the program on the microcontroller because it already writes its data to the serial port.

Figure 4: The HC-05 Bluetooth module for microcontrollers.

The breakout comes in different versions. Exactly which you have hardly matters in the end. The board typically has a button, an LED, and a voltage converter, which makes the power supply flexible. However, the signal level on the RX/TX lines is always 3.3V, so if you connect an Arduino, you should adjust the voltage with a resistance division circuit (5V to 3.3V ) between the Arduino TX pin and the module RX pin.

The HC-05 supports the command and data modes. The first mode is for configuration; for new devices, it is important to complete the steps described in the "Configuring HC-05" box. You also need to pair the Raspberry Pi and the breakout once, as described earlier, remembering the MAC address of the HC-05.

$ miniterm --eol CRLF -e /dev/ttyUSB0 38400

01 --- Miniterm on /dev/ttyUSB0  38400,8,N,1 ---
02 --- Quit: Ctrl+] | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
03 AT
04 OK
05 AT+NAME?
06 [...]
07 AT+NAME=PICO_42
08 [...]
09 AT+PSWD?
10 [...]
11 AT+PSWD="9999"
12 AT+UART?
13 [...]
14 AT+UART=115200,0,0

After completing the preparation work, the application is child's play. Connect the HC-05 to the microcontroller; it outputs its data in the usual way with a print or printf function. You now connect the Raspberry Pi to the HC-05 and redirect incoming data to a file with cat:

$ sudo rfcomm bind hci0 <aa>:<bb>:<cc>:<dd>
$ cat /dev/rfcomm0 > <MyData>.log

The value <aa>:<bb>:<cc>:<dd> is the MAC address of the breakout. If the microcontroller also processes input, commands can be sent (e.g., to /dev/rfcomm0) by calling a terminal program with the device file as an argument.

Instead of the Raspberry Pi, you can connect to the HC-05 from the Android app, which proves useful for a quick test, because you can immediately see whether the Pi Pico is sending data. Also, the Raspberry Pi or app can send data to the microcontroller (e.g., to turn LEDs and motors on/off, etc.) by the HC-05. However, the microcontroller needs to both write to the serial interface, and read from it.

Pico as a Host

The previous scenario showed how the Raspberry Pi fetched data from the Pi Pico, and it works well as long as the microcontroller is already running and the connection from the Raspberry Pi to the Pico works. However, if the microcontroller is not running, the Raspberry Pi constantly has to try to re-establish the connection in a loop.

Regardless, you have another problem: If the Pi Pico sends data only now and then (e.g., because of large intervals between reading sensors), then the Raspberry Pi can only reliably retrieve data by keeping the connection constantly open. In this case, it makes more sense to let the Raspberry Pi and the Pico swap roles so that the microcontroller actively creates the connection. The Raspberry Pi then runs a service that accepts connection requests and fields and stores the data.

For this scenario, the HC-05 must play host and establish the connection, which is precisely what can turn out to be a major obstacle because the breakout comes in different versions. Many postings online describe the procedure with firmware from 2010. However, all of my devices use a firmware version from 2017 that behaves differently when you get into the details. The procedure described here will not necessarily succeed with your device, but you can query the firmware version in command mode with the AT+VERSION? command.

Also in command mode, you can change the role of the HC-05:

AT+CMODE=0
AT+ROLE=1
AT+BIND=4D65,4D,CA6612

The first line tells the chip to only connect to a specific address (given in line 3), the second line sets the host mode, and the third line binds the target device address – the Bluetooth MAC address of the Raspberry Pi, which you query with:

echo list | bluetoothctl

However, instead of using the colon-separated format seen earlier, the BIND command requires the information in a slightly different format, so 4D:65:4D:CA:66:12 becomes 4D65,4D,CA6612.

Before rebooting, make sure the Raspberry Pi is ready to connect, and it works exactly as described in the first use case. Because microcontrollers usually send data and not commands, you do not need an application program with logic on the Raspberry Pi side, so in the simplest case, you only need a redirection to a file:

$ sudo rfcomm watch hci0 | cat /dev/rfcomm0 >> pico-data.csv &

If you now start the Pico with the HC-05 connected, all data should end up on the Raspberry Pi, unless it is not available, then the HC-05 will abort the connection. Even if the Raspberry Pi is online later, the connection is not established because the HC-05 does not retry. For a robust process, the Pi Pico needs to switch on the HC-05 before each connection attempt and then switch it off again. If the microcontroller reads and sends data from a sensor at fixed intervals, this approach also saves power.

Collecting data from multiple Picos with the Raspberry Pi requires a bit more effort. The RFCOMM server [4] could be useful as a blueprint. Note that the Raspberry Pi only maintains one connection at any given time. Theoretically, more connections would be possible with Bluetooth and the SBC, but both the HC-05 and the Android apps always contact the device on default SPP channel 1. Of course, this would be fine for sensors that only occasionally send short signals, as long as the Pi Pico quickly releases the connection again after the data has been delivered.

Conclusions

From the third generation onward, the Raspberry Pi comes with Bluetooth built in. Therefore, you can control the SBC from an external device and control any microcontroller with the Raspberry Pi. The HC-05 does a good job, because the chip removes the need for you to plumb the depths of the Bluetooth protocol. Apart from a one-off configuration, you're left with just a little wiring. Older Raspberry Pis can be upgraded for a small charge by plugging in a USB Bluetooth dongle.

Classic Bluetooth, though, has disadvantages. The power consumption in particular makes the technology uninteresting for IoT devices. Bluetooth Low Energy (BLE) steps in with a protocol variant optimized for minimal power consumption. How you get the various devices in your home IT zoo to work with it is the topic for a future article.