What is the most complicated, fullest featured way you could build out an electronic system around a Raspberry Pi Zero? This.
So, what is this monstrosity? We’ll explain it a little at a time.
Let’s start at the top. This is the power system and serial communications system. It is possible that power can be supplied via the serial communications header similar to Power over Ethernet (PoE). In this case, the “center tap” of the pulse transformer is used to extract the common-mode voltage. This power is then sent to the switch-mode power supply, mainly to buck it down from 12 V or higher down to 5 V. We also have support for battery power (implies buck-boost switch-mode power supply), and DC jack input almost directly to the regulated power mains (there must be a Schottky diode to prevent backfeeding that would violate the plug/socket power supply safety standards). There is also the recommended polyfuse on the power path to the Raspberry Pi Zero. Matter of fact, since we’re likely using 2.5 A max wires and connectors, you might consider placing it in the whole power supply circuit.
For long distance support, we use RS-422 transceivers for the serial communications. RS-485 is not much more difficult to support, so we also support that, plus it allows for a multiple-access shared bus. Finally, we also throw in a few more serial connectors on the other end like RS-232. possibly without a complete transceiver circuit to save cost (and cheap RS-232 ports will not output 12+ V anyways), pin header CMOS serial, and maybe even 3.5 mm audio connector serial.
Now, down to our Raspberry Pi, we have everything connected to it: UART, I2C, SPI, GPIO, USB hub, and Raspberry Pi Camera.
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The sole connection to SPI here is a graphics display, but there could even be a slave selector network for multiple SPI devices.
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I2C at the very minimum has a GPIO expander connected, from which we can connect all kinds of additional devices, such as DC brushed motor drivers, an low-frequency IR transceiver, 1-bit buzzer, and LEDs in particular. The idea is that we only connect devices here that require low frequency control, and high-frequency controls are directly connected to GPIOs on the Raspberry Pi.
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A 4 x 7-segment digital LED display and 4 x 4 matrix keypad is connected via GPIO with pin sharing directly to the Raspberry Pi, but this could also be connected via an I2C controller to save on directly connected GPIO pins, at the expense of more chips and higher cost. The keypad itself is designed to be multi-purpose: telephone, calculator, video game console, hexadecimal input, and so on. N-key rollover is required for video game console use.
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PWM outputs are provided on their own headers. In addition, there may be a selector network to share the two PWM outputs across multiple end points.
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“Analog inputs” are provided as GPIO pins directly connected to the Raspberry Pi. The idea is that we want the fastest possible timing in case we want to use one of the ADC methods that require realtime software timing loops. Otherwise, these are just additional GPIO pins directly connected to the Raspberry Pi for any other use.
Additionally, there could also be a “selector network” connected to the Raspberry Pi as a alternative to an I2C GPIO port expander. This allows for fast sampling from the Raspberry Pi of a larger number of inputs or outputs, so long as they do not need to be used simultaneously.
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One important device we want to connect to analog input is a force-sensing resistor (FSR) so that we can measure the weights of objects.
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Finally, of course, we need to have all kinds of fancy devices connected via USB: Ethernet, Wi-Fi, Google USB TPU, quality A/D audio output + microphone input, video capture input, and so on. We also want to have two USB on-the-go ports so that we can connect directly to the Raspberry Pi Zero rather than through the hub, and maybe auto-disable the hub connection or provide a DIP switch.
In addition, there is a 40-pin expansion header that connects directly to the 40 pins on the Raspberry Pi Zero, intended to allow you to plug in HATs as-is. Of course, you cannot use the full 40 pins in an expansion at the same time you are using all other board functions, we may even consider providing DIP switches on board to switch off the on-board peripherals.
The display-keypad is pretty good, but how could it be better?
- Separate directional arrow keypad with central “enter” button.
- L and R buttons on sides of console.
- 8 to 12 digits.
- Full characters via “union jack” display.
Indeed, if we’re thinking about connecting a much more sophisticated keypad and digit/character display, we would give it a serial protocol and connect through I2C rather than GPIO.
MORE MODULES:
- Parallel port
- Board-level reset.
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All peripherals reset, output pin from Raspberry Pi.
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Motor control selector network. The ability to select a motor to control. ADVANCED: And to hold it at a certain PWM frequency. Otherwise, you do PWM frequency select scanning in software.
Last thing you can do is to have the whole multiplexed controller connected to a serial bus like I2C rather than direct parallel connection.
And this is to say that for most practical applications, you will not need to use all such functions all at once, but onl a subset of available functions will be required.
Yes, in general, the way to build ultra-sophisticated computer controlled systems is to use serial buses. All the way down. Until you get to the bottom where you have multiplexers, shift registers, daisy chaining, and matrix networks.
- NOTE: Multiplexers are generally easier to program with than shift registers because they are stateless. For many simple circuits, they can be used with the same number of pins as would otherwise be required to control a shift register. The disadvantage, of course, is that a multiplexer has no memory, so you cannot hold multiple outputs high simultaneously.
What can a serial motor controller do? With only two-byte commands, you can command and control a full 256 motors. Wow, and that’s all through the magic of serial communications. Makes a tough challenge look tiny.
Likewise, you can have multiplexed analog inputs that can share a small number of, or even just one, ADC, if you don’t need simultaneous sampling.
Keyboards and switch matrices are easy to connect. Read state command method, 1 byte covers 256 switch states. Send delta event method, 1 byte covers 128 switch states.
Force-sensing resistor matrices require a bit of finesse, use Schottky diodes and full diode isolation, but it can be done.
Capacitive sensing matrices, well now that is hard. With diodes current can only flow in one direction. When a capacitor is charged up, current can no longer flow. But then how do you discharge it? It will just hold its charge and only leak slowly if you do nothing else.
Connections within a single box electronic system, short distance:
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If you would normally do a direct parallel connection via GPIO pins, but you’ve got too many pins to attach to the processor’s directly available GPIO pins, add a microcontroller on the I2C bus.
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If you need more speed than what I2C offers, move to SPI.
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If you need more speed than what SPI offers, move to inter-chip USB.
20191218/https://en.wikipedia.org/wiki/InterChip_USB
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If you need more speed than what USB offers, well then you’ve really got to prepare for a heck of an electrical engineering challenge with high frequency circuits. The PCI family of buses is the next faster choice.
Connections between different electronic box systems, mid distance:
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Serial communications: RS-232, RS-422, RS-485.
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CAN-Bus
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USB.
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Ethernet.
Connections between different electronic box systems, long distance:
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Serial communications: RS-422, RS-485.
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Ethernet.
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Coaxial.
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Fiber optic.