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USB Digital and SPI Interface Board We originally designed this simple, low-cost interface to control the Christmas Light Tree elsewhere in this issue. But then we realised that with a minor tweak here, a slight adjustment there, we would have a general-purpose controller which could handle up to eight digital lines from your PC, including an SPI serial interface. So here it is: use it for the LED Christmas Light Tree or anything else that comes to mind! by Tim Blythman T his small board uses a low-cost PIC16F1455 microcontroller, which incorporates a USB interface, to drive up to seven digital outputs and one input, including three used for SPI (serial peripheral interface) communications. This means that you can use it to control some external circuitry easily from your PC. The LED Christmas Tree earlier in this issue has an SPI-compatible interface and so it can be controlled using this board but there are many other ICs which also use an SPI bus. As a bonus, if the Interface Board is powered up but not connected to a computer (say, it’s connected to a USB phone charger) it will output random patterns to allow the LED Christmas Tree to be used without a computer. So if you want to develop a project around one of those ICs, this board would be a really easy way to experiment with such chips and test them out. It can even be used to drive colour TFT LCD screens as many of these are based around an SPI interface, with the addition of a few digital control pins; this board can also drive those pins, using its four extra digital outputs. You could also drive a standard alphanumeric LCD using this module. They typically require around 7 digital control pins; four for data and three for clocking/control. Luckily, that’s exactly what this board can provide. You can even use it to communicate over an I2C interface. It’s based on a PIC The PIC16F1455 used in this project is one of the smallest (and cheapest) PICs with a USB interface and impressively, it only needs three other components to work. You don’t even need to solder a USB socket onto the PCB (although there is space to do so), as we’ve made the end of the PCB into a plug that will fit into a standard USB-A socket. It isn’t completely compliant with the USB specifications, but it’s a technique that is quite widely used and it works fine. Just keep in mind that the copper tracks can wear out if you’re plugging and unplugging it a lot. In that case, a proper USB connector would be the way to go. You might remember that the PIC16F1455 was used as the basis for the popular Microbridge PIC Programmer and USB Serial Converter, from the May 2017 issue (siliconchip.com. au/Article/10648). The software we are using here is similar, in that it presents itself to the host computer as a serial port, but instead of producing a serial UART stream (compatible with RS-232), it generates an SPI stream instead. What this means is that any pro- Here the Interface Module is driving the Stackable LED Christmas Tree, using CON4 to make a direct connection. 32 Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.1: the circuit diagram for the Interface Board. IC1 is programmed to provide a USB interface via CON1, which can be either an SMD USB socket or tracks on the PCB which fit into a USB port. All of the PIC’s free pins are wired to CON3 and CON4, to provide seven programmable digital outputs as well as an SPI or I2C serial bus, to communicate with external circuitry. gram that can interface with a serial port on your computer can be used to control most SPI devices, including our Stackable LED Christmas Tree. We’ve provided a sample program in the Python language to use with our Christmas Tree but you can also use a terminal program such as the Arduino Serial Monitor to test out the commands and manually send SPI data. Two control modes The way the Digital Interface Board works is as follows. If it is configured with a board rate of 9600, then the interface works in hexadecimal SPI mode. If the baud rate is set to 19,200, then it works in binary SPI mode. A baud rate of 38,400 selects I2C mode (the data rate is 400kHz). Note that these baud rates do not affect how fast the data is clocked out; they are just a convenient way of signalling to the Digital Interface Board which mode you want to use. Table1: pin connections and control characters Hexadecimal SPI mode Three of the seven digital output pins on the board can be used as an SPI serial bus. They are labelled SCK (the serial clock), MOSI ([data] master out, slave in) and MISO ([data] master in, slave out). In hexadecimal SPI mode, the unit accepts the hex digits 0-9 and A-F (and their lowercase equivalents) over the USB serial interface. The letters T-Z and t-z are also accepted, as described below. Any hex digit received will cause four bits of SPI data to be transmitted on the MOSI and SCK pins, with the most significant bit being sent first. Any data that is received on the MISO pin (pin 10, RC0) is read back simultaneously with data being transmitted on MOSI. In this case, a hex digit is echoed back to the serial monitor. The characters T-Z and t-z can be used to set the state of the pins directly, with the uppercase character Control characters IC1 pin pin CON3 pin CON4 pin t/T u/U (input only) v/V w/W x/X y/Y z/Z 3 (RA4) 2 (RA5) 10 (RC0, MISO/DO) 9 (RC1, LT) 8 (RC2, MOSI/DI/SDA) 7 (RC3, SCK/CK/SCL) 6 (RC4) 5 (RC5) 9 10 3 4 5 6 7 8 4 5 3 6 - siliconchip.com.au Australia’s electronics magazine setting the pin to high and the lowercase character setting the pin low, as shown in Table1 below. This feature is used with the Christmas Tree to latch the data when required. Any other characters received over the USB interface are ignored. For example, using the hexadecimal mode, we can turn off all the LEDs on a single board of the LED Christmas Tree by sending the string “v00V”. This brings pin 9 low, then sends eight bits of zeros over the SPI bus, then brings pin 9 high, transferring the shifted data into the device’s output latches. Similarly, sending the string “vFFV” will turn all the LEDs on. Because other characters are ignored, line endings don’t matter and practically any terminal program can be used to send this data. Note that there are only seven pins listed in Table1 because the eighth pin, pin 10, is only used as an input and only in SPI mode. Binary SPI mode In binary SPI mode, we take advantage of the fact that USB data is sent in packets. Each time the Board receives a packet from the host, it sets LT low, clocks out the data using SCK and MOSI and then sets LT high again. It also reports serial data received on the MISO pin back to the host in binary format. November 2018  33 Parts list – USB/SPI Interface Board 1 double-sided PCB coded 16107182, 55mm x 28mm 1 PIC16F1455-I/P microcontroller programmed with 1610718A.HEX (IC1) 1 14-pin DIL IC socket (optional, for IC1) 1 mini USB type B SMD socket (CON1b; optional) 1 5-way right-angle (or straight) pin header (CON2, ICSP; optional) 1 10-way pin header or socket (CON3) 1 6-way female header socket (CON4) 2 100nF MKT capacitors 1 10k 1% or 5% resistor, 1/4W or 1/2W While this mode provides faster communications, it can only be used with a host terminal program that sends multiple bytes together, so that the data is received by the unit as a single packet. This is possible with the Arduino Serial Monitor, provided that line endings are turned off, as these will otherwise appear as binary data to the unit. If you are driving a Christmas Tree board in binary mode and see LED2 and LED4 on when you are not expecting them to be on, that indicates that you may have line endings still turned on, as this combination corresponds to the character that is used to terminate a line (line feed [LF], binary 00001010, ASCII code 10). While trickier to use manually, this mode is more convenient for writing software which delivers data to the serial port directly. Hexadecimal I2C mode To make this board even more flexible, we have also added an I2C mode. In this mode, RC2 is used as SDA while RC3 is used as SCL. To use it, you write one or more bytes to the serial port in hex format (ie, pairs of characters 0-9 or A-F), followed by a newline. When the newline character is received, the previous bytes are transmitted over the I2C bus. Alternatively, you can prefix the bytes with “S” to start communication and follow with “P” to finish. The first byte contains the 7-bit device address plus one bit to indicate read or write mode. The board scans this byte to determine whether you are doing a read or write and acts accordingly. Each byte read is followed with a “K” to indicate if an ACK signal was received or an “N” if it did not receive the ACK. In read mode, after the initial address byte, you simply send “FF” for each 34 Silicon Chip byte you wish to read back. The response will then be read back and displayed along with the ACK/NACK indicators mentioned earlier.. The Board also supports 10-bit addressing mode. In this mode, the top five bits of the address byte are 11110, and a second address byte follows. It will detect this and act accordingly. The clock rate for I2C mode is always 400kHz. Note that there are no I2C bus pullup resistors on the board. If your slave lacks pullups you will need to fit some yourself. Check the I2C specifications to determine the correct pull-up resistors to use for your circuit. Circuit description The circuit of the Digital Interface board is shown in Fig.1. A 10k pullup resistor from pin 4 (MCLR) of IC1 to VCC enables the power-on reset feature and allows for normal operation of the chip after power is applied. One 100nF capacitor between VDD (pin 1) and VSS (pin 14) provides overall supply bypassing while another capacitor from pin 11 (VUSB3V3) to ground filters the internally generated USB 3.3V supply. The proper USB socket and PCB track socket are wired in parallel, with the D- and D+ signal lines going to pins 12 and 13 of IC1 respectively. The software sets these pins to operates as USB signal lines rather than general purpose I/O pins. An optional six-pin header in-circuit serial programming (ICSP) is provided (CON2), to allow IC1 to be programmed in situ. If you’re using a pre-programmed chip, you can leave CON2 off the board. Finally, CON3 and CON4 break out the digital I/O pins. 10-pin header CON3 provides connections to GND (0V) and the USB 5V rail, as well as the eight I/Os that the unit can control (RC0-RC5 and RA4-RA5). Australia’s electronics magazine By comparison, 6-pin header CON4 only includes the four signal connections which are required for SPI or I2C communications, along with the GND and 5V connections. This suits the LED Christmas Tree board, which can be plugged straight into this header. But it could also be used in any other situation where you just need to communicate with an SPI or I2C device. As mentioned earlier, the RC0 pin on IC1 is used as an input only, in SPI mode, while the other seven pins are digital outputs. Outputs RC2 and RC3 can be used for either SPI or I2C serial communications, or as general purpose I/Os. Construction Use the PCB overlay diagram, Fig.2, as a guide during constructions. The USB Digital and SPI Interface Board is built on a PCB coded 16107182, which measures 55 x 28mm. If you intend to install the optional USB socket, we recommend doing that first, before any other components are in the way. To do this, the USB projection on the PCB needs to be snapped or cut off; otherwise, it would foul the ca ble. Firstly, score along the line of “mouse bites” to help the PCB break cleanly. This will also help to sever the PCB traces so that they don’t tear when the board comes apart. Flex the board at the score line and it should snap. Clean up any rough edges with a file. The USB socket is the only SMD component used. We recommend that you put a thin smear of flux paste on the pads before soldering. The socket has plastic pegs on its underside to locate it on the PCB. Once positioned, ensure it is flat and solder the large mechanical tabs on the sides to lock it in place. Here’s an enlargement of the USB “plug” section of the PCB, which is removed if a micro USB socket is used (as shown opposite). Score along the “mouse bite” holes before snapping this section off and clean the edge with a file. siliconchip.com.au Fig.2: it doesn’t get much easier than this. IC1 is the only polarised component; make sure to fit it with the orientation shown here. You can use a socket if you don’t want to solder the chip directly to the board. The ICSP header, CON2, is not required if your micro has already been programmed. With a clean, fine tip loaded with a bit of extra solder, carefully apply the iron to the pins and pads. The flux should draw the solder up and onto the pins. Solder all the pins and inspect them to ensure there are no bridges between adjacent pads. If there are bridges, remove them with some additional flux paste and a piece of desoldering braid (solder wick). Required components There is just one resistor on the board, so solder that in place next. Follow with the two identical capacitors. None of these components are polarised. If you are using an IC socket for IC1 (which is handy if you plan to use an external programmer), fit it next, ensuring the notch is facing towards the top of the board, as shown in Fig.2. If you will be plugging CON4 into a Stackable LED Christmas Tree board, you should ensure that it will line up nicely before soldering it. We suggest that you solder one pin in place and then check the alignment is correct before soldering the rest. You don’t need to fit CON2 if you have purchased a pre-programmed PIC. But note that even if you will be using it to program IC1, you can plug it in and hold it in place while programming the chip, then remove it. If you are programming IC1 using an external programmer, do The simplest connection method is to plug the PCB straight into a USB port, but if you fit a socket as shown here, the result is a bit more robust. It also makes the board slightly more compact. siliconchip.com.au Programming the PIC so now (see panel for instructions), then plug the programmed chip into the socket. Or, if you’re not using a socket, solder it to the board now but make sure it is orientated correctly first. Using it To use the Digital Interface Board to drive the Stackable LED Christmas Tree, plug the root board of the tree into the six-way socket on the Interface Board, with both boards facing up so that the pin names match. Plug the Interface Board into a USB port on your computer and open a terminal program such as the Arduino Serial Monitor, PuTTY or TeraTerm. Select a baud rate of 9600 (usually the default). Type “vFFV” into the terminal and press Enter. All the LEDs should light up on the root board, indicating that it’s all working properly. Typing “v00V” and pressing enter should cause all the LEDs on the root board to switch off. If your tree has multiple boards, use a longer string such as “vFFFFFFV” (which suits three boards). Each hex digit corresponds to four LEDs so you will need two hex digits for each board in the tree. If you don’t supply enough hex digits, the furthest downstream boards will be fed old data from other upstream boards. To use this board to drive a different SPI or I2C device, refer to Table.1 to figure out which connections on your device need to go to which pin on CON3 or CON4. You can then set the baud rate to any of those mentioned under the “Two control modes” cross-heading above and then use a terminal program as described to send test commands and check the responses. It’s much easier to use the hexadecimal control modes initially to test the unit out, even if you’re planning on using the binary SPI mode later. Australia’s electronics magazine If you have a blank PIC, you can program it using a PICkit 3 or PICkit 4, in conjunction with the MPLAB X IPE (Integrated Programming Environment) software. This is bundled with the MPLAB X IDE (Integrated Development Environment), which can be downloaded from siliconchip.com.au/link/aalr Having installed it, launch the IPE program. From the Setting Menu, select Advanced Mode and log in using the default password. Click the Power button on the left and ensure “Power Target Circuit from Tool” is ticked. Click the Operate button and select PIC16F1455 from the Device list, select your programmer from the Tool list and click Connect. Once it indicates success, use the Browse button to select a source HEX file and open the HEX file from the software download file. Connect the programmer to the PCB, ensuring that the arrowed pin on the programmer lines up with pin 1 (arrowed) on the PCB. Click the Program button and check that the messages in the bottom of the window indicate that IC1 was successfully programmed. To test the chip, unplug the programmer and connect the board to a USB socket. Your computer should show that a new USB serial port has been detected. Python program We have provided a small example script written in the Python programming language to drive the LED Christmas Tree using this Interface Board. You will need some Python experience (or at least some script programming experience) to modify it. The Python language can be downloaded from www.python.org/downloads/ You will also need the pyserial library to access the serial port. This can be added by running the following command from the Python command line: pip install pyserial Download the program, “Serial Tree. py”, from the SILICON CHIP website and change the port name to suit your system (eg COMx on windows, /dev/ttySx on Mac/Linux), and then run the program using the Python interpreter. It generates random patterns to give a twinkling effect. SC November 2018  35