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U s in g Che a p A s i a n ic on r t c e l E es l u d o M Par t 5 The “New Blue” 16x2 LCD module with piggy-back 2 I C serial interface by JIM ROWE This module combines a 16x2 backlit alphanumeric LCD module with a small “piggy-back” module that provides it with an I2C serial interface. This allows it to be hooked up to any of the common micros via only two wires, letting multiple displays (or other I2C devices) share the same 2-wire bus, while also freeing up some of the micro’s I/O pins for other purposes. L CD modules with two lines of 16 characters have been around for many years and we’ve used them in numerous projects. They are also now much cheaper due to being popular for use with Arduino, Micromite and the Raspberry Pi. However, many of these Arduino and other micros are a little limited when it comes to I/O pins, which means that the six or seven pins required to interface to a standard LCD module can leave you with too few pins to interface with other components. POWER LED1 CON1 4 3 2 1 2x 4.7k 3x 10k 13 SDA 15 SCL 14 1 2 I 2 C ADDRESSING 1 1 0 0 1 1 0 0 1 0 1 0 1 0 1 0 BACKLIGHT ON/OFF INT P0 P1 SCL P2 3 P3 P4 A0 P5 A1 P6 P7 A2 4 5 6 7 9 10 11 12 15 2 4 SDA IC1 PCF8574T 1 1 1 1 0 0 0 0 100nF λ 1k 16 Vdd GND HEX ADDRESS 27 26 25 24 23 22 21 20 A K VCC A2 A1 A0 This problem can be solved by using an LCD with a serial interface or alternatively, attaching a small piggy-back module to a parallel LCD to provide serial/parallel translation. By using a piggy-back module that communicates using the 2-wire I2C 6 RS EN Vdd ABL 16 x 2 LCD MODULE CONTRAST D7 D6 D5 D4 D3 D2 D1 D0 14 13 12 11 10 9 8 7 GND R/W 1 LCD CONTRAST 3 VR1 10k KBL 5 16 C E Q1 S8050 4.7k B Vss (LINKS OUT = 1 IN = 0) 8 S8050 J3Y B A2 A1 A0 NOTE: LCD MODULE HAS 100Ω RESISTOR IN SERIES WITH BACKLIGHT ANODE (PIN 15) C E Fig.1: complete circuit for the piggy-back and LCD module together. Some of these modules use a slightly different chip for IC1, that being the PCF8574AT, the main difference being the hex address range is instead between 38h and 3Fh. 82  Silicon Chip siliconchip.com.au The circuit of Fig.1 shows the LCD module at upper right, with the rest of the circuitry being that of the piggyback, which connects to the module via the usual 16-pin SIL connector along the top. All of the serial-to-parallel conversion is performed by IC1, a Philips/ NXP PCF8574T device. This is designated as a “remote 8-bit I/O expander for the I2C bus”. In other words, it accepts serial data over the I2C two-wire bus, via pins 14 and 15, and it makes the data available in parallel format at pins 4-7 and 9-12. In this case, output pin 4 is used to control the LCD’s RS (register select) control pin, while pin 6 controls the EN (enable) pin and pins 9-12 feed the character codes to pins D4-D7 of the LCD. That leaves pin 5 of IC1 to control the LCD’s R/W pin, and pin 7 to control the LCD backlight via transistor Q1. What about pins 1, 2 and 3 of IC1? They’re used to set the address of IC1 on the I2C bus. All three pins have 10kW pullup resistors connecting them to logic high (VCC) but the LCD module PCB also provides three pairs of tiny pads so that any of the pins can be tied to ground. This allows the chip’s I2C address to be set to any hexadecimal value between 0x20 (32) and 0x27 (39), just by bridging the pairs of pads, as shown in the small table at lower left in Fig.1. So the default I2C address of the piggy-back module (and thus LCD) is 0x27 with all links out but this can be changed to 0x20 simply by fitting all three links, or to any address in between by fitting one or two links. This allows a number of the LCD-piggyback combinations to be connected to the same I2C bus, with each one given a different I2C address so that the micro driving the bus can send data to any one it chooses. Other I2C devices can reside on the same bus (eg, temperature sensors, memories, other microcontrollers), as long as you ensure that no two devices have the same address. siliconchip.com.au ADC5/SCL ADC4/SDA IO2/PWM ADC3 IO3/PWM ADC2 GND IO4/PWM ADC1 VCC IO5/PWM ADC0 SDA IO6/PWM IO7 IO8 ARDUINO UNO, FREETRONICS ELEVEN OR DUINOTECH CLASSIC IO9/PWM/MOSI LCD WITH I2C SERIAL BACKPACK 16 x 2 LCD SCL VIN GND GND IO10/PWM/MISO +5V IO11/PWM/SCLK +3.3V IO12 RESET IO13 +5V GND AREF SDA SCL DC VOLTS INPUT What’s inside IO0/RXD IO1/TXD USB TYPE B MICRO protocol, you end up with an LCD that can be driven using just two wires: one for the serial data (SDA) and the other for the serial clock (SCK). That’s apart from the ground and power wires (typically +5V). There are some serial I2C LCD modules that use a slightly different chip for IC1, the PCF8574AT. This is virtually identical to the PCF8574T shown in Fig.1, except that the I2C address range is between 0x38 and 0x3F. By using a combination of the two chips, up to 16 different serial I2C LCDs to be connected to the same I2C bus, provided you use eight with the PCF8574T bridge chip and eight with the PCF8574AT chip. Fig.1 also shows that the piggy-back has a power-on indicator (LED1), a 2-pin SIL connector and jumper shunt which can be used to disable the LCD’s backlight if not required. Trimpot VR1 which can be used to adjust LCD contrast in the usual way (via pin 3). Note that the SDA and SCL lines connecting between pins 1 and 2 of CON1 and pins 14 and 15 of IC1 are each fitted with a 4.7kW pull-up resistor, as the I2C bus uses active-low log- Fig.2: pin connections for the LCD and piggy-back module to an Arduino or compatible device. Instead of passing the SDA/SCL to the ADC4/ADC5 pins on the Arduino, it can be connected to the SDA and SCL on the other side of the Arduino. For most Arduino boards, these pins are normally connected in parallel. ic. These resistors can be left in place if the module is the only slave device connected to the I2C bus. But if you’re going to be hooking up other I2C slave devices to the same bus, all but one should have the SDA and SCL pull-up resistors removed. Using it This type of module really needs to be hooked up to a micro, and that turns out to be fairly easy to do with any of the popular micros. All you have to do is connect the VCC and ground pins to a suitable voltage source (which may be the same one that’s powering the micro) and the SDA and SCL pins to the I2C bus pins on the microcontroller. Fig.2 shows how this is done with an Arduino Uno or a compatible like the Freetronics Eleven or Duinotech Classic. It couldn’t be much simpler. By the way, although the SDA Reading Hexadecimal Numbers In this article values prefixed with “0x” correspond to a hexadecimal number; you might also see values suffixed or prefixed with “h”. Reading from left-toright each character corresponds to a 4-bit long value. With 0-9 being equal to themselves and A-F (case-insensitive) are equal to 10-15 respectively. A hexadecimal value is calculated as if each character is appended to the other to form one long string of bits. Ergo, 0x5A (or 5Ah) is equivalent to 01011010 in binary and 90 in decimal form. A string of bits can be read as the sum of each individual non-zero bit, with each bit being equal to 2n-1 while n is the index of that bit starting from the right. So 0101 is equal to 23-1 + 21-1 = 4 + 1 = 5. A longer example BCDEh would just equal to 48350 in decimal and 1011110011011110 in binary. March 2017  83 The underside of the LCD module’s PCB has the piggy-back module (black) located above it. The jumper shunt located on the piggy-back module can be used to disable the LCD backlighting if it’s not needed. and SCL pins of the LCD module are shown in Fig.2 connected to the ADC4/SDA and ADC5/SCL pins at upper right on the Arduino, they could instead be connected to the pins marked SDA and SCL on the other side of the Arduino down near the USB connector. On most Arduino boards, these pin pairs are connected in parallel. It’s just as easy to connect the serial I2C LCD module to a Micromite, as you can see from Fig.3. Of course, connecting the module up to a micro is only half the story. Then you have to work out how to get the micro to send it the data you want displayed. The complicating factor here is that quite a few people have written “libraries” to make it easier to drive this kind of serial I2C LCD module from an Arduino sketch, by providing a set of simple function calls like: lcd.print(“Text”); And so on, which is all very well, but even though most of these library files have the name “LiquidCrystal_ I2C.h”, they are often different in terms of their finer details and compatibility with any particular serial I2C LCD module. Rather than you going through the same sort of hassles we did to find a suitable library, we’ll simply point you at some that we found to work. These are available at the following links: https://github.com/fdebrabander/ Arduino-LiquidCrystal-I2C-library https://github.com/marcoschwartz/ LiquidCrystal_I2C It’s possible that these are actually the same library, because in one place we found the author listed as Frank de Brabander but the maintainer as Marco Schwartz. We found both through the following website: www.arduinolibraries.info/libraries/ Anyway, these libraries do seem to work with the module shown, as +5V +3.3V 26 GND 25 VCC 24 SDA 22 SCL 21 MICROMITE 17 16 14 10 9 5 4 3 RESET 84  Silicon Chip How about a Micromite? Programming a Micromite to talk to the I2C LCD module is not quite as easy as with an Arduino, as currently the inbuilt MMBasic LCD commands only support the parallel interface. You will find a program called I2CLCD.bas in the MMBasic Library, which can be downloaded in zipped-up form from the bottom of this page: http://geoffg.net/maximite. html#Downloads However, this program was written for a piggy-back module with a different configuration than the one which most piggy-backs seem to use (and we have shown in Fig.1). Then there’s a further issue in that the I2C command syntax has changed as MMBasic has evolved. As a result, we ended up having to re-write the software completely. Changing over the program’s commands to suit the different connections between the PCF8574T bridge chip Where To Buy GND 18 you’ll find out by downloading the “Hello World” sketch (HelloWorld. ino) from the Silicon Chip website (www.siliconchip.com.au) and running it. We’ve included a copy of the library (as a ZIP file) within the package. The resulting display is shown in the adjacent photo. Incidentally don’t forget to change the I2C address shown in the sketch (0x27) to 0x3F (= 3Fh), if your piggy-back module is fitted with a PCF8574AT instead of a PCF8574T. You’ll also have to change this address if you have changed the address using the three small pairs of pads. LCD WITH I2C SERIAL BACKPACK 16 x 2 LCD (SDA) (SCL) Fig.3: pin connections for the LCD and piggy-back module to a Micromite. We’ve stocked some of these modules in our Online Shop so that you can acquire and experiment with them. Alternatively, you can find similar units (either pre-assembled or as two separate items) on eBay and AliExpress, and also 20x4 character I2C LCDs which cost very little more than the 16x2 types. The piggy-back should also work with 20x2 and 16x4 size alphanumeric LCDs, however, these are far less popular than the other two sizes. siliconchip.com.au Silicon Chip Binders REAL VALUE AT $16.95 * PLUS P & P The top of the LCD module. The screen is mounted on a PCB measuring 80 x 36mm, while the visible area of the LCD measures 64 x 14.5mm. and the LCD module itself wasn’t too hard. The major difficulty was in getting the program to initialise the LCD’s controller correctly. The correct set-up commands have to be sent to it soon after power is applied, and these commands have to be sent in a particular order, with pauses between them to allow the controller to process them before the next command arrives for correct operation. After downloading as much information as we could find regarding the correct initialisation sequence and timing for the Hitachi HD44780U and Samsung KS0066U LCD controller chips (which seem to be the two most commonly used in current alphanumeric LCD modules), we were finally able to get the program working correctly and reliably. We found this website most helpful: http://web. alfredstate.edu/weimandn/lcd/lcd/ lcd_initialization/ Basically, our program (called JRI- 2CLCD.bas) just displays a “Hello, world!” message over and over on the LCD; just like the one for the Arduino. You can download this from our website, open it in MMEdit and then upload it to your Micromite and you should get the same display as shown in the photos. As with the Arduino sketch, you may need to change the I2C address given for your display’s piggy-back, if it has some of the address links fitted or is using the PCF8574AT chip instead of the PCF8574T. Look for this line near the start: DIM AS INTEGER I2CAddr = &H27 ‘ (A2=A1=A0=1) All you need to do is change ‘&H27’ into the correct address for your module. This program provides a good starting point for writing your own MMBasic programs using an I2C LCD. It’s fairly well commented, so you should be able to see how to adapt the program to display other things. SC Serial USB-UART bridge module – another version Since writing the third article in this series (for the January 2017 issue), we’ve become aware of another popular version of the serial USB-UART bridge module based on the CP2102 device. This one is very similar to the one we discussed in the January 2017 article, but differs in two respects. One is that instead of a micro-USB socket on the USB end of the module, it is fitted with a full size type A USB plug – providing a more rugged connection and compatibility with a standard USB type A to type A extension cable. The other difference (wait for it!) is that the connections to the six pins of the SIL connector on the other end of the module are NOT the same as those on the smaller module. So make sure that you allow for the differing SIL pin connections when you connect the module to your micro or other device. siliconchip.com.au Are your copies of SILICON CHIP getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of SILICON CHIP. They feature heavy-board covers, hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for overseas prices. March 2017  85