Start • Typical MCU Tasks • Microcontroller Mode • Watch This! • Put the GP-22050 to Work • Closing the Lid • Sources and PDF

WATCH THIS!

In the case of the GP-22050, arbitrary waveforms are born from a configuration/data file and a raw hexadecimal or binary data file. The GP-22050 package includes a bunch of example files that cover all of the bases. You can enter configuration data to the best of your ability into the 8PI control panel, which is the GUI that aids in the configuration of the GP-22050 (see Photo 1, p. 63). You’ll get a configuration/data file in return by simply saving the configuration data you entered.

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Photo 1—By simply saving this configuration information, I created a configuration/data file with all of the chosen parameters laid out in text form. All that was left to do was fill in the control and data patterns needed to drive the LCD.

The entries you see in Photo 1 show that I considered everything I needed to drive the LCD in 4-bit mode using the GP-22050’s control and I/O interface. As I mentioned earlier, the 6 control/trigger bits beyond the 16 I/O bits offered by the GP-22050’s I/O interface were used for control and triggering purposes. Because I didn’t need any triggering to drive the LCD, I allocated all of the six control pins to control tasks. As it turned out, the only control signal I needed for the LCD was the Enable (E) signal, which I assigned to the least significant bit of the 6-bit control bit chain.

Initializing an HD44780-based LCD is a relatively slow process in the beginning. So, to make sure I could meet the initialization timing restrictions and still operate the LCD fairly quickly, I chose to run the GP-22050’s internal clock at 1 kHz, which resulted in 1-ms clock cycles.

To conserve microcontroller I/O, I run HD44780-based LCD panels in 4-bit mode. I do the same with the GP-22050. The least significant 4 bits of the GP-22050’s 16-bit I/O port are dedicated to the LCD’s D4–D7 data I/O pins, which the LCD uses in 4-bit mode. Bit 4 of the GP-22050’s I/O port services the LCD’s backlight by way of a ULN2003 high-current driver IC. Toggling the LCD’s RS pin differentiates commands and data sent to the LCD.  I assigned bit 5 of the GP-22050’s I/O port to LCD RS pin duty.

After saving the configuration data, I opened the resultant file (lcd_driver.ini) and found the command structure shown in Listing 1. Track through Listing 1 and compare the commands (which begin with the @ character) with the graphic configuration choices I entered in Photo 1. As you can see, Listing 1 is a skeleton template that consists of essential commands that define the GP-22050’s control and data I/O pin assignments as well as the GP-22050’s clock state, operating frequency, and trigger pattern. The next step involved filling in the binary patterns I wanted to see emitted on the GP-22050’s control and data pins.

I started by defining the LCD control sequence to toggle the LCD E pin. Initializing the HD44780-based LCD panel in 4-bit mode required a minimum of a 4.1-ms delay following the first function set initialization command. The wait time between subsequent function set commands was 100 µs. As a result, setting the minimum wait time to exceed 4.1 ms would have been overkill after the first function set wait time had expired. However, I didn’t really care in this case because LCD update speed wasn’t a factor. If faster LCD update speeds were required, I could have simply coded a separate LCD initialization configuration/data file, run it, and then loaded and run another configuration/data file, which would have performed only the LCD data transfer tasks. 

The control sequence defined in Listing 2 was performed each time data was presented on the GP-22050’s data bus. (The complete version is posted on the Circuit Cellar FTP site.) I could have included multiple control files in a configuration/data command set, but only the last control sequence coded into the command set was used. So, only one control sequence was executed per configuration/data file. 

I couldn’t have but one control sequence in my configuration/data file, so I set the dwell before and after the raising of the LCD E signal to three clock cycles, which resulted in a minimum six clock cycle (6-ms) delay between characters and commands issued to the LCD’s HD44780 chipset. That gave the LCD plenty of time to accept and execute instructions in the initialization phase and provide more than enough time for the HD44780 to put characters on the display.

The same command logic that I used in the control area applied in the data area. If you continue along the GP-22050 command chain in Listing 2, you’ll see that I didn’t set the bit to illuminate the LCD backlight until after the LCD initialization was completed. Note also that the LCD RS signal was at a low level for each instruction sent; it went high to enable communication with the LCD’s data registers. The coding of the GP-22050 commands was a bit different than writing code for a microcontroller, but the logic behind the data patterns and the way the data was transmitted to the LCD was very similar to the way a microcontroller would do it.

The control pulse chain produced by the first four @data out commands in Listing 2 is shown in Photo 2 (p. 64). I used a USB-attached, PC-based Cleverscope CS328 oscilloscope to capture both the control pin waveforms and the data presented on the GP-22050’s I/O pins that are configured as data outputs to the LCD’s data bus. 

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Photo 2—The @loop 1 3 command translates to looping the one line immediately following the @loop command three clock cycle times. Because each clock cycle is 1 ms in duration, the E signal is low for 3 ms, high for 1 ms, and low for a minimum of 3 ms following the transmission of a character or command to the LCD. The E signal pulses and data follow the commands in the LCD initialization area of Listing 2.

Analyzing the CS328 display from left to right, the first three LCD E pulses are a result of the @loop 1 3 and @data out 0000000000000011 commands. Data4 in the logic capture area of Photo 2 is physically connected to the LCD DB4 data pin and the GP-22050’s D0 pin. Data5 is connected to the LCD DB5 pin, configured as the GP-22050’s D1 pin, and so forth. The fourth LCD E pin pulse from the left was produced by the @data out 0000000000000010 command. The final two E pulses are clocking out 0x28 to the LCD data pins. In 4-bit mode, the high nibble of each ASCII character is sent first and immediately followed by the least significant nibble 7 ms later.              

I kicked off the capture session in Photo 2 using the Operation window in the 8PI control panel. I selected File mode and loaded the lcd_driver.ini file shown in Listing 2. To capture the first six commands shown in Photo 2, I set the step size value to 6 and clicked the Step button. Clicking the Start button ran the entire 48-command sequence, which resulted in what you see in Photo 3a.

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Photo 3a—Ignore the trash in the background. Focus on the GP-22050 tied to the four-line LCD panel. The GP-22050 package comes with a connector kit that includes three eight-conductor connectors and five two-conductor connectors. The quick clips are the Cleverscope logic probes, which are tied to the four LCD data lines D4–D7. b—And you thought I just sat around writing columns in an ivory tower.