The purpose of this page is to describe the software design for the Nextion display. The display is programmed in Nextion Editor, which is a configuration tool produced by Nextion. The module has a single, two-wire UART port, which is used to program the display and communicate with the STM32 PCB. The module connects to Nextion Editor with a USB to Serial converter, which is shown in Figure 1 below.

Figure 1. USB to serial converter used to program the Nextion display.

As shown above, this particular USB to Serial converter, which ships with the Nextion display has three switches. This allows the module to be used to convert USB to other forms of two-wire communication, such as RS-485 and TTL. To successfully connect to the Nextion, the switches on the USB to Serial converter must be toggled to the states shown in Figure 1: Switch 1 on, Switch 2 off, and the third switch directed away from the TTL label.

The table below describes the different variables and software objects used in the Nextion configuration file. Each object belows to the page that it was created on. Global variables can be accessed on any page and local variables can only be accessed on the page that it was defined on.

Object Name	Type	Scope	Description
page0.va0	Variable	Global	Stores the current drive mode. Identical to buttonState in Interface.cpp, 1->Direct, 2->Coast, 3->Regen, 4->Boost, 5->Pedal.
page0.va1	Variable	Global	Vehicle speed, in miles per hour, integer form. This value is used by the Plot page to chart speed vs. time.
page0.tm0	Timer	Local	This timer object has a period of 50 ms. Every 50 ms, the green bars for the speed and accumulator pressure are updated based on the current speed and pressure.
page0.n0	Number	Global	Accumulator pressure, in psi.
page0.x0	Float	Global	Vehicle speed, in miles per hour, float form. This value is only used on the home page to add resolution to the speed measurement.
page4.tm0	Timer	Local	This timer object has a period of 50 ms. It is used to update the waveform. Every 50 ms, the plot is updated with the current vehicle speed and the distance traveled is calculated.
page4.va0	Variable	Local	Stores the current vehicle speed in integer form, pulled from the value of page0.va1. This value must be reassigned to a local variable in order to correctly calculate distance traveled.
page4.va1	Variable	Local	Stores the intermediary calculation for change in distance traveled (see bullet point at the bottom of the page).
page4.x0	Float	Global	Distance traveled, in feet. As discussed in task_HALL1(), this value is displayed to four decimal places solely as a result of calculating distance traveled, as the Nextion is incapable of floating point math.

There are six pages in the Nextion interface (zero-indexed). The page IDs and descriptions are summarized in the table below.

Page ID	Description
page0	Home screen and dashboard.
page1	Help screen. It shows a QR code that directs to this wiki.
page2	Drive mode switch board. Shows five virtual switches that change the vehicle's drive mode.
page3	Hydraulic circuit. Shows the manifold schematic and indicates which valves are currently energized.
page4	Plot. Shows a plot of vehicle speed vs. time, and displays distance traveled (only while viewing the Plot page).
page5	Animation. Shows a 3D orbit of the hydraulic vehicle.

Since the Nextion display can't work with floating point numbers, we calculate distance traveled by multiplying by several factors of 10, and then shifting the decimal place of page4.x0 over appropriately. The change in distance traveled is calculated by multiplying the current vehicle speed by change in time, which is the update period of page4.tim0 (50 ms). Using the equation for distance traveled,

page4.va0 is multiplied by 735 to calculate change in distance traveled x10^4. In order to return this variable to the correct magnitude, we shift the decimal point in the simulated floating point number, page4.x0, by 4, which appears to decrease its magnitude by 10000.

Author: Jordan Kochavi

Date: March 25, 2021