Since the functions of traditional thermostats were simpler, they mainly used low-cost monochrome displays; but with the increase in color display applications and increased production of color displays, its prices have gradually become affordable and brought about rapid upgrades of thermostat display screens. The status displays for temperature, weather, and humidity, etc. have become more affinitive through the color LCD; it also allows more interactive functions including a timed switch and settings. The effects are amazing.

User requirements for mainstream color thermostats:

• Color requirement: 18-bit or 24-bit true color; the high details affect the performance of graphic colors, and further improve the user’s visual sensation.
• The dedicated protocol MODBUS RTU communication protocol can be used through the RS485 interface to perform the control of devices such as temperature switch and sensors etc.
• The touch panel screen must be smooth.

Nuvoton released a complete set of reference design solutions; not only can it be applied to thermostats for smart homes, but it can also be applied to other devices that need small-sized displays, such as the display on smart appliances and meters, etc. The hardware used for the reference design solution includes the graphic control series product N9H20 microprocessor with built-in 32 MB SDRAM memory, reducing the design threshold for the hardware. Not only is the circuit design very simple, thus reducing the area of the PCB, it also uses built-in DRAM with LQFP packaging. External signal interferences were also reduced, making it easier to process, repair, and pass EMC testing easily. Also, with the support of the capacitive 480x480 IPS color touch display, the users’ requirements can be satisfied perfectly.

Figure 1 Schematic diagram of finished thermostat

This solution provides complete hardware design, including circuit schematic, RS485 related circuit, and PCB layout document. In terms of software design, reference graphic files and function codes are provided for the thermostat, including startup screen, three-page sliding mode switch, temperature display and control, perpetual calendar, backlight control, and RS485 controls, etc. The entire reference solution uses open-source architecture, allowing users to change the control processes and images of the hardware and software based on their plans.

Hardware specifications for the entire reference solution

Figure 2 PCB appearance

Figure 3 Control interface of the thermostat

For related resources, please refer to:

https://www.nuvoton.com/applications/smart-home-appliances/tft-thermostat/