ALLPCB
Overview
Capacitive touch keys offer longer lifetime and lower cost than mechanical keys. They provide a simple structure, clean appearance, and wear resistance, and have become a mainstream option for wireless touch applications such as smart door locks. The CH583/CH582 Bluetooth SoCs, based on a RISC-V core, integrate a new generation of touch-key functionality and are supplied with a touch-key library that supports the drive shielding feature. The touch-key library can run concurrently with the Bluetooth stack and supports various wireless use cases.
Design Concept
As an example, a common 12-key touch keypad is implemented with the CH582. The CH582 supports 14 touch channels: channel 0 is configured as the drive-shield pin, while the remaining 13 channels provide 12 touch keys and one channel for battery-voltage detection.
Prototype
The prototype uses a two-layer stacked structure. The top board hosts 12 touch PADs with up to 3 mm acrylic overlay. Slots in the PADs reserve holes for backlight LEDs. The bottom board contains 12 LEDs for backlighting and houses the low-power Bluetooth chip CH582 and its RF antenna. In conventional touch detection, long traces or dense routing increase parasitic capacitance in the key loop, reducing sensitivity. This design routes the drive-shield pin around the input loop to implement drive shielding, significantly reducing the influence of parasitic parameters on sensitivity and improving detection performance.
Measured Benefits of Drive Shielding
On the demo board, a single-finger touch on the acrylic surface was measured by comparing the sampled signal value to the baseline. For key number 3, the difference was about 61 in the conventional mode (drive shielding off). After enabling drive shielding, the difference increased to 152, showing a marked signal enhancement.
Drive Shielding On
Drive Shielding Off (Conventional)
Low-Power Operation
The CH582 integrates an on-chip DC-DC converter. When enabled, system power consumption is further reduced. With a scan interval of 0.5 seconds, the measured average standby current is about 15 uA.
Schematic and Chip Support
Reference schematic and layout are provided for the design. Capacitive touch-key functions are integrated in chip families including CH32V203, 32F103 series MCUs, and CH583, CH582, CH573, CH32V208 low-power Bluetooth SoCs. Some of these chips are supplied with touch-key libraries to assist development.
