IN Brief:
- The RL78/G21 achieves 34µA/MHz operating current under specified conditions and supports a 1.6–5.5V supply range.
- Devices provide 128KB or 256KB of code flash, 8KB RAM, 4KB data flash, and ±4kV human-body-model ESD tolerance.
- New RL78/G23 configurations increase capacitive-touch capacity to 24 channels in 44-pin devices and 25 channels in 48-pin devices.
Renesas Electronics has introduced the RL78/G21, a low-power 16-bit microcontroller family for embedded system control in appliances and industrial equipment. The devices achieve a specified operating current of 34µA/MHz, approximately half the comparable figure for the established RL78/G13 family, while retaining compatibility with existing RL78 designs.
The new family targets control functions in air conditioners, refrigerators, washing machines and similar equipment. These applications require processors to monitor inputs, control outputs and communicate with peripheral devices, often within tight power and component-cost constraints. The RL78/G21 concentrates on these requirements rather than incorporating the resources of a higher-performance application processor.
Its RL78-S3 processor core operates at frequencies up to 32MHz, with devices offering 128KB or 256KB of code flash memory and 8KB of SRAM. The supply voltage range extends from 1.6V to 5.5V, while package options include 20-, 32-, 44-, 48- and 64-pin configurations.
Renesas specifies an operating current of 34µA/MHz under the conditions used for its comparison with RL78/G13 devices. The figure expresses current consumption relative to operating frequency, allowing the efficiency of the active processor to be assessed independently of the chosen clock rate. It does not establish that every completed appliance will use half as much power.
Total system consumption also depends on supply voltage, peripheral activity, external loads and the proportion of time the processor spends operating or sleeping. A control board with relays, communication interfaces or continuously energised sensors may have energy requirements dominated by those components rather than by the microcontroller core.
The RL78/G21 integrates a high-speed on-chip oscillator with specified frequency accuracy of ±1.0%. This can remove the need for an external timing component where the application permits use of the internal clock. Designs requiring different timing stability or particular communication tolerances must still be evaluated against the relevant oscillator characteristics.
Another integration feature is 4KB of data flash memory, which can hold operating parameters and configuration values without necessarily requiring a separate EEPROM. That reduces the number of external components, but software designers must account for write endurance, data integrity and recovery from interrupted programming operations.
The device also incorporates UARTA, extending its communication capabilities beyond those of the earlier RL78/G13 configuration. Serial communication is commonly used to exchange information with appliance subsystems, including sensors, user interfaces and other control processors. The available ports and supported functions vary across package and device configurations.
Pin and functional compatibility with RL78/G13 and RL78/G23 devices creates a migration route for existing designs. Reusing an established footprint can preserve portions of the PCB layout and component arrangement, although register configurations, peripheral assignments, firmware behaviour and electrical characteristics still require checking against the chosen replacement device.
Renesas has increased electrostatic discharge tolerance to ±4kV under the human body model. This component-level test assesses susceptibility to a defined discharge event and is relevant to handling, assembly and device robustness. It must be distinguished from system-level electromagnetic compatibility and immunity requirements, which involve the finished appliance, its enclosure and external connections.
The RL78/G21 also provides security functions including flash read protection, a flash shield window, unique device identification and customer identification. These mechanisms can limit certain forms of access to stored program information or support device identification, although they do not independently provide a complete secure-update or product cybersecurity architecture.
Self-diagnostic functions are supported through an IEC 60730-compliant self-test library. Such software can assist appliance developers implementing safety-related monitoring, but compliance of a library does not replace evaluation of the completed appliance against the applicable safety requirements.
Alongside the new RL78/G21, Renesas has expanded its RL78/G23 range with variants incorporating additional capacitive-touch sensing channels. The new 44-pin configuration provides 24 channels instead of 14, while the 48-pin version provides 25 instead of 16. The expanded sensing capability belongs to the RL78/G23 variants, not the RL78/G21.
Capacitive-touch controls detect changes in the electrical characteristics of sensing electrodes when a user approaches or contacts the interface. Increasing the available channel count allows more buttons, sliders or control areas to be connected without requiring an additional dedicated touch controller, subject to electrode design and the operating environment.
Renesas supports development through Smart Configurator, Fast Prototyping Boards and QE for Capacitive Touch software. The tools provide configuration and evaluation support, although the suitability of the resulting firmware and user interface remains dependent on application testing.
Both the RL78/G21 and additional RL78/G23 devices are commercially available. Renesas lists indicative RL78/G21 prices starting at US$0.56 for a 128KB, 44-pin device and US$0.77 for the equivalent 256KB configuration, based on quantities of 10,000 units. The new products give designers a choice between lower-power system control and greater integrated touch-interface capacity within the existing RL78 ecosystem.
