Powering Down the RL78/L12

Renesas’ RL78/L12 MCUs set out to integrate an LCD driver circuit alongside the CPU without sacrificing the ultra-low-power credentials of the RL78 product line—not an easy trick to accomplish. The LCD circuit adds just another 0.61 μA due to internal voltage boost, a 30% saving over competitive products.

Renesas’ new RL78/L12 Promotion Board includes a multi-part demo program that shows off many of the chip’s capabilities including:

  • Setting alarms and interrupts;
  • Moving data from DMAC to RAM;
  • Synchronizing the RTC registers with the system clock;
  • Enabling RAM/SFR guard; and
  • Measuring the MCU current drain in the various low-power modes.

It was the latter that got my attention, so I decided to check it out.

The demo program lets you put the chip into Run, Halt, Stop (with LCD on or off), and Snooze modes; by connecting an ammeter to CN5—in line with VMCU—you can measure the current drain directly in each mode. The number of active peripherals, of course, directly affects the current drain:

In this demo you can go into Halt and Stop modes—the latter with the LCD on or off—at the click of a button. Snooze Mode is technically part of Stop Mode, but for the purposes of the demonstration the user only has to press the “SNOOZE” button to activate this demonstration. Before activating SNOOZE mode, you must specify the voltage window that the device is to wake up, as well as set the trigger interval of the ADC measurements. In this demonstration the RL78 has been set to wake on an external ADC voltage (Potentiometer value) that is outside the specified voltage window.

With our multimeter connected to CN5 we measured the following:

  • Run Mode: 3.6 mA
  • Halt Mode: 0.80 mA
  • Stop Mode
    • o   With LCD on: 2 μA
    • o   With LCD off: 500 nA
  • Snooze Mode: Lower than I can measure