RL78 UART reception in sleep mode


In my application, I have to turn the CPU in sleep mode. My UART is configured to work at 115200bds.

In sleep mode, how can I wake up my CPU when I receive bytes from my UART (sleep mode). I have seen that in sleep mode (snooze mode for UART), transfer rate in the SNOOZE mode is 4800 bps only.

Thanks in advance.

  • Hi Xavier,

    Have you found a way to solve this? If the hardware manual says that the UART transfer rate in SNOOZE mode is only 4800bps, then I guess you need to set it to 4800.

    RenesasRulz Forum Moderator


  • In reply to JB:

    Hi Xavier,
    JB is correct that RL78 UART transfer speed in Snooze mode is limited to 4800 baud maximum. The issue is that the internal HOCO system/CPU oscillator is turned off in STOP mode, and when the start bit wakes up the CPU, it takes 18uSEC to 65uSEC maximum for the HOCO to start and stabilize. This needs to happen before about 0.33x start bit period has elapsed, so that the serial bits can be sampled correctly by the UART receiver using HOCO clock.

    If you can change your communications type from UART to CSI(SPI) 3-wire serial, then CSI can run up to 1Mbps in Snooze mode.

  • In reply to Mike Clodfelter:

    Hi Mike,

    Thanks for your reply. I cannot change my baudrate, and cannot use as a CSI(SPI).

  • In reply to Xavier:

    Then you cannot use STOP or SNOOZE mode but can only use HALT mode to reduce power consumption.
    in HALT mode the clock continues to run and only the CPU is switched off.
    However, power consumption in HALT mode will be much higher than in STOP mode.