Reducing current draw when UART is idle.

Question

I have an application that needs to minimize its current consumption when sleeping. From the nRF52 manual, I see that UART should consume 1uA when in idle. The total current draw (CPU sleeping + UART idle) that I'm expecting should be around 2.5uA-3.0uA given another peripheral. However, I measure around 8uA - around 5uA too high.

To narrow down this problem, I put together a small project based on the SDK's uart example. Here is the main:

int main(void)
{
   NRF_GPIO->DIRSET = 0xFFFFFFFC;  // Exclude XTAL
   NRF_GPIO->OUTCLR = 0xFFFFFFFC;

   uint32_t err_code;
   const app_uart_comm_params_t comm_params =
   {
      16, // RX
      17, // TX
      255, // RTS
      255, // CTS
      APP_UART_FLOW_CONTROL_DISABLED, 
      false,
      UART_BAUDRATE_BAUDRATE_Baud38400
   };

   APP_UART_FIFO_INIT(&comm_params,
                     UART_RX_BUF_SIZE,
                     UART_TX_BUF_SIZE,
                     uart_error_handle,
                     APP_IRQ_PRIORITY_LOW,
                     err_code);

  APP_ERROR_CHECK(err_code);


  uint8_t i;

  NRF_CLOCK->TASKS_HFCLKSTOP = 1;
  NRF_UART0->TASKS_SUSPEND = 1;
  NRF_UART0->INTENCLR = (UART_INTENCLR_TXDRDY_Clear << UART_INTENCLR_TXDRDY_Pos) | (UART_INTENCLR_RXDRDY_Clear << UART_INTENCLR_RXDRDY_Pos); // disable both ready interrupt
  NRF_UART0->INTENCLR = (UART_INTENCLR_CTS_Clear << UART_INTENCLR_CTS_Pos) | (UART_INTENCLR_RXTO_Clear << UART_INTENCLR_RXTO_Pos); // disable both ready interrupts
  
while (true)
{
   __SEV();
   __WFE();
   __WFE();
}

}

The only way I manage to get to the minimal draw is to disable UART completely using:

 NRF_UART0->ENABLE = 0;

I don't believe this is another peripheral because (a) I haven't enabled anything else, and (b) disabling UART gets it to the proper level.

How do I get the UART peripheral into the 1uA idle mode? Disabling the UART completely seems a bit drastic.

I am running this on a Rev. C nRF52 chip.

Ole Bauck · Accepted Answer

The idle current is for uart peripheral only. The uart may request some other resources that is responsible for the current draw you see. You should disable the uart when you are not using it. You can do this with the hardware flow control pins or use two separate gpio pins, see this example for nRF51.

Reducing current draw when UART is idle.

Top Replies