I am unable to get my UART to I2C dev module to go to sleep as it should, it appears that the chip will not go to sleep due to an interrupt condition that I am unable to clear, but that is my best guess after the investigation below. Everything I have tried to do to work with this chip works (i.e. reading UART data in via I2C), but it just won't go to sleep, always stays at ~4.8mA.

Current consumption is monitored via a Nordic PK2, where the 3.3Vout is going just to the module directly, bypassing the module's LDO.

Datasheet: https://www.nxp.com/docs/en/data-sheet/SC16IS740_750_760.pdf?

My "go to sleep" function:

void SC16IS750::enableSleep()
  uint8_t temp_reg;

  Serial.print("LCR(before): "); Serial.println(ReadRegister(SC16IS750_REG_LCR), HEX);
  Serial.print("EFR(before): "); Serial.println(ReadRegister(SC16IS750_REG_EFR), HEX);
  Serial.print("IER(before): "); Serial.println(ReadRegister(SC16IS750_REG_IER), HEX);
  Serial.print("FIFO(before): "); Serial.println(ReadRegister(SC16IS750_REG_FCR), HEX);
  Serial.print("IIR(before): "); Serial.println(ReadRegister(SC16IS750_REG_IIR), BIN);

  Serial.println(); Serial.println("Changing values...");
  uint8_t temp_lcr = ReadRegister(SC16IS750_REG_LCR); //set LCR to 0xBF to enable EFR
  Serial.print("LCR(before): "); Serial.println(temp_reg, HEX);
  WriteRegister(SC16IS750_REG_LCR, 0xBF);
  temp_reg = ReadRegister(SC16IS750_REG_LCR);
  Serial.print("LCR(after): "); Serial.println(temp_reg, HEX);

  temp_reg = 0x00;
  Serial.print("EFR(before): "); Serial.println(temp_reg, BIN);
  bitSet(temp_reg, 4);
  WriteRegister(SC16IS750_REG_EFR, temp_reg);
  Serial.print("EFR(after): "); Serial.println(ReadRegister(SC16IS750_REG_EFR), BIN);

  temp_reg = ReadRegister(SC16IS750_REG_IER);
  Serial.print("IER(before): "); Serial.println(temp_reg, BIN);
  bitSet(temp_reg, 4);
  WriteRegister(SC16IS750_REG_IER, temp_reg);
  Serial.print("IER(after): "); Serial.println(ReadRegister(SC16IS750_REG_IER), BIN);
  Serial.println(); Serial.println("Values Changed");


  //check sleep mode allowance
  Serial.println(); Serial.println("Sleep allowed?(BIN)");
  Serial.print("EFR[4]: (1 is good) "); Serial.println(bitRead(ReadRegister(SC16IS750_REG_EFR), 4));
  Serial.print("IER[4]: (1 is good) "); Serial.println(bitRead(ReadRegister(SC16IS750_REG_IER), 4));
  //FIFO cleared/enabled?
  Serial.print("FIFO: (1 means enabled, 0 is good) "); Serial.println(bitRead(ReadRegister(SC16IS750_REG_FCR), 0));

  //interrupt pending?
  Serial.print("IIR: (should be 1)"); Serial.println(ReadRegister(SC16IS750_REG_IIR), BIN);

  //shift register cleared?
  Serial.println(); Serial.println("Value Printouts in HEX...");
  Serial.print("LSR[5]: "); Serial.println(bitRead(ReadRegister(SC16IS750_REG_LSR), 5));
  Serial.print("LSR[6]: "); Serial.println(bitRead(ReadRegister(SC16IS750_REG_LSR), 6));

  Serial.print("LCR(after): "); Serial.println(ReadRegister(SC16IS750_REG_LCR), HEX);
  Serial.print("EFR(after): "); Serial.println(ReadRegister(SC16IS750_REG_EFR), HEX);
  Serial.print("IER(after): "); Serial.println(ReadRegister(SC16IS750_REG_IER), HEX);
  Serial.print("FIFO(after): "); Serial.println(ReadRegister(SC16IS750_REG_FCR), HEX);
  Serial.print("IIR(after): "); Serial.println(ReadRegister(SC16IS750_REG_IIR), HEX);


The serial output of running this function:

LCR(before): 3
EFR(before): 1
IER(before): 0
FIFO(before): 1
IIR(before): 1

Changing values...
LCR(before): 0
LCR(after): BF
EFR(before): 0
EFR(after): 10000
IER(before): 0
IER(after): 10000

Values Changed

Sleep allowed?(BIN)
EFR[4]: (1 is good) 1
IER[4]: (1 is good) 1
FIFO: (1 means enabled, 0 is good) 0
IIR: (should be 1)10000

Value Printouts in HEX...
LSR[5]: 0
LSR[6]: 0
LCR(after): BF
EFR(after): 10
IER(after): 10
FIFO(after): 10
IIR(after): 10

PK2 output:

enter image description here

For the life of me, I cannot figure out why the IIR is returning a value!


1 Answer 1


I have succeeded to achieve sleep mode in the SC16IS750 by following the code of NXP application note AN10571.

You can find it here: PDF AN10571

  • \$\begingroup\$ That's great! Would you be so kind as to share the snippet of code you wrote that enables this function? \$\endgroup\$
    – cMod
    Sep 29, 2022 at 21:15
  • \$\begingroup\$ Were you able to get the module to sleep without missing the first byte it receives after wakeup? \$\endgroup\$
    – cMod
    Sep 29, 2022 at 21:15
  • \$\begingroup\$ I hope you find it valuable: code \$\endgroup\$
    – David Sosa
    Sep 22 at 14:34

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