LPC54016JET100 USART3: Troubleshooting Serial Output

P0_2 and P0_3 can be assigned to FLEXCOMM3 transmit and receive functions on the LPC54016JET100. If USART3 produces no output, check the IOCON pin function, FLEXCOMM3 reset and clock controls, peripheral selection, and FLEXCOMM3 clock source before diagnosing higher-level serial code.

Configure the P0_2 and P0_3 Pin Multiplexing

Enable the IOCON and GPIO0 clocks before changing the pin configuration. Configure P0_2 as FC3_TXD and P0_3 as FC3_RXD by selecting pin function 1. The supplied configuration also enables the digital function and pull-up, disables inversion, input filtering, and open-drain operation, and selects standard slew-rate control.

#define IOCON_PIO_DIGITAL_EN    0x0100u
#define IOCON_PIO_FUNC1         0x01u
#define IOCON_PIO_INPFILT_OFF   0x0200u
#define IOCON_PIO_INV_DI        0x00u
#define IOCON_PIO_MODE_PULLUP   0x20u
#define IOCON_PIO_OPENDRAIN_DI  0x00u
#define IOCON_PIO_SLEW_STANDARD 0x00u

CLOCK_EnableClock(kCLOCK_Iocon);
CLOCK_EnableClock(kCLOCK_Gpio0);

uint32_t pin_config = IOCON_PIO_FUNC1 |
                      IOCON_PIO_MODE_PULLUP |
                      IOCON_PIO_INV_DI |
                      IOCON_PIO_DIGITAL_EN |
                      IOCON_PIO_INPFILT_OFF |
                      IOCON_PIO_SLEW_STANDARD |
                      IOCON_PIO_OPENDRAIN_DI;

IOCON_PinMuxSet(IOCON, 0, 2, pin_config); /* P0_2/FC3_TXD */
IOCON_PinMuxSet(IOCON, 0, 3, pin_config); /* P0_3/FC3_RXD */

Reset, Clock, and Select FLEXCOMM3

The supplied initialization sequence resets FLEXCOMM3 through bit 14 of PRESETCTRL[1], enables its gated clock through bit 14 of AHBCLKCTRL[1], writes 0x01 to FLEXCOMM3->PSELID, and selects FRO_12M for FLEXCOMM3 by writing 0x00 to FCLKSEL[3].

SYSCON->PRESETCTRL[1] |=  (1u << 14);
SYSCON->PRESETCTRL[1] &= ~(1u << 14);
SYSCON->AHBCLKCTRL[1] |=  (1u << 14);
FLEXCOMM3->PSELID = 0x01;
SYSCON->FCLKSEL[3] = 0x00; /* Select FRO_12M */

Resolve the Peripheral-Selection Ambiguity

The evidence assigns 0x01 to PSELID, but its adjacent comment says that FLEXCOMM3 is being set as SPI. That comment conflicts with the stated USART3 objective. Treat the value and comment as an ambiguity: verify that the selected PSELID value represents USART operation for the exact LPC54016JET100 documentation and software package before relying on the sequence.

Verify the Configuration Sequence

1. Enable the IOCON and GPIO0 clocks.
2. Write the complete function-1 digital configuration to P0_2 and P0_3.
3. Reset FLEXCOMM3 and enable its gated clock using bit 14 in the stated register banks.
4. Select the FLEXCOMM3 peripheral mode, resolving the PSELID ambiguity against the exact device documentation.
5. Select the FLEXCOMM3 clock through FCLKSEL[3], then verify the USART configuration uses that clock source.
6. Transmit a message and check P0_2, the configured FC3_TXD pin, for output.

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