LPC54018 USART Flexcomm4 non 3.3V logic

[ahmed_mostafa]

I'm using the LPC54018 IOT Module along with the mini base board. I'm currently trying to initiate a UART communication at 1200 baud rate for communication with an external HART modem. For some reason, when I connect P3_26 (port 3 pin 26) to the external HART modem, the GPIO logic is not 0-3.3V, but some lesser peak to peak voltage. When I disconnect this pin from the external device, the oscilloscope shows 0-3.3V as normal. So the issue seems to be coming from the peripheral on the LPC54018.

What could be the issue that is causing this? The TX seems to be working fine. The LPC is able to send out 0-3.3V uart signals and my external device receives it correctly.

The green is the uart signal on the RXD pin (port 3 pin 26). The yellow is the modem's HART signal which represents the data being sent, and the green is the Modem's demodulator which converts that signal to uart. The blue is just a detection signal that is HIGH whenever there is a HART signal.

[Alice_Yang]

Hello ,

P3_26 is the RX of UART, so I think first you need confirm the signal logic from HART module.

And what about the blue signal meaning?

BR

Alice

[ahmed_mostafa]

I now tried a different USART peripheral (P3_21) and also removed the external HART module, and I'm still seeing that the USART communication is not at 0-3.3V. I believe the way I'm initializing the USART peripheral may not be accurate.Can you please confirm if I am initializing my Flexomm4 peripheral properly?

I'm starting with the "lpc5018iotmodule_usart_interrupt_transfer" example from the SDK and making modifications to it.

1. Initialized Flexcomm4 as USART in pinmux.c : I inserted these two lines in "BOARD_InitPins():

const uint32_t port3_pin26_config = (
      IOCON_PIO_FUNC3 | /* Pin is configured as FC4_RXD_SDA_MOSI */
      IOCON_PIO_MODE_INACT | /* No addition pin function */
      IOCON_PIO_INV_DI | /* Input function is not inverted */
      IOCON_PIO_DIGITAL_EN | /* Enables digital function */
      IOCON_PIO_INPFILT_OFF | /* Input filter disabled */
      IOCON_PIO_SLEW_STANDARD | /* Standard mode, output slew rate control is enabled */
     IOCON_PIO_OPENDRAIN_DI /* Open drain is disabled */
);
IOCON_PinMuxSet(IOCON, PORT3_IDX, PIN26_IDX, port3_pin26_config); /* PORT3 PIN26 */

const uint32_t port3_pin27_config = (
     IOCON_PIO_FUNC3 | /* Pin is configured as FC4_TXD_SCL_MISO */
     IOCON_PIO_MODE_INACT | /* No addition pin function */
     IOCON_PIO_INV_DI | /* Input function is not inverted */
     IOCON_PIO_DIGITAL_EN | /* Enables digital function */
     IOCON_PIO_INPFILT_OFF | /* Input filter disabled */
     IOCON_PIO_SLEW_STANDARD | /* Standard mode, output slew rate control is enabled */
     IOCON_PIO_OPENDRAIN_DI /* Open drain is disabled */
);
IOCON_PinMuxSet(IOCON, PORT3_IDX, PIN27_IDX, port3_pin27_config); /* PORT3 PIN27 */

2: In board.h I defined the following variables (basically changed the default "0s" to "4"):

#define BOARD_DEBUG_UART_BASEADDR      (uint32_t) USART4
#define BOARD_DEBUG_UART_INSTANCE        4U
#define BOARD_DEBUG_UART_CLK_FREQ       CLOCK_GetFlexCommClkFreq(0U)
#define BOARD_DEBUG_UART_CLK_ATTACH   kFRO12M_to_FLEXCOMM4
#define BOARD_DEBUG_UART_RST                   kFC4_RST_SHIFT_RSTn
#define BOARD_UART_IRQ_HANDLER               FLEXCOMM4_IRQHandler
#define BOARD_UART_IRQ                                  FLEXCOMM4_IRQn

3: In usart_interrupt_transfer.c: I made the following definitions

#define DEMO_USART USART4
#define DEMO_USART_CLK_FREQ CLOCK_GetFlexCommClkFreq(4U)

4: In main() I commented out "BOARD_InitDebugConsole()"

It seems to me like the way I'm configuring the peripheral is incorrect and thus is causing a change in logic levels. Can you confirm my procedure is correct for initializing FlexComm4 for USART operation?

[frank_m]

Did you call the appropriate fsl function for the peripheral, e.g. FLEXCOMM_Init() ?

I would take a flexcomm SDK example, and compare with this code.

[ahmed_mostafa]

I managed to correct the issue on P3_21 so I will use that. P3_26 Which is Flexcomm 4 still produces issues. The issue is not with the external HART module. I have certified that it is 0-3.3V compatible.

I do have another question. Since I am using baudrate of 1200 for my communication, what is the best clock source to use for the USART? Is it ok to use the 12MHz source clock our is it better to use 32kHz RTC/ something else?

[Alice_Yang]

Hello,

Yes, 12MHZ source clock is OK.

Regards,

Alice

[frank_m]

> ...so I think first you need confirm the signal logic from HART module.

I definitely agree.

First check the datasheet of the module, if the signal levels are compatible with the MCU. Perhaps the HART module is 1,8V logic ...