LPC MicrocontrollersのトピックLPC804 USART in Master Synchronous mode always add a stop bit to transmission

LPC804 USART in Master Synchronous mode always add a stop bit to transmission

xs_graziano — Thu, 04 Sep 2025 12:20:22 GMT

Hello,

I'm using the USART in an LPC804 as Synchronous Master, sending data packets to a slave, but for every byte (8 bit long), seems the LPC add 1 stop bit.

As an example, on an 32 bit data I count 36 clock bits.

The configuration is 8 data bit, 1 stop (the choice is between 1 and 2 stops bit), no parity.

In the middle of the 9th bit there is a clock front, so it doesn't seem to be just an idle time.

Anyone was able to use this device properly?

There is any way to disable this stop bit to be added to communication?

Many thanks for any help or comment

Re: LPC804 USART in Master Synchronous mode always add a stop bit to transmission

Pablo_Ramos — Thu, 04 Sep 2025 20:42:43 GMT

Hi @xs_graziano,

The USART module on the LPC804 is designed to transmit only 7, 8, or 9 data bits per frame. It does not support sending data without framing.

The only way to transmit a frame with more bits than this would be to implement a custom protocol manually using GPIOs.

Best Regards,
Pablo

Re: LPC804 USART in Master Synchronous mode always add a stop bit to transmission

xs_graziano — Fri, 05 Sep 2025 06:52:44 GMT

Hi @Pablos_Ramos,

Thank you for your answer. So, if I right understand, there is no way to use the USART in Synchronous mode without start and stop bit for every byte.

It seems this way also in the datasheet, where there is a 'Start bit' before the first bit is transmitted... I haven't notice it before...

So, the only way to manage a Synchronous Serial Transmission with just 8 bit for every byte (without start and stop) is to use the SPI module, is that right?

Thank you for your support

Re: LPC804 USART in Master Synchronous mode always add a stop bit to transmission

Pablo_Ramos — Fri, 05 Sep 2025 20:50:44 GMT

Hi @xs_graziano,

To manage synchronous serial transmission with 8-bit data and no start/stop bits on the LPC804, you can use the SPI peripheral. Alternatively, you could implement the transmission manually using GPIOs.

Best Regards,
Pablo