LPC1768, uart using FDR at higher baud rates

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jan 10, 2018 by Alfred Lee
Content originally posted in LPCWare by pausten on Sat Jan 01 16:52:52 MST 2011

I've modified some of the uart example code to configure the baud rates using the uart fractional divider register (as per the data sheet) as I could not get comms to work at higher (> 115200 Bps) baud rates (system clock = 100MHz), I belive, due to baud rate inaccuracies.

This is working as I can now send data at 921600 Bps from the LPC1768 to a terminal emulation program on my PC and data is displayed correctly. I belive the usb-serial (TTL) adapter runs ok at 921600 as I've used it successfully using other microcontrollers at 921600 Bps. It could be pickup on the prototype board I'm using but the wire runs are quite short and I wouldn't have expected it to be a problem.

I've uploaded a couple of images from the scope showing the LPC1768 TX and RX data pins (UART0). The SW shows an ASCII 'U' character being transmitted (chosen as it has an alternating bit pattern). This shows no problems with the signal levels. This chracter was received by the PC without error. The LPC1768 SW was just sending U characters periodically in this case.

I then changed the SW on the LPC1768 so that it echo's back the character it receives. The second image shows this. This shows an incorrect character being echo'ed back to the PC, looks like the reception of the character by the LPC1768 is where the problem lies. At lower baud rates this works fine. The images show a rate of 460800 Bps.

I was wondering if anyone else has had trouble running error free over an LPC1768 serial port at higher baud rates ?