75xx xuarts

The XUART controller is a core we have included in the FPGA, as well as a userspace application called xuartctl for accessing these UARTs. Rather than using a kernel driver with the standard serial interface, we have implemented the XUARTs with features to simplify application development. The XUARTs allow you to easily use arbitrary baud rates, nonstandard modes such as DMX or 9n1, and they allow a very low latency operation. The XUART layer also uses the very low overhead TCP layer which allows you to transport serial over the network without writing any code.

The simplest example to get started is to define the port with:

xuartctl --server --port=1 --speed=115200

This will return "ttyname=/dev/pts/0", or a higher pts number. You can use this /dev/pts/# device to access the UART, but note that the pts device number can change based on other ssh, telnet or other processes. See this section for a sample script to setup the XUARTs with a predictable device name.

For more information and detailed usage, see the xuartctl page.

The XUARTs like many other standard UARTs poll the RX buffers by default. The XUARTs have large RX FIFOs so polling at 100hz is the best choice for many applications. At the expense of more CPU time you can use an IRQ to achieve a much lower latency. This board uses IRQ 29 for the XUART IRQ. You can edit the linuxrc script and change the xuartctl server to start with:

xuartctl --irq=29 --server