Skip to content

Latest commit

 

History

History
74 lines (59 loc) · 3.16 KB

uart8250.md

File metadata and controls

74 lines (59 loc) · 3.16 KB

VCML Models: UART8250

This is a TLM model of the standard 8250/16550A UARTs used for serial I/O. It models the functional aspects of the real device, so it looks like a real UART for the software. However, most of the physical aspects have been omitted (e.g., data is transmitted instantly, transmission errors cannot occur, etc.).

The model uses a tx and rx port to connect to other UARTs. A common choice would be the terminal uart model, which allows input and output via stdout and stdin from a terminal window. Input is polled once per quantum and is only fetched from the backend if the FIFOs are not full, so data is never dropped.

The model can be operated in interrupt or polling mode.

Properties

This model has the following properties:

Property Type Default Description
clock hz_t 3686400 UART clock (in Hz)
read_latency sc_time 0ns Extra read delay
write_latency sc_time 0ns Extra write delay
allow_dmi bool true Ignored
loglvl log_level info Logging threshold
trace_errors bool false Report TLM errors

Extra properties are available from the specified backends.

The properties loglvl and trace_errors require loggers.

Hardware Interface

The following ports and sockets must be connected prior to simulating:

Port Type Description
IN tlm_target_socket<> Input socket for bus requests
IRQ sc_out<bool> Interrupt port

Software Interface

The model exposes the following memory mapped registers:

Name Offset Access Width Description
THR +0x0 R/W 8 bit Rx Buffer (R) / Tx Buffer (W) Register
IER +0x1 R/W 8 bit Interrupt Enable Register
IIR +0x2 R/W 8 bit Interrupt Ident. (R) / FIFO Control (W)
LCR +0x3 R/W 8 bit Line Control Register
MCR +0x4 R/W 8 bit Modem Control Register
LSR +0x5 R 8 bit Line Status Register
MSR +0x6 R 8 bit Modem Status Register
SCR +0x7 R/W 8 bit Scratchpad Register

This device model works with Linux:

  • As early boot console: add earlycon=uart,mmio,<addr> to kernel parameters
  • As a serial device via the device tree: use compatible = "16550a";

The corresponding drivers can be found in drivers/tty/serial/8250. Below is an example that adds a UART at address 0x91000000 and assigns it to interrupt 3.

serial@91000000 {
    compatible = "ns16550a";
    clock-frequency = <3686400>; /* 3.6864MHz */
    reg = <0x91000000 0x2000>;
    interrupts = <3>;
};

Documentation December 2018