=== Introduction ===

DM814x provides up to six UART peripheral interfaces, depending on the selected pin multiplexing. Other than pin multiplexing, UARTs features a clock multiplexing: UARTs 0/1/2 have SYSCLK10 as reference clock, while UARTs 3/4/5 can be configured to have SYSCLK6, SYSCLK8 or SYSCLK10. This article describes how to configure UART3change the reference clock of UARTs 3/4/5. = Default configuration = With the default configuration, UARTs 3/4/5 are driven by the SYSCLK8 clock, which runs at 192 MHz; UARTs 0/1/2 are driven by SYSCLK10, which runs at 48 MHz. In this situation, setting the port speed (using the '''stty''' command, for example) at X baud means that UARTs 0/1/2 actually works at X baud, while UARTs 3/4/5 get a 4x multiplying factor on speed value, working at 4*X baud: * stty -F /dev/ttyOx speed Y --> port 0/1/2 is configured to run @ X baud* stty -F /dev/ttyOx speed X --> port 3/4/5 is configured to run @ 4*X baud This configuration for ports 3/4/5 can be useful when a high-speed UART is required; in other cases, a standard behavior is preferred for ports 3/4/5. = Change UARTs 3/4/5 reference clock = Register McBSP_UART_CLKSRC (see the DM814x Technical Reference Manual, http://www.ti.com/litv/pdf/sprugz8c - section 2.10, page 672) of the Power, Reset, UART4 and Clock Management (PRCM) Module allows for selection of the reference clock. This register can be set on Linux adding an initialization function (<code>init_uartX_clk(void)</code> to the <code>arch/arm/mach-omap2/board-naon.c</code> file that writes the proper value into the register.  == Example: setting SYSCLK10 as reference clock for UART5 on Linux == The following snippet shows the code to work with standard settingsadd to <code>arch/arm/mach-omap2/board-naon.c</code> for configuring SYSCLK10 (48 MHz) as reference clock for UART5: <pre>static void __init init_uart5_clk(void){ __raw_writel((1 << 7), TI814X_PLL_CMGC_MCBSP_UART_CLKSRC); /* sysclk10 as a reference clock */} static void __init naon_init(void){ init_uart5_clk(); omap_serial_init(); .... ....</pre>