topic How do I change the IMX6 UART clock speed in i.MX Processors

How do I change the IMX6 UART clock speed

sivaalagarsamy — Fri, 20 Sep 2013 01:58:20 GMT

Hi,

I have a IMX6 quad core development board. I am running Yocto linux on it. I need a custom baud rate of 1041667 on a serial port to talk to a device. With the current UART clock speed of 80MHz, I won't be able to get that particular speed. If I can change the UART clock to 50MHz, the base baud rate will be 3125000 and with a custom divisor of 3, I will get the 1041667 baud rate.
I understand that the UART clock speed is derived from a main clock that is used for CPU and other subsytem. Where do I start looking to change the clock speed for the UART?

Thanks,

Siva

Re: How do I change the IMX6 UART clock speed

johnw — Fri, 20 Sep 2013 03:44:04 GMT

To answer your question - I did some checking - I don't think the input to the UART module clock is flexible. Meaning, it comes from PLL3 which is used to create the 480MHz USB clock, and is divided down to 80 MHz for the input to the UART (integer divided by 6). This is per the reference manual of the chip - look for "CCM Clock Tree". PLL3's output is fixed at 20x the input clock to the SOC (usually 24 MHz), so this is 480 MHz.

The baud rate generator for the chip is really flexible, though, so you really shouldn't need to change the input clock to the UART module to achieve the desired baud rate. You should be able to get the exact rate you need using the binary rate multiplier in the UART (BRM). That appears to have the ability to multiply/divide the reference clock using a 16-bit numerator and denominator.

Module Clock 80

Reference Clock 26.66666667

Reference Clock Divider 3

UBMR 1599

UBIR 999

Calculated Baud Rate 1.041666667

Desired Baud Rate 1.041666667

16x desired baud rate 16.66666667

Ratio 1.6

I hope this helps.

Re: How do I change the IMX6 UART clock speed

sivaalagarsamy — Sat, 21 Sep 2013 12:36:47 GMT

Thanks for the information. I started looking into the linux serial driver to see why the custom baud rate was not working. The driver does calculate the right numerator and denominator for a given buad rate and sets it in the UBMR and UBIR registers. The only problem I see is that the driver calls uart_get_baud_rate function in the serial_core.c which in turn calls "tty_termios_baud_rate" in tty_ioctl.c. This function has a #ifdef BOTHER to allow custom baud rate. This BOTHER is not defined for the freescale kernel.

tty_ioctl.c:

305 speed_t tty_termios_baud_rate(struct ktermios *termios)

306 {

307 unsigned int cbaud;

308

309 cbaud = termios->c_cflag & CBAUD;

310

311 #ifdef BOTHER

312 /* Magic token for arbitrary speed via c_ispeed/c_ospeed */

313 if (cbaud == BOTHER)

314 return termios->c_ospeed;

315 #endif