Need advice on changing MPC5200B UART prescaler

Sylvain Lamontagne sylvain.lamontagne at
Fri Dec 18 02:57:01 EST 2009

Hi all,

I would like to be able to set a baud rate of 460800 for modem that we are
testing. With the actual prescaler of 32 and a IPB frequency of 84MHz
I got a 5.1% error (437500) vs a 0.9% error (456522) if I could use the
prescaler of 4. See MPC5200B user manual page 15-46 for the calculation
formula and page 15-12 for the CSR description.

Currently the code for the kernel we are using here, ( seams not to
take a prescaler of 4 into account.
Line 249 of mpc52xx_uart.c

/* Search for bus-frequency property in this node or a parent */
static unsigned long mpc52xx_getuartclk(void *p)
         * 5200 UARTs have a / 32 prescaler
         * but the generic serial code assumes 16
         * so return ipb freq / 2
        return mpc52xx_find_ipb_freq(p) / 2;

How could I make it use the prescaler of 4 without breaking anything that we
currently have working ?
I doubt that simply doing  return mpc52xx_find_ipb_freq(p) / 4 would do the
trick ...
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