USART to 4MBps! how? STM32L151xx

Question

How can I increase the USART baud rate to 2Mbps, 3Mbps or 4Mbps. I am using STM32L151RCT6A, I am able to run to 921600. I have set clock with PLL 32MHz. On the datasheet it is given, which shows it is possible, Has anyone ever done this?

Answer 1

The datasheet only outlined the specific part and its peripheral set and electrical characteristics. For information on how to use the device you need the Reference Manual. This gives the following equation for baud rate:

Tx/Rx baud = CK_APB1 / (8 x (2 - OVER8) x USARTDIV)

Where USARTDIV is an unsigned fixed point number that is coded on the USART_BRR register.

• When OVER8=0, the fractional part is coded on 4 bits and programmed by the DIV_fraction[3:0] bits in the USART_BRR register
• When OVER8=1, the fractional part is coded on 3 bits and programmed by the DIV_fraction[2:0] bits in the USART_BRR register, and bit DIV_fraction3 must be kept cleared.

The USARTs are on the APB1 bus, Figure 12 in the reference manual is the clock tree, which shows how the APB1 clock is derived from the PLL clock. The maximum APB1 clock is 32MHz. OVER8=1 is required for higher speeds, giving:

baud = 32X10⁶ / (USARTDIV x 8).

So USARTDIV = 32X10⁶ / (baud x 8)

For 4Mbps therefore, USARTDIV=1 (see table 138 S.No.12 for details). For 2Mps, USARTDIV=2. To achieve 3Mbps you will have to reduce the APB1 clock to 24MHz and set USARTDIV=1 (see table 131). But note that the clock rate changes for all other APB1 peripherals too.

The simplest way to correctly program the USART baud rate is via the STM32L1xx standard peripheral library. Also to determine the correct peripheral clock settings (and more), and generate initialisation code, you can use STM's MicroXplorer tool.