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dsPIC30F1010/202X

DS70178C-page 162

Preliminary

2006 Microchip Technology Inc.

15.1

UART Baud Rate Generator (BRG)

The UART module includes a dedicated 16-bit Baud

Rate Generator. The U1BRG register controls the

period of a free-running 16-bit timer. Equation 15-1

shows the formula for computation of the baud rate

with BRGH = 0.

EQUATION 15-1:

UART BAUD RATE WITH

BRGH = 0(1,2,3)

Example 15-1 shows the calculation of the baud rate

error for the following conditions:

FCY = 7.5 MHz

Desired Baud Rate = 9600

The maximum baud rate (BRGH = 0) possible is

FCY/16 (for U1BRG = 0), and the minimum baud rate

possible is FCY/(16 * 65536).

Equation 15-2 shows the formula for computation of

the baud rate with BRGH = 1.

EQUATION 15-2:

UART BAUD RATE WITH

BRGH = 1(1,2,3)

The maximum baud rate (BRGH = 1) possible is FCY/4

(for U1BRG = 0) and the minimum baud rate possible

is FCY/(4 * 65536).

Writing a new value to the U1BRG register causes the

BRG timer to be reset (cleared). This ensures the BRG

does not wait for a timer overflow before generating the

new baud rate.

EXAMPLE 15-1:

BAUD RATE ERROR CALCULATION (BRGH = 0)(1)

Note 1: FCY denotes the instruction cycle clock

frequency (FOSC/2).

2: Assuming external oscillator with fre-

quency of 15 MHz and PLL disabled,

FCY is 7.5 MHz.

3: Assuming external oscillator with fre-

quency of 15 MHz and PLL enabled,

FCY is 30 MHz.

Baud Rate =

FCY

16 (U1BRG + 1)

FCY

16 Baud Rate

U1BRG =

– 1

Baud Rate =

FCY

4 (U1BRG + 1)

FCY

4 Baud Rate

U1BRG =

– 1

Note 1: FCY denotes the instruction cycle clock

frequency.

2: Assuming external oscillator with fre-

quency of 15 MHz and PLL disabled,

FCY is 7.5 MHz.

3: Assuming external oscillator with fre-

quency of 15 MHz and PLL enabled,

FCY is 30 MHz.

Desired Baud Rate

Fcy/(16 (U1BRG + 1))

Solving for U1BRG value:

U1BRG

((FCY/Desired Baud Rate)/16) – 1

U1BRG

((7500000/9600)/16) – 1

U1BRG

Calculated Baud Rate

7500000/(16 (48 + 1))

9566

Error

(Calculated Baud Rate – Desired Baud Rate)

Desired Baud Rate

(9566 – 9600)/9600

=-0.35%

Note 1: Based on TCY = 2/FOSC, PLL are disabled.

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