Baud Rate Generator

The MSSP module has a Baud Rate Generator (BRG) available for clock generation in both I2C and SPI Host modes. The BRG reload value is placed in the SSPxADD register. When a write occurs to SSPxBUF, the BRG will automatically begin counting down. MSSP Baud Rate Generator Frequency Equation shows how the value for SSPxADD is calculated.

Once the given operation is complete, the internal clock will automatically stop counting and the clock pin will remain in its last state.

An internal Reload signal, shown in Figure 1, triggers the value from SSPxADD to be loaded into the BRG counter. This occurs twice for each oscillation of the module clock line.

Table 1 illustrates clock rates based on instruction cycles and the BRG value loaded into SSPxADD.

MSSP Baud Rate Generator Frequency Equation


Figure 1. Baud Rate Generator Block Diagram
Important: Values of 0x00, 0x01 and 0x02 are not valid for SSPxADD when used as a Baud Rate Generator for I2C. This is an implementation limitation.
Table 1. MSSP Clock Rate w/BRG
(2 Rollovers of BRG)
32 MHz 8 MHz 13h 400 kHz
32 MHz 8 MHz 19h 308 kHz
32 MHz 8 MHz 4Fh 100 kHz
16 MHz 4 MHz 09h 400 kHz
16 MHz 4 MHz 0Ch 308 kHz
16 MHz 4 MHz 27h 100 kHz
4 MHz 1 MHz 09h 100 kHz
Note: Refer to the I/O port electrical specifications in the “Electrical Specifications” chapter, Internal Oscillator Parameters, to ensure the system is designed to support all requirements.