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MAXQ7665/MAXQ7666 User’s Guide

6-15

6.4 Baud-Rate Generation

Each mode of operation has a baud-rate generation technique associated with it. The baud-rate generation is affected by certain user

options such as the power management mode enable (PMME) bit, serial mode 2 (SM2) bit, and baud-rate doubler (SMOD) bit. Table

6-3 summarizes the effects of the various user options on the UART baud clock.

Table 6-3. UART Baud-Clock Summary

The baud frequency is determined by the baud-clock generator.

6.4.1 Mode 0 Baud Rate

Baud rates for mode 0 are driven directly from the system clock divided by either 12 or 4, with the default case being divide-by-12.

The user can select the shift clock frequency using the SM2 bit in the SCON0 register. When SM2 is set to logic 0, the baud rate is

fixed at divide-by-12 of the system clock. When SM2 is set to logic 1, the baud rate is divide-by-4 of the system clock.

Mode 0 Baud Rate = System Clock Frequency x (3

SM2

/ 12)

6.4.2 Mode 2 Baud Rate

In this asynchronous mode, baud rates are also generated from the system clock source. The user can effectively double the UART

baud-clock frequency by setting the SMOD bit to logic 1. The SMOD bit is set to logic 0 on all resets, thus making divide-by-64 the

default setting. The baud rate is given by the following formula:

Mode 2 Baud Rate = System Clock Frequency x (2

SMOD

/ 64)

6.4.3 Mode 1 or 3 Baud Rate

These asynchronous modes are commonly used for communication with PCs, modems, and other similar interfaces. The baud rates

are programmable using the baud-rate generator in the UART module. The baud-clock generator is basically a phase accumulator that

generates a baud clock as the result of phase overflow into the most significant bit of the phase shifter. This baud-clock generator is

driven by the system clock or system clock divided by 4 (depending upon the state of the SMOD bit). The baud-clock generator out-

put is always divided by 16 to generate the exact baud rate.

MODE 0 MODE 2 MODES 1, 3*

SYSTEM CLOCK MODE

SM2 = 0 SM2 = 1 SMOD = 0 SMOD = 1 SMOD = 0 SMOD = 1

Divide by 1 CLK/12 CLK/4 CLK/64 CLK/32 BAUD/64 BAUD/16

Divide by 2 (default) CLK/24 CLK/8 CLK/128 CLK/64 BAUD/64 BAUD/16

Divide by 4 CLK/48 CLK/16 CLK/256 CLK/128 BAUD/64 BAUD/16

Divide by 8 CLK/96 CLK/32 CLK/512 CLK/256 BAUD/64 BAUD/16

Power Management Mode

(Divide by 256)

CLK/3072 CLK/1024 CLK/16384 CLK/8192 BAUD/64 BAUD/16

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Maxim-integrated MAXQ7666 Bedienungsanleitung Seite 233