Maxim Integrated MAXQ7666 - Manual (Page 233)

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
Maxim Integrated