Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement - Manual (Page 125)

High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
125
Mode 0
Mode 0 is synchronous, so the shift clock output frequency is the baud rate. Table 12-3 summarizes baud-rate generation as a func-
tion of the external oscillator frequency.
Table 12-3. Baud-Rate Generation, Mode 0
The default case is divide-by-12. The user can select the shift clock frequency using the SM2 bit in the associated SCON register. For
serial port 0, the SM2_0 bit is SCON0.5. For serial port 1, the SM2_1 bit is SCON1.5. For serial port 2, the SM2_2 bit is SCON2.5.
When SM2 is set to a logic 0, the baud rate is a divide-by-12 of the system clock frequency, unless power-management mode is
invoked. When operating in power-management mode with the SM2 bit clear (= 0), the serial port clock frequency is the oscillator fre-
quency divided by 3072.
When SM2 is set to a logic 1, the baud rate is generated using the system clock frequency divided by 4, unless power-management
mode is invoked. When power-management mode is used with the SM2 bit set (= 1), the serial port clock frequency tracks the system
clock frequency. Note that this use of SM2 differs from a standard 80C32. In that device, SM2 had no valid use when the UART was in
mode 0. Since the bit was generally set to a 0 and no clock multiplication was available on the 80C32, there should be no compatibil-
ity problems. The DS80C400 defaults to an oscillator divided by 12 serial port clock frequency.
Figure 12-1. Serial Port Mode 0 Block Diagram Change
1 This change is due to the addition of the third serial port (serial port 2).
2 This change is directly related to the inclusion of the clock multiplier.
MODE 0
SERIAL PORT CLOCK FREQUENCY
OSCILLATOR CYCLES PER
MACHINE CYCLE
PMR REGISTER BITS
4X/2X, CD1, CD0
SM2 = 0 (default) SM2 = 1
1 (4x mode) 100 OSC / 3 OSC / 1
2 (2x mode) 000 OSC / 6 OSC / 2
4 (default) X01, X10 OSC / 12 OSC / 4
1024 (PMM) X11 OSC / 3072 OSC / 1024
DS80C400 High-Speed Microcontroller User's Guide
SM2 = SCONx.5
(x = 0, 1, or 2) 1
DIVIDE-
BY-12
DIVIDE-
BY-4
0 1
SYSTEM CLOCK 2
DIVIDE-
BY-12
DIVIDE-
BY-4
0 1
SM2 =
SCONx.5
OSC
BAUD
CLOCK
BAUD
CLOCK
Maxim Integrated