SC68C652B datasheet(12/43 Pages) NXP

12 / 43 page

SC68C652B_2

Product data sheet

Rev. 02 — 2 November 2009

12 of 43

NXP Semiconductors

SC68C652B

Dual UART with 32-byte FIFOs and IrDA encoder/decoder

6.8 Programmable baud rate generator

The SC68C652B supports high speed modem technologies that have increased input

data rates by employing data compression schemes. For example, a 33.6 kbit/s modem

that employs data compression may require a 115.2 kbit/s input data rate. A 128.0 kbit/s

ISDN modem that supports data compression may need an input data rate of 460.8 kbit/s.

The SC68C652B can support a standard data rate of 921.6 kbit/s.

A single baud rate generator is provided for the transmitter and receiver, allowing

independent transmit/receive channel control. The programmable baud rate generator is

capable of operating with a frequency of up to 80 MHz. To obtain maximum data rate, it is

necessary to use full rail swing on the clock input. The SC68C652B can be conﬁgured for

internal or external clock operation. For internal clock oscillator operation, an industry

standard microprocessor crystal is connected externally between the XTAL1 and XTAL2

pins. Alternatively, an external clock can be connected to the XTAL1 pin to clock the

internal baud rate generator for standard or custom rates (see Table 6).

The generator divides the input 16

× clock by any divisor from 1 to (216 − 1). The

SC68C652B divides the basic external clock by 16. The basic 16

× clock provides table

rates to support standard and custom applications using the same system design. The

rate table is conﬁgured via the DLL and DLM internal register functions. Customized baud

rates can be achieved by selecting the proper divisor values for the MSB and LSB

sections of baud rate generator.

Programming the baud rate generator registers DLM (MSB) and DLL (LSB) provides a

user capability for selecting the desired ﬁnal baud rate. The example in Table 6 shows the

selectable baud rate table available when using a 1.8432 MHz external clock input.

Fig 3.

Crystal oscillator connection

002aab325

33 pF

XTAL1

XTAL2

1.8432 MHz

22 pF

SC68C652B Datasheet(PDF) 12 Page - NXP Semiconductors