
SMSC DS – LPC47M192 
Page 72 
Rev. 03/30/05 
DATASHEET 
SCRATCHPAD REGISTER (SCR)  
Address Offset =7H, DLAB =X, READ/WRITE 
This 8 bit read/write register has no effect on the operation of the Serial Port.  It is intended as a scratchpad register 
to be used by the programmer to hold data temporarily.  
PROGRAMMABLE BAUD RATE GENERATOR (AND DIVISOR LATCHES DLH, DLL)  
The Serial Port contains a programmable Baud Rate Generator that is capable of dividing the internal PLL clock by 
any divisor from 1 to 65535.  The internal PLL clock is divided down to generate a 1.8462MHz frequency for Baud 
Rates less than 38.4k, a 1.8432MHz frequency for 115.2k, a 3.6864MHz frequency for 230.4k and a 7.3728MHz 
frequency for 460.8k.  This output frequency of the Baud Rate Generator is 16x the Baud rate.  Two 8 bit latches 
store the divisor in 16 bit binary format.  These Divisor Latches must be loaded during initialization in order to insure 
desired operation of the Baud Rate Generator.  Upon loading either of the Divisor Latches, a 16 bit Baud counter is 
immediately loaded.  This prevents long counts on initial load.  If a 0 is loaded into the BRG registers the output 
divides the clock by the number 3.  If a 1 is loaded the output is the inverse of the input oscillator.  If a two is loaded 
the output is a divide by 2 signal with a 50% duty cycle.  If a 3 or greater is loaded the output is low for 2 bits and high 
for the remainder of the count.  The input clock to the BRG is a 1.8462 MHz clock. 
Table 30 shows the baud rates possible. 
Effect Of The Reset on Register File 
The Reset Function (details the effect of the Reset input on each of the registers of the Serial Port. 
FIFO INTERRUPT MODE OPERATION 
When the RCVR FIFO and receiver interrupts are enabled (FCR bit 0 = “1”, IER bit 0 = “1”), RCVR interrupts occur as 
follows: 
A. The receive data available interrupt will be issued when the FIFO has reached its programmed trigger 
level; it is cleared as soon as the FIFO drops below its programmed trigger level. 
B. The IIR receive data available indication also occurs when the FIFO trigger level is reached.  It is 
cleared when the FIFO drops below the trigger level. 
C. The receiver line status interrupt (IIR=06H), has higher priority than the received data available 
(IIR=04H) interrupt. 
D. The data ready bit (LSR bit 0) is set as soon as a character is transferred from the shift register to the 
RCVR FIFO.  It is reset when the FIFO is empty. 
When RCVR FIFO and receiver interrupts are enabled, RCVR FIFO timeout interrupts occur as follows: 
A. A FIFO timeout interrupt occurs if all the following conditions exist:   
At least one character is in the FIFO. 
The most recent serial character received was longer than 4 continuous character times ago. (If 2 stop 
bits are programmed, the second one is included in this time delay). 
The most recent CPU read of the FIFO was longer than 4 continuous character times ago. 
This will cause a maximum character received to interrupt issued delay of 160 msec at 300 BAUD with a 12 
bit character. 
B. Character times are calculated by using the RCLK input for a clock signal (this makes the delay 
proportional to the baudrate). 
C. When a timeout interrupt has occurred it is cleared and the timer reset when the CPU reads one 
character from the RCVR FIFO. 
D. When a timeout interrupt has not occurred the timeout timer is reset after a new character is received or 
after the CPU reads the RCVR FIFO. 
When the XMIT FIFO and transmitter interrupts are enabled (FCR bit 0 = “1”, IER bit 1 = “1”), XMIT interrupts occur 
as follows: 
A. The transmitter holding register interrupt (02H) occurs when the XMIT FIFO is empty; it is cleared as 
soon as the transmitter holding register is written to (1 of 16 characters may be written to the XMIT 
FIFO while servicing this interrupt) or the IIR is read. 
B. The transmitter FIFO empty indications will be delayed 1 character time minus the last stop bit time 
whenever the following occurs:  THRE=1 and there have not been at least two bytes at the same time in 
the transmitter FIFO since the last THRE=1. The transmitter interrupt after changing FCR0 will be 
immediate, if it is enabled.