PC87334VLJ datasheet(69/102 Pages) NSC

69 / 102 page

60 Serial Ports (Continued)

64 LINE STATUS REGISTER (LSR)

This 8-bit register provides status information to the CPU

concerning data transfers Table 6-2 shows the contents of

the Line Status Register Details on each bit follow

Bit 0

This bit is the receiver Data Ready (DR) indicator It

is set to 1 whenever a complete incoming character

has been received and transferred into the Receiver

Buffer Register or the FIFO It is reset to 0 by read-

ing the data in the Receiver Buffer Register or the

FIFO

Bit 1

This bit is the Overrun Error (OE) indicator It indi-

cates that data in the Receiver Buffer Register was

not read by the CPU before the next character was

transferred into the Receiver Buffer Register there-

by destroying the previous character The OE indica-

tor is set to 1 upon detection of an overrun condition

and reset whenever the CPU reads the contents of

the Line Status Register If the FIFO mode data con-

tinues to fill the FIFO beyond the trigger level an

Overrun error will occur only after the FIFO is com-

pletely full and the next character has been received

in the shift register OE is indicated to the CPU as

soon as it happens The character in the shift regis-

ter is overwritten but it is not transferred to the

FIFO

Bit 2

This bit is the Parity Error (PE) indicator It indicates

that the received data character does not have the

correct parity as selected by the even parity select

bit The PE bit is set to 1 upon detection of a parity

error and is reset to 0 whenever the CPU reads the

contents of the Line Status Register In the FIFO

mode this error is associated with the particular

character that it applies to in the FIFO This error is

revealed to the CPU when its associated character is

at the top of the FIFO

Bit 3

This bit is the Framing Error (FE) indicator It indi-

cates that the received character did not have a val-

id Stop bit It is set to 1 whenever the Stop bit follow-

ing the last data bit or parity bit is a 0 (Spacing level)

The FE indicator is reset whenever the CPU reads

the contents of the Line Status Register In the FIFO

mode this error is associated with the particular

character that it applies to in the FIFO This error is

revealed to the CPU when its associated character is

at the top of the FIFO The UART will try to resyn-

chronize after a framing error by assuming that the

error was due to the next start bit It samples this

‘‘start’’ bit twice and then takes in the bits following it

as the rest of the frame

Bit 4

This bit is the Break Interrupt (BI) indicator It is set

to 1 whenever the received data input is held in the

Spacing (0) state for longer than a full word trans-

mission time (ie the total time of Start bit a data

bits a Parity a Stop bits) The BI indicator is reset

whenever the CPU reads the contents of the Line

Status Register In the FIFO mode this error is asso-

ciated with the particular character that it applies to

in the FIFO This error is revealed to the CPU when

its associated character is at the top of the FIFO

When a break occurs only one character is loaded

into the FIFO To Restart after a break is received

the SIN pin must be 1 for at least one half bit time

Note

Bits 1 through 4 are the error conditions that produce a

Receiver Line Status interrupt whenever any of the corre-

sponding conditions are detected and the interrupt is en-

abled

Bit 5

This bit is the Transmitter Holding Register Empty

(THRE) indicator It indicates that the UART is ready

to accept a new character for transmission In addi-

tion it causes the UART to issue an interrupt to the

CPU when the Transmit Holding Register Empty In-

terrupt enable is set high The THRE bit is set to 1

when a character is transferred from the Transmitter

Holding Register into the Transmitter Shift Register

The bit is reset to 0 whenever the CPU loads the

Transmitter Holding Register In the FIFO mode it is

set when the XMIT FIFO is empty it is cleared when

at least 1 byte is written to the XMIT FIFO

Bit 6

This bit is the Transmitter Empty (TEMT) indicator It

is set to 1 whenever the Transmitter Holding Regis-

ter (THR) and the Transmitter Shift Register (TSR)

are both empty It is reset to 0 if either the THR or

TSR contains a data character In the FIFO mode

this bit is set to 1 whenever the transmitter FIFO and

the shift register are both empty

Bit 7

In the NS16450 Mode this is 0 In the FIFO Mode

this bit is set when there is at least one parity error

framing error or break indication in the FIFO It is

cleared when the CPU reads the LSR if there are no

subsequent errors in the FIFO

Note

The Line Status Register is intended for read operations

only Writing to this register is not recommended as this

operation is only used for factory testing In the FIFO mode

the software must load a data byte in the Rx FIFO via the

Loopback Mode in order to write to LSR2–LSR4 LSR0 and

LSR7 can’t be written to in the FIFO Mode

65 FIFO CONTROL REGISTER (FCR)

This is a write-only register at the same location as the IIR

(the IIR is a read-only register) This register is used to en-

able the FIFOs clear the FIFOs and to set the RCVR FIFO

trigger level

Bit 0

Writinga1to FCR0 enables both the XMIT and

RCVR FIFOs Resetting FCR0 clears all bytes in

both FIFOs When changing from FIFO Mode to

NS16450 Mode and vice versa data is automati-

cally cleared from the FIFOs This bit must al-

ready be 1 when other FCR bits are written to or

they will not be programmed

Bit 1

Writing 1 to FCR1 clears all bytes in the RCVR

FIFO and resets its counter logic to 0 The shift

bit position is self-clearing

Bit 2

Writing 1 to FCR2 clears all bytes in the XMIT

FIFO and resets its counter logic to 0 The shift

bit position is self-clearing

Bit 3

Writing to FCR3 does not change UART opera-

tions

Bits 4 5

FCR4 to FCR5 are reserved for future use

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