ST16C552 datasheet(21/39 Pages) EXAR

21 / 39 page

ST16C552/552A

Rev. 3.40

ISR BIT-0:

Logic 0 = An interrupt is pending and the ISR contents

may be used as a pointer to the appropriate interrupt

service routine.

Logic 1 = No interrupt pending. (normal default condi-

tion)

ISR BIT 1-3: (logic 0 or cleared is the default condition)

These bits indicate the source for a pending interrupt

at interrupt priority levels 1, 2, and 3 (See Interrupt

Source Table).

ISR BIT 4-5: (logic 0 or cleared is the default condition)

Not Used - initialized to a logic 0.

ISR BIT 6-7: (logic 0 or cleared is the default condition)

These bits are set to a logic 0 when the FIFO’s are not

being used in the 16C450 mode. They are set to a logic

1 when the FIFO’s are enabled in the 16C552/552A

mode.

Line Control Register (LCR)

The Line Control Register is used to specify the

asynchronous data communication format. The word

length, the number of stop bits, and the parity are

selected by writing the appropriate bits in this register.

LCR BIT 0-1: (logic 0 or cleared is the default condi-

tion)

These two bits specify the word length to be transmit-

ted or received.

BIT-1

BIT-0

Word length

LCR BIT-2: (logic 0 or cleared is the default condition)

The length of stop bit is specified by this bit in

conjunction with the programmed word length.

BIT-2

Word length

Stop bit

length

(Bit time(s))

5,6,7,8

1-1/2

6,7,8

LCR BIT-3:

Parity or no parity can be selected via this bit.

Logic 0 = No parity. (normal default condition)

Logic 1 = A parity bit is generated during the transmis-

sion, receiver checks the data and parity for transmis-

sion errors.

LCR BIT-4:

If the parity bit is enabled with LCR bit-3 set to a logic

1, LCR BIT-4 selects the even or odd parity format.

Logic 0 = ODD Parity is generated by forcing an odd

number of logic 1’s in the transmitted data. The

receiver must be programmed to check the same

format. (normal default condition)

Logic 1 = EVEN Parity is generated by forcing an even

the number of logic 1’s in the transmitted. The receiver

must be programmed to check the same format.

LCR BIT-5:

If the parity bit is enabled, LCR BIT-5 selects the

forced parity format.

LCR BIT-5 = logic 0, parity is not forced. (normal

default condition)

LCR BIT-5 = logic 1 and LCR BIT-4 = logic 0, parity bit

is forced to a logical 1 for the transmit and receive

data.

LCR BIT-5 = logic 1 and LCR BIT-4 = logic 1, parity bit

is forced to a logical 0 for the transmit and receive

data.

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