XR16C854 Datasheet, PDF (27/51 Pages) Exar Corporation – QUAD UART WITH RX/TX FIFO COUNTERS,128-BYTE FIFO

XR16C854
FCR BIT-3:
Logic 0 = Set DMA mode “0”. (normal default condi-
tion)
Logic 1 = Set DMA mode “1.”
Transmit operation in mode “0”:
When the 854 is in the ST16C450 mode (FIFO’s
disabled, FCR bit-0 = logic 0) or in the FIFO mode
(FIFO’s enabled, FCR bit-0 = logic 1, FCR bit-3 = logic
0) and when there are no characters in the transmit
FIFO or transmit holding register, the -TXRDY pin will
be a logic 0. Once active the -TXRDY pin will go to a
logic 1 after the first character is loaded into the
transmit holding register.
Receive operation in mode “0”:
When the 854 is in mode “0” (FCR bit-0 = logic 0) or
in the FIFO mode (FCR bit-0 = logic 1, FCR bit-3 =
logic 0) and there is at least one character in the
receive FIFO, the -RXRDY pin will be a logic 0. Once
active the -RXRDY pin will go to a logic 1 when there
are no more characters in the receiver.
Transmit operation in mode “1”:
When the 854 is in FIFO mode ( FCR bit-0 = logic 1,
FCR bit-3 = logic 1 ), the -TXRDY pin will be a logic 1
when the transmit FIFO is completely full. It will be a
logic 0 if one or more FIFO locations are empty.
Receive operation in mode “1”:
When the 854 is in FIFO mode (FCR bit-0 = logic 1,
FCR bit-3 = logic 1) and the trigger level has been
reached, or a Receive Time Out has occurred, the -
RXRDY pin will go to a logic 0. Once activated, it will
go to a logic 1 after there are no more characters in the
FIFO.
FCR BIT 4-5: (logic 0 or cleared is the default condi-
tion, TX trigger level = none)
The XR16C854 provide 4 user selectable trigger
levels, The FCTR Bits 4-5 selects one of the following
table. These bits are used to set the trigger level for the
transmit FIFO interrupt. The XR16C854 will issue a
transmit empty interrupt when number of characters in
FIFO drops below the selected trigger level.
TRIGGER TABLE-A (Transmit)
“Default setting after reset ST16C550 mode”
BIT-5 BIT-4
FIFO trigger level
X
X
None
TRIGGER TABLE-B (Transmit)
BIT-5 BIT-4
FIFO trigger level
0
0
16
0
1
8
1
0
24
1
1
30
TRIGGER TABLE-C (Transmit)
BIT-5 BIT-4
FIFO trigger level
0
0
8
0
1
16
1
0
32
1
1
56
TRIGGER TABLE-D (Transmit)
BIT-5 BIT-4
FIFO trigger level
X
X
User programmable
Trigger levels
Rev. 1.00P
27