XR16V2550 Datasheet, PDF (8/48 Pages) Exar Corporation – HIGH PERFORMANCE DUART WITH 16-BYTE FIFO

XR16V2550
PRELIMINARY
HIGH PERFORMANCE DUART WITH 16-BYTE FIFO
REV. P1.0.0
registers, but do not attempt to read from both uarts simultaneously. Individual channel select functions are
shown in Table 1.
TABLE 1: CHANNEL A AND B SELECT
CSA#
1
0
1
0
CSB#
1
1
0
0
FUNCTION
UART de-selected
Channel A selected
Channel B selected
Channel A and B selected
2.6 Channel A and B Internal Registers
Each UART channel in the V2550 has a set of enhanced registers for control, monitoring and data loading and
unloading. The configuration register set is compatible to those already available in the standard single
16C550 and dual ST16C2550. These registers function as data holding registers (THR/RHR), interrupt status
and control registers (ISR/IER), a FIFO control register (FCR), receive line status and control registers (LSR/
LCR), modem status and control registers (MSR/MCR), programmable data rate (clock) divisor registers (DLL/
DLM/DLD), and a user accessible Scratchpad Register (SPR).
Beyond the general 16C2550 features and capabilities, the V2550 offers enhanced feature registers (EFR,
Xon/Xoff 1, Xon/Xoff 2) that provide automatic RTS and CTS hardware flow control, and Xon/Xoff software flow
control. All the register functions are discussed in full detail later in “Section 3.0, UART INTERNAL
REGISTERS” on page 21.
2.7 DMA Mode
The device does not support direct memory access. The DMA Mode (a legacy term) in this document doesn’t
mean “direct memory access” but refers to data block transfer operation. The DMA mode affects the state of
the RXRDY# A/B and TXRDY# A/B output pins. The transmit and receive FIFO trigger levels provide additional
flexibility to the user for block mode operation. The LSR bits 5-6 provide an indication when the transmitter is
empty or has an empty location(s) for more data. The user can optionally operate the transmit and receive
FIFO in the DMA mode (FCR bit-3=1). When the transmit and receive FIFO are enabled and the DMA mode is
disabled (FCR bit-3 = 0), the V2550 is placed in single-character mode for data transmit or receive operation.
When DMA mode is enabled (FCR bit-3 = 1), the user takes advantage of block mode operation by loading or
unloading the FIFO in a block sequence determined by the selected trigger level. In this mode, the V2550 sets
the TXRDY# pin when the transmit FIFO becomes full, and sets the RXRDY# pin when the receive FIFO
becomes empty. The following table shows their behavior. Also see Figures 17through 22.
TABLE 2: TXRDY# AND RXRDY# OUTPUTS IN FIFO AND DMA MODE
PINS
FCR BIT-0=0
(FIFO DISABLED)
FCR BIT-0=1 (FIFO ENABLED)
FCR BIT-3 = 0
(DMA MODE DISABLED)
FCR BIT-3 = 1
(DMA MODE ENABLED)
RXRDY# A/B LOW = 1 byte
HIGH = no data
LOW = at least 1 byte in FIFO
HIGH = FIFO empty
HIGH to LOW transition when FIFO reaches the
trigger level, or time-out occurs
LOW to HIGH transition when FIFO empties
TXRDY# A/B LOW = THR empty LOW = FIFO empty
LOW = FIFO has at least 1 empty location
HIGH = byte in THR HIGH = at least 1 byte in FIFO HIGH = FIFO is full
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