Data Sheet

ISM14585-L35 Specification
DOC-DS-14585-201807-3.0
Confidential Inventek Systems
Page 54
Figure 24 Flowchart of Interrupt Generation for Programmable THRE Interrupt Mode
This threshold level is programmed into FCR[5:4]. The available empty thresholds are:
empty, 2, ¼ and ½. See UART_FCR_REG for threshold setting details. Selection of the
best threshold value depends on the system's ability to begin a new transmission
sequence in a timely manner. However, one of these thresholds should prove optimum
in increasing system performance by preventing the transmitter FIFO from running
empty.
In addition to the interrupt change, Line Status Register (LSR[5]) also switches function
from indicating transmitter FIFO empty, to FIFO full. This allows software to fill the FIFO
each transmit sequence by polling LSR[5] before writing another character. The flow
then becomes, “fill transmitter FIFO whenever an interrupt occurs and there is data to
transmit”, instead of waiting until the FIFO is completely empty. Waiting until the FIFO is
empty causes a performance hit whenever the system is too busy to respond
immediately.
Even if everything else is selected and enabled, if the FIFOs are disabled via FCR[0],
the Programmable THRE Interrupt mode is also disabled. When not selected or
disabled, THRE interrupts and LSR[5] function normally (both reflecting an empty THR