SC16C850 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.
Product data sheet Rev. 2 — 11 November 2010 24 of 55
NXP Semiconductors
SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
7.2.1 IER versus Transmit/Receive FIFO interrupt mode operation
When the receive FIFO is enabled (FCR[0] = logic 1), and receive interrupts
(IER[0] = logic 1) are enabled, the receive interrupts and register status will reflect the
following:
• The receive RXRDY interrupt (Level 2 ISR interrupt) is issued to the external CPU
when the receive FIFO has reached the programmed trigger level. It will be cleared
when the receive FIFO drops below the programmed trigger level.
• Receive FIFO status will also be reflected in the user accessible ISR register when
the receive FIFO trigger level is reached. Both the ISR register receive status bit and
the interrupt will be cleared when the FIFO drops below the trigger level.
• The receive data ready bit (LSR[0]) is set as soon as a character is transferred from
the shift register (RSR) to the receive FIFO. It is reset when the FIFO is empty.
• When the Transmit FIFO and interrupts are enabled, an interrupt is generated when
the transmit FIFO is empty due to the unloading of the data by the TSR and UART for
transmission via the transmission media. The interrupt is cleared either by reading the
ISR, or by loading the THR with new data characters.
7.2.2 IER versus Receive/Transmit FIFO polled mode operation
When FCR[0] = logic 1, setting IER[3:0] puts the SC16C850 in the FIFO polled mode of
operation. In this mode, interrupts are not generated and the user must poll the LSR
register for TX and/or RX data status. Since the receiver and transmitter have separate
bits in the LSR either or both can be used in the polled mode by selecting respective
transmit or receive control bit(s).
• LSR[0] will be a logic 1 as long as there is one byte in the receive FIFO.
• LSR[4:1] will provide the type of receive errors, or a receive break, if encountered.
• LSR[5] will indicate when the transmit FIFO is empty.
• LSR[6] will indicate when both the transmit FIFO and transmit shift register are empty.
• LSR[7] will show if any FIFO data errors occurred.
1 IER[1] Transmit Holding Register interrupt. In the non-FIFO mode, this interrupt will be
issued whenever the THR is empty, and is associated with LSR[5]. In the FIFO
modes, this interrupt will be issued whenever the FIFO is empty.
logic 0 = disable the Transmit Holding Register Empty (TXRDY) interrupt
(normal default condition)
logic 1 = enable the TXRDY (ISR level 3) interrupt
0 IER[0] Receive Holding Register interrupt. In the non-FIFO mode, this interrupt will be
issued when the RHR has data, or is cleared when the RHR is empty. In the FIFO
mode, this interrupt will be issued when the FIFO has reached the programmed
trigger level or is cleared when the FIFO drops below the trigger level.
logic 0 = disable the receiver ready (ISR level 2, RXRDY) interrupt (normal
default condition)
logic 1 = enable the RXRDY (ISR level 2) interrupt
Table 9. Interrupt Enable Register bits description
…continued
Bit Symbol Description
