OXCB950
OXFORD SEMICONDUCTOR LTD.
7.4 Transmitter and receiver FIFOs
Both the transmitter and receiver have associated holding
registers (FIFOs), referred to as the transmitter holding
register (THR) and receiver holding register (RHR)
respectively.
Byte mode and a FIFO mode. This bit will return to zero
after clearing the FIFOs.
FCR[2]: Flush THR
logic 0 ⇒ No change.
In normal operation, when the transmitter finishes
transmitting a byte it will remove the next data from the top
of the THR and proceed to transmit it. If the THR is empty,
it will wait until data is written into it. If THR is empty and
the last character being transmitted has been completed
(i.e. the transmitter shift register is empty) the transmitter is
said to be idle. Similarly, when the receiver finishes
receiving a byte, it will transfer it to the bottom of the RHR.
If the RHR is full, an overrun condition will occur (see
section 7.5.3).
logic 1 ⇒Flushes the contents of the THR, in the same
manner as FCR[1] does for the RHR.
THR Trigger levels:
FCR[3]: Tx trigger level enable
logic 0 ⇒Transmit trigger levels enabled
logic 1 ⇒Transmit trigger levels disabled
When FCR[3]=0, the transmitter trigger level is always set
to 1, thus ignoring FCR[5:4]. Alternatively, 950-mode
trigger levels can be set using ACR[5].
Data is written into the bottom of the THR queue and read
from the top of the RHR queue completely asynchronously
to the operation of the transmitter and receiver.
FCR[5:4]: Compatible trigger levels
The size of the FIFOs is dependent on the setting of the
FCR register. When in Byte mode, these FIFOs only
accept one byte at a time before indicating that they are
full; this is compatible with the 16C450. When in a FIFO
mode, the size of the FIFOs is either 16 (compatible with
the 16C550) or 128.
450, 550 and extended 550 modes:
The transmitter interrupt trigger levels are set to 1 and
FCR[5:4] are ignored.
650 mode:
In 650 mode the transmitter interrupt trigger levels can be
set to the following values:
Data written to the THR when it is full is lost. Data read
from the RHR when it is empty is invalid. The empty or full
status of the FIFOs are indicated in the Line Status
Register ‘LSR’ (see section 7.5.3). Interrupts are generated
when the UART is ready for data transfer to/from the
FIFOs. The number of items in each FIFO may also be
read back from the transmitter FIFO level (TFL) and
receiver FIFO level (RFL) registers (see section 7.11.2).
FCR[5:4]
Transmit Interrupt Trigger level
00
01
10
11
16
32
64
112
Table 13: Transmit Interrupt Trigger Levels
These levels only apply when in Enhanced mode and when
FCR[3] is set, otherwise the trigger level is set to 1. A
transmitter empty interrupt will be generated (if enabled) if
the TFL falls below the trigger level.
7.4.1 FIFO Control Register ‘FCR’
FIFO setup:
FCR[0]: Enable FIFO mode
logic 0 ⇒Byte mode.
750 Mode:
logic 1 ⇒FIFO mode.
In 750 compatible mode, transmitter trigger level is set to 1,
FCR[4] is unused and FCR[5] defines the FIFO depth as
follows:
This bit should be enabled before setting the FIFO trigger
levels.
FCR[5]=0: FIFO size is 16 bytes.
FCR[5]=1: FIFO size is 128 bytes.
FCR[1]: Flush RHR
logic 0 ⇒No change.
logic 1 ⇒Flushes the contents of the RHR
In non-Enhanced mode and when FIFOSEL pin is low (tied
low internally on the OXCB950) FCR[5] is writable only
when LCR[7] is set. Note that in Enhanced mode, the FIFO
size is increased to 128 bytes when FCR[0] is set.
This is only operative when already in a FIFO mode. The
RHR is automatically flushed whenever changing between
DS-0033 Sep 05
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