OXCF950 datasheet(36/66 Pages) OXFORD

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Page 36

OXCF950 rev B DATA SHEET V 1.0

OXFORD SEMICONDUCTOR LTD.

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

the16C550) or 128.

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

generated or DMA signals can be used to transfer data

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

6.11.2).

6.4.1

FIFO Control Register ‘FCR’

FCR[0]: Enable FIFO mode

logic 0 ⇒ Byte mode.

logic 1 ⇒ FIFO mode.

This bit should be enabled before setting the FIFO trigger

levels.

FCR[1]: Flush RHR

logic 0 ⇒ No change.

logic 1 ⇒ Flushes the contents of the RHR

This is only operative when already in a FIFO mode. The

RHR is automatically flushed whenever changing between

Byte mode and a FIFO mode. This bit will return to zero

after clearing the FIFOs.

FCR[2]: Flush THR

logic 0 ⇒ No change.

logic 1 ⇒ Flushes the contents of the THR, in the same

manner as FCR[1] does for the RHR.

DMA Transfer Signalling:

FCR[3]: DMA signalling mode / Tx trigger level enable

logic 0 ⇒ DMA mode '0'.

logic 1 ⇒ DMA mode '1'.

DMA signals are not bonded out in the OXCF950, so this

control only affects the transmitter trigger level in DMA

mode 0.

FCR[5:4]: THR trigger level

Generally in 450, 550, extended 550 and 950 modes these

bits are unused (see section 6.1 for mode definition). In

650 mode they define the transmitter interrupt trigger levels

and in 750 mode FCR[5] increases the FIFO size.

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 are set

to the following values:

FCR[5:4]

Transmit Interrupt Trigger level

112

Table 29: Transmit Interrupt Trigger Levels

These levels only apply when in Enhanced mode and in

DMA mode 1 (FCR[3] = 1), 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.

750 Mode:

In 750 compatible non-Enhanced (EFR[4]=0) mode,

transmitter trigger level is set to 1, FCR[4] is unused and

FCR[5] defines the FIFO depth as follows:

FCR[5]=0 Transmitter and receiver FIFO size is 16 bytes.

FCR[5]=1 Transmitter and receiver FIFO size is 128 bytes.

In non-Enhanced mode FCR[5] is only writable when

LCR[7] is set. Note that in Enhanced mode, the FIFO size

is also increased to 128 bytes when FCR[0] is set.

950 mode:

Setting ACR[5]=1 enables arbitrary transmitter trigger level

setting using the TTL register (see section 6.11.4), hence

FCR[5:4] are ignored.

FCR[7:6]: RHR trigger level

In 550, extended 550, 650 and 750 modes, the receiver

FIFO trigger levels are defined using FCR[7:6]. The

interrupt trigger level and upper flow control trigger level

where appropriate are defined by L2 in the ta ble below. L1

defines the lower flow control trigger level where

applicable. Separate upper and lower flow control trigger

levels introduce a hysteresis element in in-band and out-of-

band flow control (see section 6.9).

Mode

650

FIFO Size 128

750

FIFO Size 128

550

FIFO Size 16

FCR

[7:6]

n/a

112

n/a

112 120

112

n/a

Table 30: Receiver Trigger Levels

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