XR16C2850
FIFO Operation
Hardware Flow Control
The 128 byte transmit and receive data FIFO are
enabled by the FIFO Control Register (FCR) bit-0.
With 16C2550 devices, the user can set the receive
trigger level but not the transmit trigger level. The
2850 provides independent trigger levels for both
receiver and transmitter. To remain compatible with
ST16C2550, the transmit interrupt trigger level is set
to 16 following a reset. It should be noted that the user
cansetthetransmittriggerlevelsbywritingtotheFCR
register, but activation will not take place until EFR bit-
4issettoalogic1. ThereceiverFIFOsectionincludes
a time-out function to ensure data is delivered to the
external CPU. An interrupt is generated whenever the
Receive Holding Register (RHR) has not been read
following the loading of a character or the receive
triggerlevelhasnotbeenreached. (seehardwareflow
control for a description of this timing).
When automatic hardware flow control is enabled, the
2850 monitors the -CTS pin for a remote buffer
overflow indication and controls the -RTS pin for local
buffer overflows. Automatic hardware flow control is
selected by setting bits 6 (RTS) and 7 (CTS) of the
EFR register to a logic 1. If -CTS transitions from a
logic0toalogic1indicatingaflowcontrolrequest,ISR
bit-5 will be set to a logic 1 (if enabled via IER bit 6-7),
and the 2850 will suspend TX transmissions as soon
asthestopbitofthecharacterinprocessisshiftedout.
Transmission is resumed after the -CTS input returns
to a logic 0, indicating more data may be sent.
With the Auto RTS function enabled, an interrupt is
generated when the receive FIFO reaches the pro-
grammed trigger level. The -RTS pin will not be forced
to a logic 1 (RTS Off), until the receive FIFO reaches
the next trigger level. The-RTSpinwillreturntoalogic
0 after the data buffer (FIFO) is unloaded to the next
trigger level below the programmed trigger level.
Under the above described conditions the 2850 will
continue to accept data until the receive FIFO is full.
Example of 650 trigger level selection
Selected
Trigger
INT
Pin
-RTS
Logic 1
-RTS
Logic 0
Level
Activation
(characters) (characters)
(characters)
8
8
16
24
28
28
0
8
16
24
16
24
28
16
24
28
Rev. 1.00P
10
EXAR [ EXAR CORPORATION ]
