SB16C1058_Data Sheet_EN

SB16C1058
OCTAL UART WITH 256-BYTE FIFO
JULY 2013 REV 1.04
17
6.3 FIFO Operation
SB16C1058’s FIFO has two modes, 64-byte FIFO mode and 256-byte FIFO mode.
Setting FCR[0] to ‘1’ enables FIFO, and if AFR[0] is set to ‘0’, it operates in 64-byte FIFO
mode(default). In this mode, Transmit Data FIFO, Receive Data and Receive Status FIFO
are 64 bytes. 64-byte FIFO mode allows you to select the Transmit Interrupt Trigger Level
from 8, 16, 32, or 56. You can verify this Interrupt Trigger Level by TTR and RTR. In this
mode TTR and RTR are Read Only.
And by FCR[5:4], XOFF Trigger Level can be selected to either 8, 16, 56, or 60, and XON
Trigger Level to either 0, 8, 16, or 56 by FCR[7:6]. You can verify XON and XOFF Trigger
Level by FUR and FLR. In 64-byte FIFO mode TTR and RTR are Read Only.
If you select 256-byte FIFO mode, you can experience more powerful features of
SB16C1058. Setting both FCR[0] and AFR[0] to ‘1’ will enable this mode. In this mode,
Transmit Data FIFO, Receive Data and Receive Status FIFO are 256 bytes. Interrupt
Trigger Level and XON, XOFF Trigger Level are controlled by TTR, RTR, FUR and FLR,
not by FCR[7:4]. That is, TTR, RTR, FUR and FLR can both read and write. You can
verify free space of Transmit FIFO and the number of characters received in Receive
FIFO by TCR, RCR and ISR[7:6].
While TX FIFO is full, the value sent to THR by CPU disappears. And while RX FIFO is
full, the data coming from external devices disappear as well, provided that flow control
function is not used.
For more information, refer to Register Description.
6.4 Hardware Flow Control
Hardware flow control is executed by Auto-RTS and Auto-CTS. Auto-RTS and Auto-CTS
can be enabled/disabled independently by programming EFR[7:6]. If Auto-RTS is
enabled, it reports that it cannot receive more data by asserting RTS# when the amount
of received data in RX FIFO exceeds the written value in FUR. Then after the data stored
in RX FIFO is read by CPU, it reports that it can receive new data by deasseting RTS#
when the amount of existing data in RX FIFO is less than the written value in FLR.
When Auto-CTS is enabled and CTS# is cleared to ‘0’, transmitting data to TX FIFO has
to be suspended because external device has reported that it cannot accept more data.
When data transmission has been suspended and CTS# is set to ‘1’, data in TX FIFO is
retransmitted because external device has reported that it can accept more data. These
operations prevent overrun during communication and if hardware flow control is disabled
and transmit data rate exceeds RX FIFO service latency, overrun error occurs.
6.4.1 Auto-RTS
To enable Auto-RTS, EFR[6] should be set to ‘1’. Once enabled, RTS# outputs ‘0’. If the
number of received data in RX FIFO is larger than the value stored in FUR, RTS# will be
changed to ‘1’ and if not, holds ‘0’. This state indicates that RX FIFO can accept more
data. After RTS# changed to ‘1’ and reported to the CPU that it cannot accept more data,
the CPU reads the data in RX FIFO and then the amount of data in RX FIFO reduces.
When the amount of data in RX FIFO equals the value written in FLR, RTS# changes to
‘0’ and reports that it can accept more data. That is, if RTS# is ‘0’ now, RTS# is not
changed to ‘1’ until the amount in RX FIFO exceeds the value set in FUR. But if RTS# is