IN16C1054
6.2.1 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
‘1’ now, RTS# is not changed to ‘0’ until the amount in RX FIFO equals the value written
in FLR.
The value of FUR and FLR is determined by FIFO mode. If FCR[7:6] holds ‘00’, ’01’, ‘10’,
and ‘11’, FUR stores 8, 16, 56, and 60, respectively. And if FCR[5:4] holds ‘00’, ’01’, ‘10’,
and ‘11’, FLR stores 0, 8, 16, and 56, respectively in 64-byte FIFO. In 256-byte FIFO
mode, users can write FUR and FLR values as they want and use them. But the value of
FUR must be larger than that of FLR. While Auto-RTS is enabled, you can verify if RTS#
is ‘0’ or ‘1’ by FSR[5]. If FSR[5] is ‘0’, RTS# is ‘0’ and if ‘1’, RTS# is ‘1’, too.
When IER[6] is set to ‘1’ and RTS# is changed from ‘0’ to ‘1’ by Auto-RTS function,
interrupt occurs and it is displayed on ISR[5:0]. Interrupts by Auto-RTS function are
removed if MSR is read. RTS# is changed from ‘0’ to ‘1’ after the first STOP bit is
received. Figure 4 shows the RTS# timing chart while Auto-RTS is enabled.
In Figure 4, Data Byte n-1 is received and RTS# is deasserted when the amount of data
in RX FIFO is larger than the value written in FUR. UART completes transmitting new
data (DATA BYTE n) which has started being transmitted even though external UART
recognizes RTS# has been deasserted. After that, the device stops transmitting more
data. If CPU reads data of RX FIFO, the value of RCR decreases and then if that value
equals that of FLR, RTS# is asserted for external UART to transmit new data.
START
DATA BYTE n-1
STOP
START
DATA BYTE n
DATA BYTE 2
FUR -0
STOP
START
RXD
RTS#
DATA BYTE 1
DATA BYTE n
IOR#
FUR + 0
FUR + 1
FUR - 1
FUR - 0
FLR + 1
FLR + 0
RCR[7:0]
Figure 4: RTS# Functional Timing
Rev. 00
IKSEMICON [ IK SEMICON CO., LTD ]
