Samsung S3C2451X User Manual page 352

UART
Auto Flow Control (AFC)
The S3C2451X's UART 0, UART 1 and UART 2 support auto flow control with nRTS and nCTS signals. In case, it
can be connected to external UARTs. If users want to connect a UART to a Modem, disable auto flow control bit
in UMCONn register and control the signal of nRTS by software.
In AFC, nRTS depends on the condition of the receiver and nCTS signals control the operation of the transmitter.
The UART's transmitter transfers the data in FIFO only when nCTS signals are activated(Low) (in AFC, nCTS
means that other UART's FIFO is ready to receive data). Before the UART receives data, nRTS has to be
activated(Low) when its receive FIFO has a spare more than 32-byte and has to be inactivated(High) when its
receive FIFO has a spare under 32-byte (in AFC, nRTS means that its own receive FIFO is ready to receive data).
Transmission Case in UART A
UART A
NOTE:
UART 3 does not support AFC function, because the S3C2451X has no nRTS 3 and nCTS 3.
Example of Non Auto-Flow Control (Controlling nRTS and nCTS by Software)
Rx Operation with FIFO
1. Select receive mode (Interrupt or DMA mode).
2. Check the value of Rx FIFO count in UFSTATn register. If the value is less than 32, users have to set the
value of UMCONn[0] to '1' (activating nRTS), and if it is equal or larger than 32 users have to set the value to
'0' (inactivating nRTS).
3. Repeat the Step 2.
Tx Operation with FIFO
1. Select transmit mode (Interrupt or DMA mode).
2. Check the value of UMSTATn[0]. If the value is '1' (activating nCTS), users write the data to Tx FIFO register.
3. Repeat the Step 2.
Fifo Contains
33 byte
32 byte
31 byte
30 byte
Preliminary product information describe products that are in development,
for which full characterization data and associated errata are not yet available.
15-4
Specifications and information herein are subject to change without notice.
UART B
TxD RxD
nCTS nRTS
Figure 15-2. UART AFC interface
RX side
Fifo Space nRTS
31 byte High
32 byte High
33 byte Low
34 byte Low
Reception Case in UART A
UART A
nRTS meaning nCTS
Not ready to receive High
Not ready to receive High
Ready to Receive Low
Ready to Receive Low
S3C2451 RISC MICROPROCESSOR
UART B
RxD TxD
nRTS nCTS
TX side
nCTS meaning
Don't send data to RX
Don't send data to RX
Send data to RX
Send data to RX