CP2102N Datasheet, PDF (18/44 Pages) Silicon Laboratories – USBXpress Family

CP2102N Data Sheet
Functional Description
4.3.7 Hardware Handshaking (RTS and CTS)
To utilize the functionality of the RTS and CTS pins of the CP2102N, the device must be configured to use hardware flow control on the
USB host.
RTS, or Ready To Send, is an active-low output from the CP2102N and indicates to the external UART device that the CP2102N’s
UART RX FIFO has not reached the FLOW OFF watermark level of 448 bytes and is ready to accept more data. When the amount of
data in the RX FIFO reaches the watermark, the CP2102N pulls RTS high to indicate to the external UART device to stop sending data.
The CP2102N does not pull RTS low again until the UART RX FIFO is at the FLOW ON watermark level of 384 bytes (at least 128 free
bytes). This hysteresis allows for optimal operation. These RTS watermark levels are configurable using Xpress Configurator in Simplic-
ity Studio.
CTS, or Clear To Send, is an active-low input to the CP2102N and is used by the external UART device to indicate to the CP2102N
when the external UART device’s RX FIFO is getting full. The CP2102N will not send more than two bytes of data once CTS is pulled
high.
Hardware handshaking allows for optimal continuous transmission speeds at high baud rates (greater than 1 MBaud). The effective
throughput depends on USB bus loading and host USB stack efficiency. The typical maximum continuous bidirectional data transfer is
> 450 kbytes/s at 3 Mbaud.
CP2102N
TX
RX
RTS
CTS
RS232
System
TX
RX
RTS
CTS
Figure 4.1. Using Hardware Flow Control with the CP2102N
4.3.8 Software Handshaking
The CP2102N also supports software handshaking using the XON and XOFF event characters. The characters used for XON/XOFF is
set by the host software.
If the CP2102N receives an XOFF request, it will stop transmission, even if the CP2102N receiver needs to transmit an XOFF over
UART. This can potentially allow an overflow to occur or a deadlock condition if both the CP2102N and the connected UART device
transmit XOFF at the same time. The XOFF_CONTINUE setting allows the CP2102N transmitter to send XOFF/XON requests even if it
has received an XOFF request from the connected UART device. Once the connected UART device transmits XON, normal transmis-
sion from the CP2102N resumes.
Software handshaking uses the same watermark levels as hardware handshaking and can be configured dynamically by host software.
Watermark levels greater than 512 are converted to an XON limit of 448 bytes and an XOFF limit of 512 bytes. If the XON limit crosses
over the XOFF limit, the XON limit will automatically be modified to not cross over the XOFF limit. An XOFF limit of 0 is converted to 64
to guarantee buffer space is available until the UART end device stops transmission. When setting the XON and XOFF limits, it's rec-
ommended to use values where the XON limit added to the XOFF limit is less than 512 bytes, like 192/192 or 128/128. CP2102N
testing shows that the XON limit set to 192 and XOFF limit set to 192 provides optimal software flow control behavior.
CP2102N
CP2102N
receives XON
TX 0x64 0x87 0x51
CP2102N
receives XOFF
0xF0 0x23 0x40
RS232
System
RX
Figure 4.2. Software Flow Control Timing Diagram
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