CP2102N Datasheet, PDF (19/44 Pages) Silicon Laboratories – USBXpress Family

CP2102N Data Sheet
Functional Description
4.3.9 Data Throughput Optimization
Effective throughput depends on several factors:
• CP2102N placement on the physical USB device tree
• USB bus load from other devices
• Host OS USB stack efficiency
• CP2102N configuration options
Handshaking is required at high baud rates (greater than 1 MBaud) to avoid receiver overrun. A request to stop transmission is only
initiated once the RX FIFO has reached the FLOW OFF watermark level. Once the USB bus lowers the RX FIFO level below the FLOW
ON watermark, a request to continue transmission is sent.
Hardware handshaking allows for optimal continuous transmission speeds at high baud rates. Using a Windows host PC, the
CP2102N's typical maximum continuous bidirectional throughput is > 450 kbytes/s at 3 Mbaud (> 70% efficiency).
Software handshaking using XON/XOFF transmission requires more overhead. Using a Windows host PC, the CP2102N's typical maxi-
mum continuous bidirectional throughput is > 330 kbytes/s at 3 Mbaud (> 55% efficiency).
For these performance numbers, the CP2102N is placed on a USB hub connected to the Windows host PC with a third party UART
adapter. The only significant USB traffic is generated by the USB to UART devices. The Windows host PC is running automated tests
with minimal CPU load.
Certain conditions will reduce the maximum throughput at high baud rates (> 1Mbaud):
• Using DSR, DTR, or DCD handshaking signals lowers maximum performance. Use hardware CTS/RTS only for peak performance.
• Embedded events or error character insertion requires free space in the UART RX FIFO to post events to the host. At high baud
rates with continuous data reception, this space may not be available. Limit maximum baud rates with continuous data reception to 1
MBaud when using embedded events or error character insertion to guarantee reception of events or the error character.
• Transmitting an immediate character momentarily causes lower bidirectional throughput as the character forces a bypass of the cur-
rent transmit FIFO. Once the character has been transmitted, the typical bidirectional throughput is restored.
Using Remote Wakeup, Charge Enable, Clock Out, or the GPIOs will not impact UART throughput.
4.3.10 Transmit and Receive LED Toggles (TXT and RXT)
The TX and RX LED toggle pins will toggle on and off at a fixed rate specified in Table 3.7 GPIO on page 10 whenever a byte is trans-
mitted or received by the CP2102N. These pins are logic high whenever a device is not transmitting or receiving data and can directly
drive basic LEDs within the device specification limits.
VIO
CP2102N
TXT
RXT
Figure 4.3. Transmit and Receive Toggle
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