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UART has been implemented in two different fashions for Pmods. The reason for this is originally PIC32 microcontrollers had arranged the UART pins on a different location on the microcontroller, but after July 2010 the UART pins were rearranged, hence the need for the crossover cable.

Pmod Interface Type 3 (UART) This provides a UART interface with optional RTS/CTS handshaking. When this interface is placed on a twelve-pin connector on a host, pins 1-6 should be used (i.e. the upper row of pins). The RTS/CTS handshaking signals are optional, and some Pmods do not use them. In this case, pins 1 & 2 are not used. NOTE: This interface definition is deprecated. System boards and peripheral module boards designed before 07/2010 use this signal definition. This has been superseded by Pmod Interface Type 4 below. When making connections between a Type 3 UART connection and a Type 4 UART connection, a flying lead cable or crossover cable is required.

Pin Signal Direction 1 CTS Out 2 RTS In 3 RXD In 4 TXD Out 5 GND 6 VCC CTS - Permission for peripheral to send data to host RTS - Request from peripheral to send data to host RXD - Data from peripheral to host TXD - Data from host to peripheral Pmod Interface Type 4 (UART) This provides a UART interface with optional hardware flow control. When this interface is placed on a twelve-pin connector on a host, pins 1-6 should be used (i.e. the upper row of pins). The hardware flow control signals are optional, and some Pmods do not use them. In this case pins 1 & 4 are not used. The hardware flow control signal names are defined from the system board perspective. The RTS signal is an output indicating that the device (host or peripheral) is ready to receive data. The device asserts this signal low when it is ready to receive data. The CTS signal is an input to the device (host or peripheral). The device will only transmit data when the CTS input is asserted low. A peripheral module that uses hardware flow control will connect the host’s RTS signal to its internal CTS input and the host’s CTS signal to its internal RTS output. NOTE: This interface type supersedes the Type 3 UART interface. System and peripheral module boards designed after 07/2010 use this signal definition for UART connectors. When making connections between the Type 3 UART interface and the Type 4 UART interface, a flying lead cable or crossover cable is required. The change in UART signal definition from Type 3 UART to Type 4 UART was motivated by the fact that some microcontroller families (notable some Microchip PIC32 parts) have serial interface controllers that support both SPI operation and UART operation. This signal definition allows system boards using those microcontroller chips to have Pmod connectors that can support both SPI and UART on the same connector. Digilent Pmod™ Interface Specification www.digilentinc.com page 9 of 11 Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Pin Signal Direction 1 CTS In 2 TXD Out 3 RXD In 4 RTS Out 5 GND 6 VCC CTS - Device will only transmit when this signal is asserted RTS - Device is ready to receive data RXD - Data from peripheral to host TXD - Data from host to peripheral