STV0681 Hardware Interfaces

STV0681

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ADCS 7283313C

4.4

UART module for RS232 interface

The UART module on STV0681 provides a 115200 baud full duplex serial interface to an external

host.

4.4.1

RS232 physical interface

STV0681 does not support direct connection to a RS232 serial link. Physical line driver circuitry is

required, e.g. using a standard RS232 transceiver chip, or by using a small number of low cost

discretes (refer to Reference Design for details). Unscreened cable can be used, a suitable low cost

connector is a 3.5mm stereo audio jack plug.

4.4.2

UART module overview

Receiving data

The UART module in STV0681 receives serial data through the RXD pin. Data reception is initiated

by a 1-to-0 transition on RXD, and the received data is sampled every 8.66

µs.

If the RXD input is not 0 when the incoming data is first sampled, the UART module goes back to

look for another 1-to-0 transition. This is to provide rejection of false start bits. If the start bit proves

valid, reception of the rest of the frame proceeds.

Transmitting data

When a transmission is activated by STV0681, the contents of an internal transmit shift register are

shifted onto the TXD pin, every 8.66

µs.

The UART operates at 12MHz, which is not an integer multiple of 115200, so the actual baud rate is

115232 +/- 0.3%, which is well within the requirements of a typical 16x oversampling UART, which

can tolerate a 3.75% error in baud rate.