STV0681 Hardware Interfaces
STV0681
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 8.66µs.
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 8.66µs.
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.
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ADCS 7283313C
STMICROELECTRONICS [ ST ]
