STM8S datasheet(346/467 Pages) STMICROELECTRONICS

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Universal asynchronous receiver transmitter (UART)

RM0016

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DocID14587 Rev 14

The SIR receive decoder demodulates the return-to-zero bit stream from the infrared

detector and outputs the received NRZ serial bit stream to UART. The decoder input is

normally HIGH (marking state) in the idle state. The transmit encoder output has the

opposite polarity to the decoder input. A start bit is detected when the decoder input is low.

•

IrDA is a half duplex communication protocol. If the Transmitter is busy (i.e. the UART

is sending data to the IrDA encoder), any data on the IrDA receive line will be ignored

by the IrDA decoder and if the Receiver is busy (UART is receiving decoded data from

the UART), data on the TX from the UART to IrDA will not be encoded by IrDA. While

receiving data, transmission should be avoided as the data to be transmitted could be

corrupted.

•

A ’0’ is transmitted as a high pulse and a ’1’ is transmitted as a ’0’. The width of the

pulse is specified as 3/16th of the selected bit period in normal mode (see Figure 129).

•

The SIR decoder converts the IrDA compliant receive signal into a bit stream for the

UART.

•

The SIR receive logic interprets a high state as a logic one and low pulses as logic

zeros.

•

The transmit encoder output has the opposite polarity to the decoder input. The SIR

output is in low state when idle.

•

The IrDA specification requires the acceptance of pulses greater than 1.41 us. The

acceptable pulse width is programmable. Glitch detection logic on the receiver end

filters out pulses of width less than 2 PSC periods (PSC is the prescaler value

programmed in UART_PSCR). Pulses of width less than 1 PSC period are always

rejected, but those of width greater than one and less than two periods may be

accepted or rejected, those greater than 2 periods will be accepted as a pulse. The

IrDA encoder/decoder doesn’t work when PSC=0.

•

The receiver can communicate with a low-power transmitter.

•

In IrDA mode, the STOP bits in the UART_CR2 register must be configured to “1 stop

bit”.

IrDA low-power mode

The IrDA can be used either in normal mode or in Low Power mode. The Low Power mode

is selected by setting the IRLP bit in UART_CR5 register.

Transmitter:

In low-power mode the pulse width is not maintained at 3/16 of the bit period. Instead, the

width of the pulse is 3 times the low-power baud rate which can be a minimum of 1.42 MHz.

Generally this value is 1.8432 MHz (1.42 MHz < PSC< 2.12 MHz). A low-power mode

programmable divisor divides the system clock to achieve this value.

Receiver:

Receiving in low-power mode is similar to receiving in normal mode. For glitch detection the

UART should discard pulses of duration shorter than 1/PSC. A valid low is accepted only if

its duration is greater than 2 periods of the IrDA low-power Baud clock (PSC value in

UART_PSCR).

Note:

A pulse of width less than two and greater than one PSC period(s) may or may not be

rejected.

The receiver set up time should be managed by software. The IrDA physical layer

specification specifies a minimum of 10 ms delay between transmission and reception (IrDA

is a half duplex protocol).

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