Functional Description
In transmission, the LH software must exercise an element of real time control for transmitting data
packets; they must each be emitted at a constant delay from the start bits of each of the individual packets
which means when sending a series of packets, the packet to packet delay must respect a specific delay.
To control this delay 2 methods can be used:
•
By filling the TX FIFO with a number of zero bit which is transmitted with a T period.
•
By using an external system timer which controls the delay either between each start of frame or
between the end of a frame and the start of the next one. This can be performed:
–
By controlling the start of the frame through the configuration register MDR1[5] and ACREG[2]
depending on the timer status (in case of control the delay between each start of frame).
–
By using the TX_STATUS interrupt IIR[5] to pre-load the next frame in the TX FIFO and to control
the start of the timer (in case of control the delay between end of frame and start of next frame).
In reception, there are two ways to stop it :
•
The LH can disable the reception by setting the ACREG[5] to 1 when it considers that the reception is
finished because a large number of 0 has been received. To receive a new frame, the ACREG[5] must
be set to 0.
•
A specific mechanism, depending on the value set in the BOF length register (EBLR), allows for
automatically stopping the reception. If the value set in the EBLR register is different than 0, this
features is enabled and count a number of bit received at 0. When the counter achieves the value
defined in the EBLR register, the reception is automatically stopped and RX_STOP_IT (IIR[2]) is set.
When a 1 is detected on the RCRX pin, the reception is automatically enabled.
Note: There's a limitation when receiving data in UART CIR mode. The IrDA transceivers on the market
have a common characteristic that shrinks the hold time of the received modulation pulse. The UART
filtering schema on receiving is based on the same encoding mechanism used in transmission.
For the following scenario:
•
Shift register period: 0.9 us
•
Modulation frequency: 36 Khz
•
Duty cycle: 1/4 of a modulation frequency period
The data sent in these conditions would look like 7us pulses within 28us period. The UART expects to
receive similar incoming data on receive, but available transceiver timing characteristics typically send 2us
modulated pulses. Those will be filtered out and RX FIFO won’t receive any data.
This does not affect UART CIR mode in transmission.
Note: The CIR RX demodulation can be bypassed by setting the MDR3[0] register bit.
19.3.8.3.3 Carrier Modulation
Looking closer at the actual modulation pulses of the infrared data stream, it should be noted that each
modulated pulse that constitutes a digit is in fact a train of on/off pulses.
3492
Universal Asynchronous Receiver/Transmitter (UART)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated