PC87309VLJ datasheet(106/192 Pages) NSC

106 / 192 page

Enhanced Serial Port with IR -UART2 (Logical Device 2)

106

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5.7.2

Consumer-IR Reception

The Consumer-IR receiver is significantly different from a

UART receiver in two ways. Firstly, the incoming infrared

signals are DASK modulated. Therefore, demodulation may

be necessary. Secondly, there are no start bits in the incom-

ing data stream.

Whenever an infrared signal is detected, receiver opera-

tions depend on whether or not receiver demodulation is en-

abled. If demodulation is disabled, the receiver immediately

becomes active. If demodulation is enabled, the receiver

checks the carrier frequency of the incoming signal, and be-

comes active only if the frequency is within the programmed

range. Otherwise, the signal is ignored and no other action

is taken.

When the receiver enters the active state, the RXACT bit in

the ASCR register is set to 1. Once in the active state, the

receiver keeps sampling the infrared input signal and gen-

erates a bit string where a logic 1 indicates an idle condition

and a logic 0 indicates the presence of infrared energy. The

infrared input is sampled regardless of the presence of in-

frared pulses at a rate determined by the value loaded into

the Baud Generator Divisor Registers. The received bit

string is either de-serialized and assembled into 8-bit char-

acters, or it is converted to run-length encoded values. The

resulting data bytes are then transferred into the RX_FIFO.

The receiver also sets the RXWDG bit in the ASCR register

each time an infrared pulse signal is detected. This bit is au-

tomatically cleared when the ASCR register is read, and it

is intended to assist the software in determining when the

infrared link has been idle for a certain time. The software

can then stop the data reception by writing a 1 into the RX-

ACT bit to clear it and return the receiver to the inactive

state.

The frequency bandwidth for the incoming modulated infra-

red signal is selected by the DFR and DBW fields in the IR-

RXDC register.

There are two Consumer-IR reception data modes: “Over-

sampled” and “Programmed T Period” mode. For either

mode the sampling rate is determined by the setting of the

Baud Generator Divisor Registers.

The “Over-sampled” mode can be used with the receiver

demodulator either enabled or disabled. It should be used

with the demodulator disabled when a detailed snapshot of

the incoming signal is needed, for example to determine the

period of the carrier signal. If the demodulator is enabled,

the stream of samples can be used to reconstruct the in-

coming bit string. To obtain good resolution, a fairly high

sampling rate should be selected.

The “Programmed-T-Period” mode should be used with the

receiver demodulator enabled. The T Period represents

one half bit time for protocols using biphase encoding, or

the basic unit of pulse distance for protocols using pulse dis-

tance encoding. The baud is usually programmed to match

the T Period. For long periods of logic low or high, the re-

ceiver samples the demodulated signal at the programmed

sampling rate.

Whenever a new infrared energy pulse is detected, the re-

ceiver synchronizes the sampling process to the incoming

signal timing. This reduces timing related errors and elimi-

nates the possibility of missing short infrared pulse se-

quences, especially with the RECS 80 protocol.

In addition, the “Programmed-T-Period” sampling minimiz-

es the amount of data used to represent the incoming infra-

red signal, therefore reducing the processing overhead in

the host CPU.

5.8

FIFO TIME-OUTS

Time-out mechanisms prevent received data from remain-

ing in the RX_FIFO indefinitely, if the programmed interrupt

or DMA thresholds are not reached.

An RX_FIFO time-out generates a Receiver Data Ready in-

terrupt and/or a receiver DMA request if bit 0 of IER and/or

bit 2 of MCR (in Extended mode) are set to 1 respectively.

An RX_FIFO time-out also sets bit 0 of ASCR to 1 if the

RX_FIFO is below the threshold. When a Receiver Data

Ready interrupt occurs, this bit is tested by the software to

determine whether a number of bytes indicated by the

RX_FIFO threshold can be read without checking bit 0 of

the LSR register.

The conditions that must exist for a time-out to occur in the

various modes of operation are described below.

When a time-out has occurred, it can only be reset when the

FIFO is read by the CPU or DMA controller.

5.8.1

UART, SIR or Sharp-IR Mode Time-Out Conditions

Two timers (timer1 and timer 2) are used to generate two

different time-out events (A and B, respectively). Timer 1

times out after 64

µsec. Timer 2 times out after four charac-

ter times.

Time-out event A generates an interrupt and sets the

RXF_TOUT bit (bit 0 of ASCR) when all of the following are

true:

At least one byte is in the RX_FIFO, and

More than 64

µsec or four character times, whichever is

greater, have elapsed since the last byte was loaded

into the RX_FIFO from the receiver logic, and

More than 64

µsec or four character times, whichever is

greater, have elapsed since the last byte was read from

the RX_FIFO by the CPU or DMA controller.

Time-out event B activates the receiver DMA request and is

invisible to the software. It occurs when all of the following

are true:

At least one byte is in the RX_FIFO, and

More than 64

µsec or four character times, whichever is

smaller, have elapsed since the last byte was loaded

into the RX_FIFO from the receiver logic, and

More than 64

µsec or four character times, whichever is

smaller, have elapsed since the last byte was read from

the RX_FIFO by the CPU or DMA controller.

5.8.2

Consumer-IR Mode Time-Out Conditions

The RX_FIFO time-out, in Consumer-IR mode, is disabled

while the receiver is active. It occurs when all of the follow-

ing are true:

At least one byte has been in the RX_FIFO for 64

µsec

or more, and

The receiver has been inactive (RXACT = 0) for 64

µsec

or more, and

More than 64

µsec have elapsed since the last byte was

read from the RX_FIFO by the CPU or DMA controller.

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