TMPR3911 datasheet(236/462 Pages) TOSHIBA

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Chapter 10 IR Module

10-6

10.4 Carrier Detect State Machine

10.4.1

Requirements

The external communication IR analog circuitry contains a circuit which detects if there is a valid

carrier present. It is not desirable to leave this circuit enabled all the time because it dissipates too much

power. Thus the circuit is powered on and off with the RXPWR pin. It is possible for the CPU to

periodically enable the RXPWR pin, then wait to see if a carrier is present (CARDET pin

= “1”), but

this requires a lot of processor overhead. In order to alleviate the overhead, the IR Module contains a

circuit that periodically enables the RXPWR pin, waits a specified period of time, samples the

CARDET pin, then disables the RXPWR pin. If the CARDET pin is a “1” when sampled then an

interrupt is issued to the CPU indicating that there is a valid carrier present. After enabling the RXPWR

pin the CARDET pin cannot be sampled for a specified amount of time because the Carrier Detect

Logic in the external communication IR analog circuitry takes time to settle. The time between

successive enabling of the RXPWR pin is programmable depending on the desired sample rate.

Wait Value

Delay Value

Valid Carrier

No Valid Carrier

Settle Time

CARDET

RXPWR

Figure 10.4.1 Carrier Detect State Timing

10.4.2

Block Diagram

RXPWR

8-Bit

WAIT

DOWN

COUNTER

8-Bit

DELAY

DOWN

COUNTER

7.8 ms CLOCK

WAITVAL (7:0)

DELAYVAL (7:0)

Figure 10.4.2 Carrier Detect Block Diagram

TMPR3911 Datasheet(PDF) 236 Page - Toshiba Semiconductor