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MAX3212EAI Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX3212EAI Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Table 2 summarizes the MAX3212 operating modes. FORCEON and FORCEOFF override the automatic cir- cuitry and force the transceiver into its normal operat- ing state or into its low-power standby state. When neither control is asserted, the IC selects between these states automatically based on receiver input lev- els. Figure 4 depicts valid and invalid RS-232 receiver levels. The MAX3212 shuts down after sensing invalid RS-232 levels for greater than 30µs, ensuring the AutoShutdown mode is not enabled for slow-moving signals (1V/µs). A mouse or another system with AutoShutdown may need a period of time to wake up. Figure 5 shows a cir- cuit that forces the transmitters on for 100ms after start- up, allowing enough time for the other system to realize that the MAX3212 system is awake. If the other system outputs valid RS-232 signals within that time, the RS- 232 ports on both systems remain enabled. Transition Detector The MAX3212 also has an on-board transition detector that monitors activity on the receiver inputs. In systems with a sleep mode, the transition detector output (TRAN) can be used to wake up the system when activity at the receiver inputs is detected. Before putting the system to sleep, set TRAN low by cycling EN high and then low. TRAN remains low as long as no activity is detected on the receiver inputs. When any receiver is toggled, TRAN latches high. Connect TRAN to a microprocessor inter- rupt, or if the system wakes up periodically TRAN can be polled. Transition detection is useful when valid RS- 232 levels are present at the receiver inputs but no data is being sent. For example, if a printer is attached to the serial port but is not in use, the microprocessor senses this and forces the MAX3212 off. +2.7V to +3.6V-Powered, 1µA Supply Current, 3-Driver/5-Receiver, True RS-232 Transceiver 8 _______________________________________________________________________________________ INVALID IS AN INTERNALLY GENERATED SIGNAL THAT IS USED BY THE AUTOSHUTDOWN LOGIC AND APPEARS AS AN OUTPUT OF THE DEVICE. POWER DOWN IS ONLY AN INTERNAL SIGNAL. IT CONTROLS THE OPERATIONAL STATUS OF THE TRANSMITTERS AND THE POWER SUPPLIES. FORCEOFF 0 0 0 0 1 1 1 1 FORCEON 0 0 1 1 0 0 1 1 INVALID 0 1 0 1 0 1 0 1 POWER DOWN 0 0 0 0 0 1 1 1 POWER DOWN FORCEOFF FORCEON INVALID + - + - Rx RIN_ EN ROUT_ 30µs DELAY • • • • • • +0.3V to +3.0V -0.3V to -3.0V Figure 3. AutoShutdown Logic |
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