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TI's MAX3243C auto-powerdown pin question

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brisk

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It's a USB -> FT2232H (channel 2) ->Max3243C.
Does anyone know if I connect MAX3243C's auto-powerdown to low, how long it takes for Max3243 to shut it down if not RS232 signal is detected? datasheet doesn't mention it.
The problem I am facing is, if I enable auto-powerdown (active low), sometimes it is disabled when I power it on. In normal condition, Pin 1 should be 5.8V. If max3243 is in shut down mode, Pin 1's voltage is 3.3v. I supply 3.3v to it.
for example in a loopback test, if auto-powerdown is enabled, un-short RX and TX will cause max3243 to shutdown. However, If I disable auto-powerdown (connect to 3.3v), it won't go down even if I unshort RX and TX in the loopback test.

Another strange problem is, loopback of max3243 will pass if it is hot, when it cools down to room temperature, it stops to work. Auto-powerdown is enabled. What could be the problem? Bad max3243? I bought from digikey and randomly test 5 boards, all have the same problem.
Or, PCB's problem? but same pcb worked before.
 

brisk

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Did a few more tests:

In both tests FORCEON is connected to GND, which means allow auto-powerdown.

MAX3243C bought from element14 last year:
The chip is in normal working status when I power it on, this can be determined by checking voltage of pin 1, it is 5.8v.
During the loopback test, if I disconnect RX and TX, it goes to shutdown mode, unless I give a logic high to any of RS232 input pins, e.g pin 4.

MAX3243C bought from Digikey a few days ago:
The chip is in power down status when I power it on, this can be determined by checking the voltage of pin 1, it is Vcc (3.3v).
I need to give a logic high to any of RS232 input pins, e.g pin 4, and it will go to normal working condition (pin 1 is 5.8v).
Another way to make it in normal operation status is, unplug the power, use heat gun or solder iron to heat the chip, then power it on, it will go to normal working condition, if I just wait and let it cool down, it will go back to shutdown mode.

Anyone has any experience on MAX3243? especially the FORCEON pin? Datasheet did not mention what is the state of the chip when you first power it on. But to me, it looks at the digikey ones are defective, because I can change the statue by heating the chip.

This is what the datasheet says:

Disconnecting the serial port or turning off the peripheral drivers causes the auto-powerdown condition to occur.
I think it implies that it should be in working state when I power it up. it will go to sleep mode only if I disconnect the serial port or turnoff the peripheral drivers.

Thanks
 
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