Special king of one-shot circuit

Apr 16, 2007 10 Replies
E

Hi,



I'm working on a circuit that drives a termal printer. The printer has 8 output signals which activate the heaters. This signals connected to the CPU through a latch (74HC573). The sequence of the process is to enable the OE signal of the latch for about 2ms (at which the heaters are activated), then disable the OE and write new information to the printer via serial interface (takes about 100uS) and activate the OE again...



I want to add a protection circuit that will recognize a situation that the OE of the latch is enabled for too much time (more then 4mS - that might burn the heaters) and will disable it.



Of course, I'm looking for the simplest circuit possible.



Any ideas?



Thanks! Eyal.



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R

Latch??? Why not a one-shot for the driver?

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E

On-shot is not a good solution for a driver because the 2mS is software-controlled. it depends on the head temprature, blackness of the printing, paper type and more.

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J

You could design the one-shot so it is "transparent". In other words so that the output disasserts when the software commands it to but also disasserts when 2msec have elapsed and the software failed to do so. For example by using a regular one-shot plus AND logic at the output.

Regards, Joerg http://www.analogconsultants.com
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E

I think that I have found the simplest solution.

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Thanks for your help. Eyal.

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B

Just avoid the ST version of that chip.

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J

I guess he meant SGS Thomson.

Regards, Joerg http://www.analogconsultants.com
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R

Or "ST Microelectronics", as they now call themselves. Why should their version be avoided? Just this chip, or the whole 4xxx series (or ST altogether)?

robert

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J

No idea what he meant with that. So far I've not had any bad experience with ST.

Regards, Joerg http://www.analogconsultants.com
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B

yep, ST Microelectronics aka SGS-Thompson.

Had a batch of ten products going through final test containing

4017's from one of two manufacturers - ST Micro and On-Semi. Five out of the batch failed a particular test, which all previous batches had passed. These chips were driving devices with Moto MC14174B or Toshiba HD4174BP in a test jig.

After some head-scratching, it transpired that all five failing units had HCF4017BE by STM while all the passing units were fitted with On-Semi. The STM chips were pulled and replaced with On-Semi parts, problem disappeared.

I asked my supplier which brands comprised his current 4017 stock. His reply was: "4017 all stock is On Semi sorry about the problems. I generally don't get ST parts because of strange behaviours, the worst example is the 4538 Monostable".

On seeing the reference to the 4538 in this thread, it flagged red to me.

While one would be entitled to assume that an "industry standard" 4000-series chip can drive the inputs to another "industry standard" 4000-series chip, that appears to not always be the case.

As always, YMMV, but I intend to steer clear of the ST branded stuff wherever possible.

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