Why is there a transistor?
- Thread starter edmunds
- Start date
tea monster
New Member
looks like power-on reset to me
This is the data sheet for the module. I believe tea monster has what is going on. Pages 18 and 19 of the data sheet(s) show that schematic. They show Q1 as an 809 and also as a AME8500 which are uP supervisory devices, here is the data sheet for the AME8500 family.
Ron
Which pretty much comes down to what tea monster said give or take several dozen words.
Ron
Ron,
Great links, thank you. I did have the data sheet of both the module and a bare chip (which I'm playing with). Can it be somehow summarised into something a less experienced person without a degree in electronics engineering could grasp?
Thank you for your input,
Edmunds
Great links, thank you. I did have the data sheet of both the module and a bare chip (which I'm playing with). Can it be somehow summarised into something a less experienced person without a degree in electronics engineering could grasp?
Thank you for your input,
Edmunds
Yes, in a nutshell, in this application, if the supply voltage dips the monitor throws the chip into a reset state and holds it there preventing damage to the uP. When the power voltage returns it continues to hold a reset state for a period of time.Ron,
Great links, thank you. I did have the data sheet of both the module and a bare chip (which I'm playing with). Can it be somehow summarised into something a less experienced person without a degree in electronics engineering could grasp?
Thank you for your input,
Edmunds
Notice it is only shown twice and all subsequent RESET is accomplished with a single push button switch to Ground. For an actual circuit application I wouldn't worry about it but even though drawn as a transistor, that is what it is.
Ron
inglewoodpete
Senior Member
In simple words, the "transistor" is not a transistor. It is a micro-processor/-controller resetting device. It is attractive due to its small size. As drawn in the circuit in post #1, it does not make sense.
In the old days, we would use a resistor and a small electrolytic capacitor to hold the microprocessor in a reset state until all the peripherals had powered up. That method is much bulkier (and more expensive in mass production).
In the old days, we would use a resistor and a small electrolytic capacitor to hold the microprocessor in a reset state until all the peripherals had powered up. That method is much bulkier (and more expensive in mass production).
eggdweather
Senior Member
At power on, the capacitor (104) is initially discharged and will hold the RESET pin low, then very quickly thereafter, the transistor will begin conducting and charge the capacitor to Vcc (3.3-~0.2volts) and the RESET pin will be held high. If the power should fail momentarily, the C will hold the reset pin high (noting it will start to discharge through the reset pin and the reversed bias transistor junction) and if power returns within a yet to be determined time-constant of the C and some R, the PIC will not have been reset, eventually if power is off for long enough the C will be discharged and the process of power-on reset can resume. It is a form of brown out device as it speeds up the reset transition time during power perturbations.
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fernando_g
Senior Member
That is a Microchip TCM809 voltage supervisor-reset circuit in a 3 pin package.
Very dumb -if you ask me- to draw it as a NPN transistor.
Now the TCM809 has several variants, depending on the VDD threshold to enable the reset. For 3v3, it is probably a "T" or "S" variant.
Very dumb -if you ask me- to draw it as a NPN transistor.
Now the TCM809 has several variants, depending on the VDD threshold to enable the reset. For 3v3, it is probably a "T" or "S" variant.