I am baffled by a problem that (to me) is very strange.
The setup is a simple board with a MSP430 on it, and the only other IC is a
MAX232. There are separate 5v and 3.3v rails.
When I power on the 5v rail (*only*) for the MAX232, and connect the Tx/Rx lines
of the MSPs UART (UART1) to the MAX232, a voltage appears on the Vcc pins of
the MSP! (3.8v or so!). Meaning, there is no voltage applied to the 3.3v rails
(in fact I have cut the PCB line from the MSP to the rail) and still this
voltage appears!
Is there any logical reason why this should happen?! The MSP, by the way, seems
to work fine when connected back to the rails (at least I can flash it, if that
is any indication).
Help!
MSP powers on from UART lines! - ?!?
Started by ●March 13, 2009
Reply by ●March 13, 20092009-03-13
MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
powered from this too, but below 3.8V.
M.
"merapcb" :
> I am baffled by a problem that (to me) is very strange.
>
> The setup is a simple board with a MSP430 on it, and the only other IC
> is a MAX232. There are separate 5v and 3.3v rails.
>
> When I power on the 5v rail (*only*) for the MAX232, and connect the
> Tx/Rx lines of the MSPs UART (UART1) to the MAX232, a voltage appears
> on the Vcc pins of the MSP! (3.8v or so!). Meaning, there is no voltage
> applied to the 3.3v rails (in fact I have cut the PCB line from the MSP
> to the rail) and still this voltage appears!
>
> Is there any logical reason why this should happen?! The MSP, by the
> way, seems to work fine when connected back to the rails (at least I can
> flash it, if that is any indication).
>
> Help!
MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
powered from this too, but below 3.8V.
M.
"merapcb" :
> I am baffled by a problem that (to me) is very strange.
>
> The setup is a simple board with a MSP430 on it, and the only other IC
> is a MAX232. There are separate 5v and 3.3v rails.
>
> When I power on the 5v rail (*only*) for the MAX232, and connect the
> Tx/Rx lines of the MSPs UART (UART1) to the MAX232, a voltage appears
> on the Vcc pins of the MSP! (3.8v or so!). Meaning, there is no voltage
> applied to the 3.3v rails (in fact I have cut the PCB line from the MSP
> to the rail) and still this voltage appears!
>
> Is there any logical reason why this should happen?! The MSP, by the
> way, seems to work fine when connected back to the rails (at least I can
> flash it, if that is any indication).
>
> Help!
Reply by ●March 13, 20092009-03-13
--- In m..., Matthias Weingart wrote:
>
> MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> powered from this too, but below 3.8V.
>
> M.
This one is not good?
http://focus.ti.com/docs/prod/folders/print/max232.html
>
> MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> powered from this too, but below 3.8V.
>
> M.
This one is not good?
http://focus.ti.com/docs/prod/folders/print/max232.html
Reply by ●March 13, 20092009-03-13
"merapcb" :
> This one is not good?
> http://focus.ti.com/docs/prod/folders/print/max232.html
No. Just read: "Supply Voltage(s)(V) 5"
Use this: http://focus.ti.com/docs/prod/folders/print/max3232e.html
M.
> This one is not good?
> http://focus.ti.com/docs/prod/folders/print/max232.html
No. Just read: "Supply Voltage(s)(V) 5"
Use this: http://focus.ti.com/docs/prod/folders/print/max3232e.html
M.
Reply by ●March 13, 20092009-03-13
Yes, and that is exactly what I am giving it, 5v......
Duh?
--- In m..., Matthias Weingart wrote:
>
> "merapcb" :
>
> > This one is not good?
> > http://focus.ti.com/docs/prod/folders/print/max232.html
>
> No. Just read: "Supply Voltage(s)(V) 5"
> Use this: http://focus.ti.com/docs/prod/folders/print/max3232e.html
>
> M.
>
Duh?
--- In m..., Matthias Weingart wrote:
>
> "merapcb" :
>
> > This one is not good?
> > http://focus.ti.com/docs/prod/folders/print/max232.html
>
> No. Just read: "Supply Voltage(s)(V) 5"
> Use this: http://focus.ti.com/docs/prod/folders/print/max3232e.html
>
> M.
>
Reply by ●March 13, 20092009-03-13
On Fri, 2009-03-13 at 12:56 +0000, merapcb wrote:
> --- In m..., Matthias Weingart wrote:
> >
> > MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> > MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> > You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> > powered from this too, but below 3.8V.
> >
> > M.
> >
> > This one is not good?
> http://focus.ti.com/docs/prod/folders/print/max232.html
That is correct, it is not a good design practice.
You have a processor with I/O expecting logic levels between 0 and 3.3V.
And You have a RS232 interface that runs at min 4.5V to max 5.5V (see
page 3). So the logic lines go from 0 to 5V on it.
Do you think it is a good idea to pump 5V into a micro-controller
expecting 3.3V? It is not.
The MAX3232 part is the same part as the MAX232 but runs on 3.3V not 5V.
Kip
--
Kipton Moravec AE5IB .- . ..... .. -...
=============================================Four Way Test
Is it the Truth?
Is it Fair to all concerned?
Will it build Goodwill and Better Friendships?
Will it be Beneficial to all concerned?
- Herbert J Taylor (1932)
> --- In m..., Matthias Weingart wrote:
> >
> > MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> > MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> > You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> > powered from this too, but below 3.8V.
> >
> > M.
> >
> > This one is not good?
> http://focus.ti.com/docs/prod/folders/print/max232.html
That is correct, it is not a good design practice.
You have a processor with I/O expecting logic levels between 0 and 3.3V.
And You have a RS232 interface that runs at min 4.5V to max 5.5V (see
page 3). So the logic lines go from 0 to 5V on it.
Do you think it is a good idea to pump 5V into a micro-controller
expecting 3.3V? It is not.
The MAX3232 part is the same part as the MAX232 but runs on 3.3V not 5V.
Kip
--
Kipton Moravec AE5IB .- . ..... .. -...
=============================================Four Way Test
Is it the Truth?
Is it Fair to all concerned?
Will it build Goodwill and Better Friendships?
Will it be Beneficial to all concerned?
- Herbert J Taylor (1932)
Reply by ●March 13, 20092009-03-13
MAX232 has 5V level output (3.5V minimum) and MSP430 is 3V. Of course you can
use a simple resistor between MAX232's output and MSP430's input (like
2K2 or a little bigger) to limit the current flowing in the MSP430's input
protection diodes. But mind that these diodes drain the excess of voltage
transfering the current to the MSP430's VCC rail. Therefore, if the current
is enough (and MSP430 doesn't need much current to run) it can supply the
MCU even with the protection resistor between MAX232 and MSP.
One possible solution to avoid MSP running this way is to use a reverse polarized diode in the 3V regulator, connected between input and output. Normaly the regulator's input is higher than output and the diode is not conducting. If the regulator input is lower (may be zero Volts when you have no power at all) the diode starts conducting draining any current coming from MSP430's VCC rail. If the regulator's input is zero Volts a regular silicon diode would bring the VCC rail to around 0.7V and the MSP430 wouldn't run.
Since you seems to already have the circuit with MAX232 I recomend just to include this resistor to protect MSP's input. Otherwise use the MAX3232 that works with 3V.
The diode in the regulator is allways a good idea to avoid reverse voltage in the regulator. Some regulators don't like it.
Check your regulator's datasheet. Maybe it already have one.
Note: The diode won't help much if the regulator's input is at high impedance (for example an open connection from an external power supply). Even if it is a waste of energy some design require another low value resitor between power input and GND to drain the current from that diode.
-Augusto
De:m...
Para:m...
Cia:
Data:Fri, 13 Mar 2009 12:56:20 -0000
Assunto:[msp430] Re: MSP powers on from UART lines! - ?!?
--- In m..., Matthias Weingart wrote:
>
> MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> powered from this too, but below 3.8V.
>
> M.
>
>
This one is not good?
http://focus.ti.com/docs/prod/folders/print/max232.html
One possible solution to avoid MSP running this way is to use a reverse polarized diode in the 3V regulator, connected between input and output. Normaly the regulator's input is higher than output and the diode is not conducting. If the regulator input is lower (may be zero Volts when you have no power at all) the diode starts conducting draining any current coming from MSP430's VCC rail. If the regulator's input is zero Volts a regular silicon diode would bring the VCC rail to around 0.7V and the MSP430 wouldn't run.
Since you seems to already have the circuit with MAX232 I recomend just to include this resistor to protect MSP's input. Otherwise use the MAX3232 that works with 3V.
The diode in the regulator is allways a good idea to avoid reverse voltage in the regulator. Some regulators don't like it.
Check your regulator's datasheet. Maybe it already have one.
Note: The diode won't help much if the regulator's input is at high impedance (for example an open connection from an external power supply). Even if it is a waste of energy some design require another low value resitor between power input and GND to drain the current from that diode.
-Augusto
De:m...
Para:m...
Cia:
Data:Fri, 13 Mar 2009 12:56:20 -0000
Assunto:[msp430] Re: MSP powers on from UART lines! - ?!?
--- In m..., Matthias Weingart wrote:
>
> MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> powered from this too, but below 3.8V.
>
> M.
>
>
This one is not good?
http://focus.ti.com/docs/prod/folders/print/max232.html
Reply by ●March 13, 20092009-03-13
I am the OP of this thread (trivial to most I guess) and just wanted to say
thank you.
I did search the datasheet and could not find whether the inputs of the MSP are 5v tolerant, I guess they are not...
Thank you
--- In m..., "Augusto Einsfeldt" wrote:
>
> MAX232 has 5V level output (3.5V minimum) and MSP430 is 3V. Of course you can use a simple resistor between MAX232's output and MSP430's input (like 2K2 or a little bigger) to limit the current flowing in the MSP430's input protection diodes. But mind that these diodes drain the excess of voltage transfering the current to the MSP430's VCC rail. Therefore, if the current is enough (and MSP430 doesn't need much current to run) it can supply the MCU even with the protection resistor between MAX232 and MSP.
> One possible solution to avoid MSP running this way is to use a reverse polarized diode in the 3V regulator, connected between input and output. Normaly the regulator's input is higher than output and the diode is not conducting. If the regulator input is lower (may be zero Volts when you have no power at all) the diode starts conducting draining any current coming from MSP430's VCC rail. If the regulator's input is zero Volts a regular silicon diode would bring the VCC rail to around 0.7V and the MSP430 wouldn't run.
>
> Since you seems to already have the circuit with MAX232 I recomend just to include this resistor to protect MSP's input. Otherwise use the MAX3232 that works with 3V.
> The diode in the regulator is allways a good idea to avoid reverse voltage in the regulator. Some regulators don't like it.
> Check your regulator's datasheet. Maybe it already have one.
> Note: The diode won't help much if the regulator's input is at high impedance (for example an open connection from an external power supply). Even if it is a waste of energy some design require another low value resitor between power input and GND to drain the current from that diode.
> -Augusto
>
> De:m...
>
> Para:m...
>
> Cia:
>
> Data:Fri, 13 Mar 2009 12:56:20 -0000
>
> Assunto:[msp430] Re: MSP powers on from UART lines! - ?!?
>
> --- In m..., Matthias Weingart wrote:
> >
> > MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> > MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> > You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> > powered from this too, but below 3.8V.
> >
> > M.
> >
> >
>
> This one is not good?
> http://focus.ti.com/docs/prod/folders/print/max232.html
>
>
>
>
>
>
>
I did search the datasheet and could not find whether the inputs of the MSP are 5v tolerant, I guess they are not...
Thank you
--- In m..., "Augusto Einsfeldt" wrote:
>
> MAX232 has 5V level output (3.5V minimum) and MSP430 is 3V. Of course you can use a simple resistor between MAX232's output and MSP430's input (like 2K2 or a little bigger) to limit the current flowing in the MSP430's input protection diodes. But mind that these diodes drain the excess of voltage transfering the current to the MSP430's VCC rail. Therefore, if the current is enough (and MSP430 doesn't need much current to run) it can supply the MCU even with the protection resistor between MAX232 and MSP.
> One possible solution to avoid MSP running this way is to use a reverse polarized diode in the 3V regulator, connected between input and output. Normaly the regulator's input is higher than output and the diode is not conducting. If the regulator input is lower (may be zero Volts when you have no power at all) the diode starts conducting draining any current coming from MSP430's VCC rail. If the regulator's input is zero Volts a regular silicon diode would bring the VCC rail to around 0.7V and the MSP430 wouldn't run.
>
> Since you seems to already have the circuit with MAX232 I recomend just to include this resistor to protect MSP's input. Otherwise use the MAX3232 that works with 3V.
> The diode in the regulator is allways a good idea to avoid reverse voltage in the regulator. Some regulators don't like it.
> Check your regulator's datasheet. Maybe it already have one.
> Note: The diode won't help much if the regulator's input is at high impedance (for example an open connection from an external power supply). Even if it is a waste of energy some design require another low value resitor between power input and GND to drain the current from that diode.
> -Augusto
>
> De:m...
>
> Para:m...
>
> Cia:
>
> Data:Fri, 13 Mar 2009 12:56:20 -0000
>
> Assunto:[msp430] Re: MSP powers on from UART lines! - ?!?
>
> --- In m..., Matthias Weingart wrote:
> >
> > MAX232 is supplied by 5V!? The RXD Pin of the MSP get's the 5V from the
> > MAX232 and put it on the MSP VCC-Rail via the internal protection diode.
> > You should use MAX3232 or similar 3V Levelshifter. However the MSP get's
> > powered from this too, but below 3.8V.
> >
> > M.
> >
> >
>
> This one is not good?
> http://focus.ti.com/docs/prod/folders/print/max232.html
>
>
>
>
>
>
>
