8-bit Microcontrollers中的主题 Re: ADC input broke MCU,why?

ADC input broke MCU,why?

BrightBao — Thu, 29 Jun 2006 15:55:33 GMT

In my project,I use MC9S08AW16.

ADC input voltage range is from 5V~26V.

and I use 47K (series) and 11K(shunt) by ADC. It works well.

but when I changes 11K to 47K,the MCU is broken when the input voltage is 13.5V.

why?

Re: ADC input broke MCU,why?

Pingu — Thu, 29 Jun 2006 19:33:26 GMT

Hi Bright Bao,

The maximum voltage you can send in to the AW16 is about 6V (see datasheet Table A-2). So you have broken the MCU by sending in 13V. Any ADC voltage above 5V will read $FF/$3FF. Also remember, the ADC pins are shared with general purpose I/O so the pads aren't meant to cope with such a large voltages.

Pingu

Re: ADC input broke MCU,why?

john_suchyta — Fri, 30 Jun 2006 04:47:46 GMT

Hi, I understand the 26V with 47k series and 11k shunt, which should give you about 5V at the pin. And 47k series with 47k shunt will give half of what the applied voltage is. However, the MCU pins have diode clamps on each pin, so the input voltage (at the pin) will be no more than a diode drop above VDD. If your VDD is 5.0V, the max pin voltage would be about 5.7V, assuming you limit the current.

I tried a simple experiment on multiple port B pins and have not seen any problems. With VDD at 5.0V, the source voltage at 26V, and 47k series with 47k shunt, I measure 5.66V at the pins. The pins are still functional.

The AW parts can allow a certain amount of injected current - current that flows into the pin when its voltage is higher than VDD. This current is on the order of 1 - 2mA. In my experiments the injected current was around 0.3mA. That's not enough to destroy the pin.

What do you mean when you say the MCU is broken?

Re: ADC input broke MCU,why?

BrightBao — Mon, 03 Jul 2006 08:59:39 GMT

Hi,jsuchyta :

I received the PCB from product department and the MCU has not be programmed.I power on the PCB, then check the voltage on the PCB and found that everything is right.

But afer I program the MCU on the PCB through the BDM and repower the PCB,the MCU is broken and the current goes up to 200mA. The temperature of the MCU is increase about 20 degree.

I tried it for two times.

So,try to program the MCU (with the AD port run) and run it.

Re: ADC input broke MCU,why?

john_suchyta — Mon, 03 Jul 2006 20:51:43 GMT

OK, it appears that you do have a problem, but there isn't enough info to say what the problem is and what caused it. Can you submit a ticket on the Tech Support link (on Freescale web site) so we can look at it? Please submit schematic and source files.

Thanks,

John

Re: ADC input broke MCU,why?

BrightBao — Tue, 04 Jul 2006 10:18:14 GMT

Hi,jsuchyta :

You are right.

I clean the PCB,and retest the circuit.

I use 47K+47K .the voltages at Input and port are listed below

Input Voltage Voltage on AD Port

11.82V 5.51V

13.6V 5.53V

14.5V 5.56V

15.45V 5.58V

16.5V 5.59V

17.3V 5.60V

18.27V 5.61V

19.6V 5.62V

20.6V 5.63V

21.6V 5.63V

22.5V 5.64V

23.3V 5.64V

24.4V 5.64V

25.5V 5.65V

26.1V 5.65V

so, the port has a clamp diode to GND. Clamp voltage is 5.6V.