Reset

[DanielMorley]

I am programing a MC9S08GT16A to operate an allegro driver turning on a 12 v driver.  At random times  I get a reset.  By disabling stop mode seems to make the internal resets less common but they still happen.  I can not find a maximum current draw on the allegro part but the freescale data sheets says it can source upto 25 mA. Just before an interupt occurs the output on the motor driver drops.  I am pretty sure this is a hardware isssue but I need some input on what is going on. 

[peg]

Hi Daniel,

We need a bit more info on your hardware than you have given, but....

I use a ULN2803A or a UDN2981A on a HC08GP and a ULN2801A with 1k5 base resistor on a GT16 without any problems. These devices impose a loading of only a couple on mA on the MCU outputs. Which is no problem at all. I suspect the problem is noise from the other side of these drivers.

Am I on the right track? If not more info please.

[DanielMorley]

Your theory reguarding the noise is what I currently belive.  I am not sure of what info you are needing.  The Allegro device does not specify the requiring driving current just a minimum voltage.  I am waiting on the answer from a field engineer.  The motor gives out at random times which to me leans much more toward your noise theory.  Also I am wondering if it is related to vibrations of the motor.  They are about two feet apart however it is very noisy.   The outputs of the processor are hooked directly to the processor and I used them with internal pull ups. 

[peg]

Hi Daniel,

By more info I mean description of hardware between MPU and motor, inclusive. Part No.s and connection details.

[DanielMorley]

A3949SLPTR-T

 is the only hardware attched to microcontroller.  I currently do not have and amplifier in between.  The pull ups are internal to the microcontroller. 

[peg]

Hi Daniel,

Now I understand what you have!

The logic input currents are quoted but they are, as expected, in the order of 10's of uA. This is not where you are in trouble.

As the ground is common here you have to be very careful about getting ground currents of the motor into the MPU i/o and power supply. You need a short low-resistance path from the SENSE/GND of the chip to your power supply.

One thing to try would be to temporarily replace your motor with a very small non-inductive load and see if it works like that

[bigmac]

Hello Daniel,

You do not say whether you are using a DEMO board, or a board of your own design.  If your own design, do you have adequate bypassing of the supply rail, with the high frequency capacitors and the larger tantalum capacitor, as recommended in the data sheet?  Do you also have a 100nF capacitor connected between the reset pin and Vss, and an external pullup resistor to Vdd?

Is the motor driver device currently on the same board as the MCU?  If so, the layout of ground tracks may be quite critical, as Peg has already suggested.  You do not mention the current draw of the motor - do you have sufficient track width for all the tracks handling high current, and in particular the ground track between the power supply feed and the SENSE/GND pins of the motor driver.  For example, assuming 0.5 oz PCB laminate, for a current capacity of 3.5 A, the track width would need to be at least 0.1" (2.5 mm).  The high current tracks should be as short as possible, and kept well away from low current components, including the MCU.

Regards,

Mac

[DanielMorley]

You do not say whether you are using a DEMO board, or a board of your own design. 

My own design

If your own design, do you have adequate bypassing of the supply rail, with the high frequency capacitors and the larger tantalum capacitor, as recommended in the data sheet? 

After looking more closely on the schiematic I seem to have place all of the caps that is described on the analog supply for the MCU.  The power supply design does have a .1 cap but it is not high frequency and far away from the pin.     I

Do you also have a 100nF capacitor connected between the reset pin and Vss, and an external pullup resistor to Vdd? yes

Is the motor driver device currently on the same board as the MCU?  If so, the layout of ground tracks may be quite critical, as Peg has already suggested.  You do not mention the current draw of the motor - do you have sufficient track width for all the tracks handling high current, and in particular the ground track between the power supply feed and the SENSE/GND pins of the motor driver.  For example, assuming 0.5 oz PCB laminate, for a current capacity of 3.5 A, the track width would need to be at least 0.1" (2.5 mm).  The high current tracks should be as short as possible, and kept well away from low current components, including the MCU.

The curret draw is only about 200 mA at current load and the track with is about 20mills thickness.   I am using jumper wires to connect the motor.