topic Re: Vtst on HC908QT1/2 in 8-bit Microcontrollers

Vtst on HC908QT1/2

Impudence — Fri, 19 Sep 2008 07:24:08 GMT

I am having some troubles with getting into monitor mode with the high voltage. When I apply Vtst to the IRQ pin (I am using 8.5V for Vtst and 5V for Vdd), the chip starts to heat up surprisingly quickly and reaches over 100C. This seems to be due to a large amount of current being drawn by the device via the IRQ pin.

I have two questions. Is this normal? If it is, how long can the device survive 8.5V on the IRQ pin?

I'll have to order some more of these chips before I can try anything else, but I thought I'd ask here for advice.

Re: Vtst on HC908QT1/2

peg — Fri, 19 Sep 2008 07:48:44 GMT

Hello and welcome to the fora Impudence,

This is definitely not normal! I would say you have damaged the device here. There should not be any noticeable heating of the chip when Vtst is applied to it.
Accidently applying the high voltage to the wrong pin could do this. Normally one would use a series resistor with this voltage source to limit the possibility damage in this case.

Re: Vtst on HC908QT1/2

Ake — Fri, 19 Sep 2008 15:11:28 GMT

Hi,

You should put a 10 KOhm resistor in series with the Vdd.

If you connect it directly, the chip might latchup and burn.

Insert the resistor, value not too critical, and try it once more.

If it still draws a lot of current, measure the voltage on the Vdd input, then you will have to replace the HC908QT MCU.

Regards,

Ake

Re: Vtst on HC908QT1/2

Impudence — Mon, 22 Sep 2008 10:18:44 GMT

Thanks for the advice, peg and Ake. 10k seems a bit large to put on Vdd, did you mean to put 10k on Vtst? I'll order some more parts tonight and check it out.

Thanks again!

Re: Vtst on HC908QT1/2

peg — Mon, 22 Sep 2008 10:52:53 GMT

Hi,

Yes I presume he means Vtst as well. 10k seems a little high to me as well but if it works, it is safer. Actually I checked my programmer and it only has 180R but the Vtst is stolen from the MAX232 so it is already a fairly high impedance source.

I checked a couple of manuals and they all show 10k in the example circuit for MON08 programming.

Re: Vtst on HC908QT1/2

Ake — Tue, 23 Sep 2008 14:10:04 GMT

Hi,

Ohoh, I am sorry, I got the resistor values mixed up.

Look at Figure 15-11 in the MC68HC908QY4/D, there should be a 9.1 V applied to /IRQ.

This is done with a 1 K to +12 V and a 9.1 V zener diode.

The 10 K resistor I was talking about, is just a pullup on the /RST pin.

This will always put the HC908QY/QT in Monitor mode

The other way is to use the schematic in Figure 15-12, or 15-13 depending if you want to use the external/internal clock. But this is only good for first time programming, that is the reset vector must be 0xff, 0xff.

Regards,

Ake