topic Re: HCS08 : 3.3v MCU and 5v logic ? in 8-bit Microcontrollers

HCS08 : 3.3v MCU and 5v logic ?

BC109 — Mon, 08 May 2006 19:59:21 GMT

Hi,

I'm a brand new user of HCS08 family (HCS08QG8). This MCU is running at 3.3v, and I have no experience about how to connect 5v peripherals. I was previously using HC11 at 5v.

I read lots of things about 5v-tolerant inputs, direct-driving of 5v inputs from 3.3v outputs, and various level converters (74LVC, 74ALVT, etc...) As I'm completely novice with 3.3v logic, could someone point me out some kind of tutorial or 3.3v beginner's guide in relation with HCS08 MCU ?

Also, are there anny Application Notes about that ?

Thank you in advance,
Toussaint OTTAVI

Re: HCS08 : 3.3v MCU and 5v logic ?

glork — Mon, 08 May 2006 20:30:54 GMT

BC 109 wrote:
Hi,

I'm a brand new user of HCS08 family (HCS08QG8). This MCU is running at 3.3v, and I have no experience about how to connect 5v peripherals. I was previously using HC11 at 5v.

I read lots of things about 5v-tolerant inputs, direct-driving of 5v inputs from 3.3v outputs, and various level converters (74LVC, 74ALVT, etc...) As I'm completely novice with 3.3v logic, could someone point me out some kind of tutorial or 3.3v beginner's guide in relation with HCS08 MCU ?

Also, are there anny Application Notes about that ?

Thank you in advance,
Toussaint OTTAVI

Good morning.
Although Freescale makes some 3.3V devices which are 5V tolerant on some I/Os the 9S08QG family isn't amoung them. If you must use this device with 5V peripherals you will need to use appropriate level conversion circuitry.
Sorry, I'm not aware of any tutorials on the use of 3.3V logic but a google search might turn up some.
ron

Re: HCS08 : 3.3v MCU and 5v logic ?

rocco — Tue, 09 May 2006 04:42:20 GMT

Hello, Toussaint:

Welcome to the world of mixed voltages. I avoided 3 volts for years, because of the very issues you express now. I finally had to take the plunge when Motorola discontinues the DSP56002, and replaced it with the 3 volt DSP56303. But at least the DSP56303 had many 5 volt-tolerant I/Os.

Here are two tricks that I learned:
You can use 74HCTxxx parts as buffers to turn 3 volt outputs into 5 volt outputs. The 74HCT parts use TTL-level input thresholds, which are near 2 volts for the minimum high state, and 3 volts outputs can satisfy that threshold.

Terminating resistor packs, often used in SCSI systems, make excellent voltage divides. Specifically, the packs that contain 220ohm/330ohm pairs. 5 volts in becomes 3 volts out. A disadvantage is that it puts a hefty (550ohm) load on the 5 volt output, and it will increase propagation delay if the 3 volt input has a capacitive component.

Then again, there are chips designed specifically for your problem.

The simplest approach, though not always feasible, it to switch everything over to 3 volts.

Re: HCS08 : 3.3v MCU and 5v logic ?

BobMac — Tue, 09 May 2006 08:31:17 GMT

Hi there, BC 109. I had the same problem when faced with talking to some SD memory modules. As I remember, TI has a great app note on just this subject of voltage level shifting. I can't find my old notes now, but I sure remember the source because of my frustration at the time.
Regards, Bob

PS, I have a "HAM" license and in the early 1950's I owned a Military radio out of a B17, the BC628. It used tubes (vacuum state) but worked quite well, as I remember.

Re: HCS08 : 3.3v MCU and 5v logic ?

bigmac — Tue, 09 May 2006 14:43:40 GMT

Hello Toussaint,

Provided you don't need a lot of speed, the use of open drain N-channel MOSFETs might be suitable. The MOSFET type should be capable of switching with 3V logic levels at its gate (and would also suitable for 5V levels). The drain would connect to the logic input, and would also require a suitable pull-up resistor to either 3V or 5V, depending on the conversion direction. The resistor value will depend to some extent on the speed you require.

Of course your code will need to allow for inverted polarity through the device.

An alternative to the MOSFET could be the use of bi-polar NPN transistors, but these would need an additional resistor in series with the base.

Regards,
Mac

Re: HCS08 : 3.3v MCU and 5v logic ?

Nabla69 — Tue, 09 May 2006 16:48:06 GMT

Hi Toussaint,

Please let me add my two cents worth.
There is AN2433 - 5V to 3V Considerations that I could recommend reading.

Cheers,
Alvin.

(Alban fixed link)

Message Edited by Alban on 05-09-2006 10:51 AM

Re: HCS08 : 3.3v MCU and 5v logic ?

UK_CF_FAE — Thu, 11 May 2006 03:14:26 GMT

Guys - check out the MC9S08AW family of parts (16k to 60k flash) which are 1.8 to 5.5 volt operation.

OK, a bit larger packages than the QG8. But fully featured. What will you do with those 2-off SCI UARTs?

Mark

Message Edited by UK_CF_FAE on 05-10-200603:15 PM

Re: HCS08 : 3.3v MCU and 5v logic ?

BC109 — Sun, 21 May 2006 19:21:15 GMT

Hi,

Thank you everybody for the answers.

Of course, the perfect MCU for me would be a QG8 with the 5v capability of the AW family, and with the 'HC11 full assembly language. Hey, Mr Freescale, could you build it for me ? :smileyhappy:

It's only for small amateur applications, and in most of the cases, I will need very few I/O, that's why the QG8 PDIP package is perfect, because it avoids complex PCB design using high pin count devices.

Thanks again for the help, I'll try to play around with it ASAP.

Best regards from Corsica,
Toussaint
(ham radio, TK1BI)