topic Re: Different supply voltage in 8-bit Microcontrollers

Different supply voltage - MC9S08AW16

mimco — Mon, 31 Dec 2007 06:00:31 GMT

Hello,

Please i have one qustion about connection 2 MCU with different supply voltage. First MCU MC9S08AW16 has supply voltage 5V and is connected to the MC9S12NE64 demonstration board. MC9S12NE64 has a voltage level for IO pins around 3V. In the datasheet are these Absolute Maximum Ratings:
                                                                                     min        max
I/O, Regulator and Analog Supply Voltage (VDD3) : -0,3    4,5V
Digital Logic Supply Voltage (VDD) :                          -0,3        3V
...
Digital I/O Input Voltage (VIN) :                                   -0,3        6,5V

I need connect these MCUs with SPI. It is possible when voltage level of the MC9S12NE64 pins is 3V and voltage level of MC9S08AW16 is 5V?

Thanks

Added p/n to subject.

Regards
mimco

Message Edited by NLFSJ on 2007-12-31 06:52 AM

Message Edited by NLFSJ on 2007-12-31 07:15 AM

Re: Different supply voltage

bigmac — Mon, 31 Dec 2007 11:58:32 GMT

Hello mimco,

You cannot directly connect the SPI pins of two MCUs operating at different supply voltages.

Where a 5 volt output connects to a 3V input the voltage swing at the input must be limited by one of a number of possible methods, so as not cause damage to the 3V device. Where a 3V output connects to a 5V input, is is quite possible there will be insufficient voltage swing to reliably switch the 5V device. In this case the voltage swing will need to be boosted to meet the requirements of the 5V input.

You do not specify which device is the SPI master. The master has three outputs and a single input, whereas the slave has three inputs and a single output.

Special buffer devices are available for voltage translation purposes, with some capable of bi-directional operation. Alternatively, for high-to-low voltage translation, a simple resistive voltage divider may suffice. Yet another possibility is to use MOSFETs as inverters. The MOSFET must reliably switch at less than 3 volts, a relatively low value pull-up resistor (perhaps 1k to 4k7) will be required at the drain, and the logic inversion will also need to be taken into account.

It two separate power sources are used for each MCU, it is also possible for only one source to be present, and the other switched off. Under these circumstances, the buffering method must also ensure that excessive current cannot flow in any of the interconnections - perhaps not exceeding 0.2 mA.

Regards,

Mac

Re: Different supply voltage

mimco — Mon, 31 Dec 2007 17:56:33 GMT

Hello Mac,

Thanks for you reply. I thought it, but i was not sure, becouse between absolute maximum ratings is
Digital I/O Input Voltage: -0,3 to 6,5V, what confust me.

Thanks and Happy New Year

Regards,
mimco

Re: Different supply voltage

peg — Wed, 02 Jan 2008 05:53:46 GMT

Hi mimco,

Take a look at AN2433 for more details on this subject. (put AN2433 in the keyword search at top left).

Re: Different supply voltage - MC9S08AW16

UcTechnoGeek — Wed, 02 Jan 2008 12:00:04 GMT

You could always use something like the Maxim MAX3390E.

It can work as an SPI level translator.

uCTechnoGeek

Re: Different supply voltage - MC9S08AW16

mimco — Sat, 05 Jan 2008 19:16:27 GMT

Hi,

I have look at AN2433 and other contributions in forum about SPI. I would like try IC max3390 or max3377 (NE64 is a master and aw16 is a slave). But in my country (Slovakia) is not a distributor, that would deliver this IC's. I can try ask for samples.

Other solution is a supply also AW16 with 3V (AW16 has VCCmin=2,7V) or use GT16A, but i don't know whether has a AW16 different requirements on crystal with supply voltage 5 and 3V. I would use 8MHz crystal. On the board with AW16 is a power H bridge (compound of 4 STD35nf3LL NMOS transistors) to drive BDC motor with encoder. Encoder works with supply voltage 5V, so i must use resistive voltage divider to cut-down voltage from 5 to 3V.

Thanks.

Regards mimco