Power consumption prediction of MCU

[samuelwang]

Hi all,

I am using 9S12G128 series MCU in my project and need you fellows' recommendation how to define the maximum power consumption. As shown in user manual page 1193, the maximum run mode consumes 21mA, and no further information about it. Please help reply where I could find detailed information about the current consumption when MCU is in running mode. All replies will be appreciated. Thanks in advance.

BR,

Samuel

[lama]

Hi,

Diana is right, however I would like to add a few more comments I collected from my PC.

The device is CMOS which means current consumption dependence from BUSCLK is linear.

The current consumption does not depend on used voltage within required range because of internal voltage regulator.

It also depends on loads connected to the pins. Let’s take Table A-15. Run and Wait Current Characteristics and consider no load connected to the MCU. We consider only consumption of the MCU. The maximum current consumption presented in the data sheet is 22mA for 25 MHz BUSCLK and PLL on. If we simplify consideration and take stop current to be 0 mA then we can say:

I[mA] = 0.9 * BUSCLK[MHz]

Of course, this approximation isn’t really accurate.

When we use external oscillator, we should add 0.4mA to maximum consumption as worst case.

The MCU is able to work in static mode if clock monitor is disabled (not in case S12G) – you can clock it by push button connected to the EXTAL. However, some BUSCLK dependent peripherals (like SCI, SPI,…) will not be able to be set and work. It is also valid for periodical clock source with frequency lower than minimal defined by the data sheet. This could be used as a solution how to decrease consumption if speed is not required.

The second solution is to use either STOP or PSEUDO STOP mode of the MCU and wake up the MCU only for required time period. Current consumption for STOP and PSEUDO STOP mode is presented in the Table A-12. Full Stop Current Characteristics and Table A-13. Pseudo Stop Current Characteristics.

Note that the current consumption in the data sheet takes into account worst condition together with some safety gap.

Moreover:

Instantaneous current for one I/O pin is +/-25mA. This current is guaranteed for permanent pin load (thermal and dynamical influence of the current is taken into consideration). This value is maximum immediate value and it is not allowed to be calculated as average value in the case of PWM pin. Beyond absolute maximum ratings device might be damaged.

However, this value is valid if and only if maximum value of MCU power consumption does not exceed maximum allowable current and temperature (power losses) of the MCU.

- 25mA is a current limit and in general it is not recommended to use more than +/-20mA DC current through any IO pin.

For standard pin:

Rdson can be calculated (Table A-9 5V I/O Characteristics – items either 7 or 8):

Rdson = 0.8V/0.004A = 200 ohm   (voltage drop/defined current)

For example, you use 6 * 4mA loads, LQFP 100, double sided PCB. Table A-5 of the device guide (Thermal package characteristics) -> (36) Thermal resistance=61 K/W.

Let ambient temperature is 50 deg. C then chip temperature is:

50 + 61*6*(0.8*0.004A)=51.17 deg.C.  This temperature has to be less than max. operating junction temperature.

(Table A-4 of the device guide.)

Moreover, I would like to bring into your notice that the data sheet defines voltage drop 0.8V at 4mA load. It means that higher current could cause higher voltage drop and you can get out of voltage level specification.

The current we can take through the power supply ring on the silicon itself is surprisingly low (60mA). It is a limit per Vddx and Vssx rail individually. It means you can drive a few loads to log.1 and few loads to log.0 in order to increase number of loads. 

(Let’s suppose 6x10mA diodes switched to log.0 and 6x10mA diodes switched to log.1 (supplied directly from MCU pins). In this case 60mA flows from Vddx through diodes to gnd. Next 60mA flows from external source through diodes to VSSX. In this case we are able to drive 600mA but we are not allowed to add any load. If higher current than 60mA flows through VDDX or VSSX it can destroy this pin because of current density.)

So the umber of VDDX/VSSX pairs has influence on the current the MCU can manage from pin loads point of view.

Let's say an automotive dashboard manufacturer wants to drive 10 LEDs with the micro (driven to log. 1 – Vddx supplies diodes), 10mA each. That is 100mA of current, 0.8V drop across the I/O structure => 80mW power dissipation. That will take the device temperature up by 4.9degC (at 61K/W). That is not too much in terms of temperature rise, but the current is the 40mA over limit!

Moreover, I would like to bring into your notice the document  AN2434 - Input_Output (I_O) Pin Drivers to see information about Voh and Vol dependence on load current.

http://www.freescale.com/files/microcontrollers/doc/app_note/AN2434.pdf

It is older application node created for S12 MCUs at 250nm technology, therefore absolute value doesn’t need to fits, but general information is valid.

Note:  Do not forget at maximum injection current. It is not allowed to be higher than +/-2.5 mA for one pin and sum of all injection currents is not allowed to exceed +/-25mA. Each pin is connected to some of the VDD sources (see Table 2-1. Signal Properties Summary) through ESD protection diode. If the current is higher the diode can be destroyed.

There are two types of diode failing – short-circuit and disconnection.

1. a) In both cases can happen that diode in failure can burn out also area close to it.
2. b) In the case of short circuit you get direct connection between two independent sources which can on the base of short-circuit parameters cause local overheating.
3. c) In the second case you will lose ESD protection which does not have to affect MCU behavior but nobody will guarantee that it will not cause any problems in the near or far future.

VDDX, VSSX – each VDDX and VSSX pin is able to conduct 60mA. Entire current consumed by VDDX, VSSX loads is not allowed to exceed 60mA to each side due to electromigration limit – you can drive 60mA to log1 (current flows from VDDX to the load) and 60mA to log.0 (current flows from load to the VSSX of the MCU) => 120mA. (plus we can multiply this value by a number of VDDX/VSSX pairs)

There is a Table 1-22 100-Pin LQFP Pinout for S12G96 and S12G128 in the spec which denotes which pins are supplied by what supply.

Best regards,

Ladislav

[samuelwang]

Hi Lama,

Thanks for reply, and I have few concerns as following. 

1. Each power pin can conduct 60mA, and we have VDDA, VRH, VDDX, and VDDR for this MCU which means that maximum current is rated 240mA? I assume it's operated under an extreme condition, but I won't put it into consideration.

2. I know the instantaneous max current for any I/O is 25mA at Table A-1, but it's not applicable to power calculation for a normal usage. However, from Table A-6 the injection current limit per pin is 2.5mA or total limit 25mA of all pins is what I can use for current consumption calculation, right? Moreover, is the 25mA is only for one I/O module or all?

3. From Diana's reply, Table-A15 shows 21mA @5.5V is a reference current consumption for 9S12G128. Regarding my design is operated at 3.3V, 21mA*5.5V/3.3 = 35mA@3.3V, right?

4. Total power consumption is 35mA + 25mA = 60mA@3.3V

Please let me know if you have any idea. Thanks.

BR,

Samuel

[lama]

Hi,

1) No, you have to take into consideration which voltage pin supplies which pin. As I have mentioned you should look into a data sheet, for example, Table 1-20. 48-Pin LQFP Pinout for S12G96 and S12G128  … column “power supply”.

2) Injection current and max. current are different terms.
Max. current is a current which is pin able to manage without damage but it is recommended to use nominal current at the pin (defined voltage drop). The sum of loads at the pins connected to given supply voltage are not allowed to exceed 60mA multiplied by an amount of supply pins.
example: we have for example 3 Vddx supply pins. So we can supply 3*60mA loads connected to the pins. (for example 18 diodes each consuming 10mA). The more supply pins the more load can be connected to the GPIO pins.
In order to understand injection current I would like to direct you to:

- https://www.nxp.com/docs/en/application-note/AN2434.pdf

 - https://www.nxp.com/docs/en/application-note/AN4731.pdf

 But, of course sum of injection currents on entire MCU is not allowed to exceed 25mA. (25 mA , in this case, is only randomly the same value as max. load current for one pin )

3) No, voltage level of supply voltage does not play any role in consumption. The current consumption of the MCU (without additional load caused by loads connected to pins) is dependent on the BUSCLK. The consumption presented in the table is for BUSCLK=25MHz. So, your consumption is:

     Simplification if fbus=0 then Imcu=0. (I have set stop current is zero)

     Then    I = f(fbus) = fbus * I_nom/fbus_nom = fbus * 22mA/25Mhz = 0.88 * fbus

     Fbus is the frequency of the bus you are using

     I_nom has been taken from table A-15 for code executed from flash. The f_nom (25MHz) generates I_nom.

4) ??? I do not undwerstand. However total current consumption is :
I = 0.88 * fbus  +  sum of all connected loads

Best regards,

Ladislav

[samuelwang]

Hi Lama,

Thanks for reply. Per the conversation, I = 0.88 * fbus + sum of loading pins of VDDA & VDDX modules. VDDA/VDDX should not exceed 60mA max and I/O should not exceed total 25mA per module, am I right?

BR,

Samuel

[lama]

Hi,

You are right partially.

VDDA/VDDX as well as VSSA/VSSX should not exceed 60mA for each supply rail. (You have to think that Ivssx does not have to be equal to Vvddx in the case we have positive and negative loads connected to the pins) If you have 3 x Vddx pins  and 3xVsssx pins pin then you can consume +180mA and -180mA. (100 pin LQFP package contains 3 pair of supply pins)

25mA is still value when the pin is not damaged but functionality is not guaranteed. In order to fit voltage drop at the pin which guarantees correct logical level you should not exceed nominal operating current which is +/- 4mA for 5V Vdd and +/-1.75mA for 3.3V Vddx..

Best regards,

Ladislav

[dianabatrlova]

Hi Samuel,

We don't have detailed information about current consumption for run mode, because it depends on your application and I/Os. Power consumption also depends on core frequency.

Regarding maximum run mode consumption in Table A-15 of the MC9S12GRMV1 (21 mA), this value was measured at specific conditions, which you can see in reference manual Chapter A.3. Table A-14.

Best Regards,

Diana