IMX6 IOMUXC Pad Control Drive Strength Field (DSE) - current vs voltage

[timharvey]

KevinWong

arpan_chakravarty

partner

Gateworks

We provide several IMX6 GPIO pins to connectors for off-board use and are continually asked what drive strength these pins can support.  The IMX6DQRM documents the DSE field of the various iomuxc pad control registers as impedance values (HIZ, 240ohm, 120ohm, 80ohm, 60ohm, 48ohm, 40ohm, 34ohm for the pads we use).  In our configuration the pads we use are on 3.3V rails.  We have been told that in order to provide an output voltage vs current chart for the IO cells for our customers we need to use the IBIS models for the IMX6 IO cells.    Has anyone done this or can anyone shed some light on the subject of what 'output' voltage vs current is possible at the various DSE's?

Thanks,

Tim

[Yuri]

1.
To simplify considerations one can use the equation below regarding output impedance

(mentioned in the DSE field)  :

Zo = (VCC – VIOH) / Iout

Here Zo (output impedance) is considered as a resistance between VCC and output pin, that

drives high level (VIOH). Note, the resistance between pin (VIOH) and GND is considered as load

one.

2.
For detailed calculations simulation should be applied. This requires to build a model, including
for example, i.MX6 output pin, PCB trace, input pin of an external device. Simulation tool can provide

signal wave forms using IBIS models of the affected pins.

3.
As for volt ampere curves, when considering current using IBIS it is needed to take into account external
resistive load R. The current I may be defined as solution of equation :

Vol = OVDD – I * R

  (at cross point of IBIS Volt-Ampere curve and straight line I = (OVDD – Vol) / R)

Please take a look at the figure.

Please use typical I(typ) of IBIS volt-ampere curves.

[timharvey]

Yuri,

Thanks for the info.  I'm aware of the standard 'Zo = (VCC – VIOH) / Iout' equation but this assumes you have a constant VIOH.  Customers want to know what Ioh and Iol are at various voltages.  The IMX6DQIEC Table 22 only shows Ioh and Iol for OVDD-0.15 and 0.15 respectively.  Furthermore, Table 22 seems to indicate that there are really only two categories of drive strength (DSE 001, 010 vs DSE 011,100,101,110,111) which makes me even more confused at what DSE really does.  What we are interested in is a Voltage/Current graph which you wouldn't need to know anything about the load being drive for (because the load there would simply be V/I).

We have not encountered datasheets with I/O cells requiring you to use IBIS models for simple V/I curves and quite frankly I don't have the software/knowledge to run a simulation on an IBIS model.  Does this really need to be that complicated?  Perhaps the datasheet can provide some additional current conditions such as Ioh @ VDD - 0.8V and Voh @ 0.8V

Thanks,

Tim

[Yuri · Accepted solution]

Please look at my comments below :

> Customers want to know what Ioh and Iol are at various voltages. 

   It is needed to sovle the non-linear equation - as mentioned earlier - for it.

Basically it is possible to calculate some typical applications as use it in table form,
but Freescale does not provide this, assuming customers can use IBIS models

for thier specific cases.

> The IMX6DQIEC Table 22 only shows Ioh and Iol for OVDD-0.15 and 0.15 respectively.

The information in the Datasheet(s) is interpretted as following (Voh as an example):

High-level output voltage (Voh) is guaranteed as value greater or equal OVDD-0.15 if current

Ioh is no more than specified value (say 1 mA).

[manojprabahar]

Hi Yuri,

I came through your explanation about drive strength. Its very helpful. But i have a query on it.

The test conditions in the mentioned table says even for multiple drive strengths, to keep the stated Voh, the test current is not more than 1 mA!

But if i solve for maximum drive strength (DSE-111) Iout=(OVDD(3.3)-0.15 ) / Zo111(20ohm) = 7.5 mA.

So, will you recommend me to use this drive strength for driving a chip LED with If < 7mA?

Thanks,

Manoj

[Yuri]

  Strictly speaking the i.MX6 signals are not intended for high current load,
nevertheless the value of 7.5 mA for single pin (assuming other ones are under
low current) is not very critical. Sorry, but we do not specify what is really critical
current load..

~Yuri

[timharvey]

Yuri,

Thanks for your reply.  I received the following information from our FAE that I figured I would pass along in case others need it.

Below are typical values for the respective low/high voltages. This is using the model gpiohv_ds001_sr110 for a 3.3V pad. High voltage mode, low drive strength, fast slew rate

Iol

0.2V = 1.4459mA (0.14V)

0.6V = 5.2275mA (0.58V)

1.0V = 7.9481mA (1.02V)

1.4V = 9.3609mA (1.35V)

Ioh

3.0V = 13.1360mA (3.02V)

2.6V = 12.8743mA (2.58V)

2.2V = 12.4030mA (2.25V)

1.8V =  11.2155mA (1.81V)

This information along with your reply gives us enough to go on.

Thanks!

Tim