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MicroZed PG_1V8 or VCCIO_EN logic sense?

Former Member

What is the logic sense of the MicroZed PG_1V8 or VCCIO_EN signal? Is it active high? Meaning that when this signal is about 1.5V, then our carrier board design needs to generate VCCIO for the PL I/O banks?

  • 0

    Take a look at the MicroZed Carrier Design Guide located here: http://www.zedboard.org/documentation/1519

     

    You can also use the MicroZed IOCC carrier design has an example. The design files for the IOCC Carrier card are here: http://www.zedboard.org/documentation/1522

     

    -Gary

  • 0 in reply to Former Member

    Related to VCCIO_EN on MicroZed and differences that might be seen between some F revisions and Rev G:

     

    MicroZed has a handshaking signal that communicates from the SOM to the Carrier to tell the Carrier that it is OK to enable the VCCIO supplies and meet the Zynq-7000 sequencing requirements. On Avnet's MicroZed Carriers and in the MicroZed Carrier Design Guide, this signal is called VCCIO_EN. On the MicroZed, the signal is tied to the Power Good from the 1.8V circuit, signal name PG_1V8.

     

    The Carrier Design Guide states:

    All three Vccio banks that receive power from the carrier can be independent, or tied together depending on the specific

    design needs. To maintain proper start up sequencing, these Vccio supplies should be enabled by the VCCIO_EN signal

    tied to JX2 pin 10. Note that this enable signal is the PGOOD for the 1.8V supply on MicroZed. It is vital to ensure that

    the enable threshold for the regulators chosen is compatible with a 1.8V signal. If 1.8V is not high enough to reliably

    enable the device, an external circuit must be used to boost this voltage. An example of such a circuit can be found in

    the schematic for the FMC Carrier card.

     

    Electrically, PG_1V8/VCCIO_EN is an open-drain output controlled by the Power Good signal coming out of the SOM's 1.8V regulator. This signal was never designed to drive all the way to 1.8V. The 1.8V rail on MicroZed is generated from device U15, which is a TI TLV62130RGT. As stated in the TLV62130 datasheet, the PG output is open drain and requires a pull-up resistor. The implementation on MicroZed includes a strong pull-up and a weak pull-down. The weak pull-down was added to more quickly shut down the circuitry in a power-off event.

     

    As you can see in the schematics, this strong pull-up / weak pull-down combination is resistor-divider R35/R91 that will limit the Voh-max to 1.8*(R91/(R91+R35)). On Revisions prior to F-06, Voh-max is 1.5V. On Rev F-06 and Rev G with R35=1.5K, the Voh-max = 1.35V. In practical measurements, we’ve seen the Vouts be about 100 to 150mV less than the theoretical.

     

    On Avnet's own carriers, the VCCIO regulators had input enables whose Vih-min < 1.35V, so they worked fine regardless of the Revision. However, a custom carrier board that was actively monitoring VCCIO_EN / PG_1V8 and expecting ~1.5V will work with the older Rev F's and then fail with Rev G. Therefore, make sure the threshold of any comparator on a Carrier's VCCIO_EN will trigger high at 1.2V or greater.

     

    Bryan