AR# 67577

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Calculate internal pull-up and pull-down strength. Calculating external pull-down with internal pull-up enabled.

Description

How do I calculate the strength of the internal pull-up and pull-down resistors.

How do I calculate the external pull-down resistor with an internal pull-up enabled.

Solution

To calculate the internal pull-up or pull-down you will need Irpu (max) or Irpd (max) from the device DC and AC Switching Characteristics Datasheet.

This Irpu / Irpd value is based on the Vcco voltage. With this information you can calculate Rpu and Rpd using the equations below.

  • Rpu = Vcco / Irpu
  • Rpd = Vcco / Irpd

For example:

From Virtex UltraScale (DS893) Irpu at Vin = 0, Vcco = 1.8V, Irpu(max) = 678uA

Rpu = 1.8V / 678uA = 2.564 k


If you need to add an external pull-down you will need Irpu (max) and the maximum Vin Low (Vil) input level for the I/O standard being used. 

For this calculation we will treat Irpu (max) as a constant current source.

With the pull-down (Rpd-ext) the line level must be less than Vil for the I/O Standard.


Rpd-ext * Irpu (max) < Vil (max)

Rpd-ext < Vil / Irpu


For example:

From Zynq-7000 PL (DS191) Irpu at Vin = 0V, Vcco = 3.3V, Irpu (max) = 330uA

LVCMOS33 Vil (max) = 0.8V

Rpd-ext < 0.8V / 330uA

Rpd-ext < 2.424 k

AR# 67577
Date 03/22/2017
Status Active
Type General Article
Devices More Less
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