$ crontab -l
*/1 * * * * /usr/bin/php /home/user/prj/mage2/bin/magento cron:run >> /home/user/prj/mage2/var/log/cron.log

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/crontab.xsd">
    <group id="default">
        <job name="my_own" instance="VendorModuleCronTask" method="execute">
            <schedule>...</schedule>
        </job>
    </group>
</config>

<schedule>* * * * *</schedule>
<schedule>*/1 * * * *</schedule>