Generation of 18k square wave

Easy: just toggle a port pin once every 28ms!

Easy: just toggle a port pin once every 28ms!

Now, here are the links to the so-called "bible" for the 8051 - see what you can find that might enable you to toggle a port pin once every 28ms:

Chapter 1 - 80C51 Family Architecture:
www.nxp.com/.../80C51_FAM_ARCH_1.pdf

Chapter 2 - 80C51 Family Programmer's Guide and Instruction Set:
www.nxp.com/.../80C51_FAM_PROG_GUIDE_1.pdf

Chapter 3 - 80C51 Family Hardware Description:
www.nxp.com/.../80C51_FAM_HARDWARE_1.pdf

Also check the datasheet for the particular chip that you're using - it may have other useful features...

28uS :-)

Actually, 28us !! ;-)

even easier:
use the PCA
of course, if you insist on using some antique or 'simple' chip, that does not apply

Erik

reference:
The PCA cookbook
www.intel.com/.../270609.htm