The CSN pin on the TNT4882 is the chip select pin. This pin must be
asserted (driven low) during a register access to the TNT4882. The
purpose of a chip select pin is to allow multiple devices to occupy the
same address space and be able to uniquely address them. Whenever RDN
or WRN is asserted, and CSN is asserted, the TNT4882 interprets this as
a register access. Asserting RDN or WRN without CSN asserted will be
ignored by the TNT4882.
The exact implementation of the CSN pin is unique to each system. The
most common method is to connect it to an address pin beyond the range
of what the TNT4882 uses. For example, the TNT4882 uses 5 address
lines. If the host microprocessor has 10 address lines, the 6th address
line could be connected to the CSN input of the TNT4882 through an
inverter. In this case, whenever the TNT4882 is accessed, the 6th
address bit must be driven high, to cause the inverter to drive the CSN
signal low. Accesses to other devices must be made with the 6th address
bit low which will drive CSN high.
If all accesses on the interface bus will be to the TNT4882 (no other
internal or external devices) it is possible to tie CSN directly to
ground. If you do this it is still advisable to provide some external
circuit for asserting CSN even if it is not used.
In the example you mentioned, PSEN is a chip select signal for an
external EEPROM. The circuit will assert CSN only when PSEN is
unasserted (high) and address 5 is low. In this circuit address 5 must
be low to access the TNT4882.
