18 Am28F256A
This detailed information is for your reference. It may
prove easier to always issue the Reset command two
consecutive times. This eliminates the need to deter-
mine if you are in the Setup Program state or not.
In-System Programming Considerations
Flash memories can be programmed in-system or in a
standard PROM programmer. The device may be sol-
dered to the circuit board upon rece ipt of shipment and
programmed in-system. Alternatively, the device may
initially be programmed in a PROM programmer prior
to soldering the device to the circuit board.
Auto Select Command
AMD’s Flash memories are designed for use in appli-
cations where the local CPU alters memory contents.
In order to correctly program any Flash memories
in-system, manufacturer and device codes must be
accessible while the device resides in the target
system. PROM programmers typically access the sig-
nature c odes by rais ing A9 to a high v oltage . Howe v er ,
multiplexing high voltage onto address lines is not a
generally desired system design practice.
The device contains an A uto Select operation to supple-
ment traditional PROM programming methodologies.
The operation is ini tiated by wr iting 80h or 90h into the
command register. Following this command, a read
cycle address 0000h retrie v es the manufacturer code of
01h (AMD). A read cycle from address 0001h returns
the device cod e (see the Auto Select C ode table of the
corresponding device data sheet). To ter minate the op-
eration, it is necessar y to w r ite another valid command,
such as Reset (00h or FFh), into the register.