SA25F020 Advanced Information
SAIFUN
32
Release from Software
Protection and Read Electronic
Signature (RES)
Once the device has entered SP mode, all
instructions are ignored except the RES
mode. The instruction can also be used to
read the 8-bit Electronic Signature of the
device on the SO pin.
The RES instruction always provides
access to the Electronic Signature of the
device (except while an Erase, Program or
Write Status Register cycle is in progress),
and can be applied even if SP mode has
not been entered. Any RES instruction
executed while an Erase, Program or Write
Status Register cycle is in progress is not
decoded, and has no effect on the cycle in
progress.
This instruction serves a second purpose
as well. The device features an 8-bit
Electronic Signature, whose value for the
SA25F020 is 11h. This can be read using
the RES instruction.
The device is first selected by driving CSb
low. The instruction code is followed by
three dummy bytes, each bit being latched
in on SI during the rising edge of SCK.
The 8-bit Electronic Signature, which
stored in the memory, is then shifted out on
SO, with each bit being shifted out during
the falling edge of SCK. The instruction
sequence is shown in Figure 22.
Driving CSb high after the Electronic
Signature has been read at least once
terminates the RES instruction. Sending
additional clock cycles on SCK, while CSb
is driven low causes the Electronic
Signature to be output repeatedly.
When CSb is driven high, the device is put
into Standby mode. The transition to
Standby mode is delayed by tRES, and CSb
must remain high for at least tRES(max), as
specified in Table 4 on page 10. Once in
Standby mode, the device waits to be
selected, so that it can receive, decode and
execute instructions.
1
3
210
9
8
7
6
5
4
0
31
30
29
28
Instruction
3 Dummy
Bytes
23
21
22
1
32
0
36
35
34
33
32
39
38
37
SCK
SI
SO
High Impedance
MSB
Electonic ID
MSB
t
RES
Software Protect Mode
Standby Mode
1
32
0
765
4
CS
Figure 22. Release from Software Protection and Read Electronic Signature (RES) Instruction
Sequence
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