M25P64 Datasheet, PDF (36/50 Pages) STMicroelectronics – 64 Mbit, Low Voltage, Serial Flash Memory With 50MHz SPI Bus Interface

Instructions
M25P64
6.11
Read Electronic Signature (RES)
The instruction is used to read, on Serial Data Output (Q), the old-style 8-bit Electronic
Signature, whose value for the M25P64 is 16h.
Please note that this is not the same as, or even a subset of, the JEDEC 16-bit Electronic
Signature that is read by the Read Identifier (RDID) instruction. The old-style Electronic
Signature is supported for reasons of backward compatibility, only, and should not be used
for new designs. New designs should, instead, make use of the JEDEC 16-bit Electronic
Signature, and the Read Identifier (RDID) instruction.
The device is first selected by driving Chip Select (S) Low. The instruction code is followed
by 3 dummy bytes, each bit being latched-in on Serial Data Input (D) during the rising edge
of Serial Clock (C). Then, the old-style 8-bit Electronic Signature, stored in the memory, is
shifted out on Serial Data Output (Q), each bit being shifted out during the falling edge of
Serial Clock (C).
The instruction sequence is shown in Figure 18.
The Read Electronic Signature (RES) instruction is terminated by driving Chip Select (S)
High after the Electronic Signature has been read at least once. Sending additional clock
cycles on Serial Clock (C), while Chip Select (S) is driven Low, cause the Electronic
Signature to be output repeatedly.
When Chip Select (S) is driven High, the device is put in the Standby Power mode. Once in
the Standby Power mode, the device waits to be selected, so that it can receive, decode and
execute instructions.
Driving Chip Select (S) High after the 8-bit instruction byte has been received by the device,
but before the whole of the 8-bit Electronic Signature has been transmitted for the first time,
still ensures that the device is put into Standby Power mode. Once in the Standby Power
mode, the device waits to be selected, so that it can receive, decode and execute
instructions.
Figure 18. Read Electronic Signature (RES) instruction sequence and data-out
sequence
S
0 1 2 3 4 5 6 7 8 9 10 28 29 30 31 32 33 34 35 36 37 38
C
Instruction
3 Dummy Bytes
D
23 22 21 3 2 1 0
MSB
High Impedance
Q
Electronic Signature Out
76543210
MSB
1. The value of the 8-bit Electronic Signature, for the M25P64, is 16h.
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