FM25V20 – 2Mb SPI F-RAM
without another WREN command. Figure 5 below
illustrates the WREN command bus configuration.
Power Up to First Access
The FM25V20 is not accessible for a period of time
(tPU) after power up. Users must comply with the
timing parameter tPU, which is the minimum time
from VDD (min) to the first /S low.
S
0
0
1
0
2
0
3
0
4
0
5
1
6
1
7
0
C
Data Transfer
All data transfers to and from the FM25V20 occur in
8-bit groups. They are synchronized to the clock
signal (C), and they transfer most significant bit
(MSB) first. Serial inputs are registered on the rising
edge of C. Outputs are driven from the falling edge of
clock C.
D
Q
Hi-Z
Figure 5. WREN Bus Configuration
Command Structure
There are nine commands called op-codes that can be
issued by the bus master to the FM25V20. They are
listed in the table below. These op-codes control the
functions performed by the memory. They can be
divided into three categories. First, there are
commands that have no subsequent operations. They
perform a single function, such as to enable a write
operation. Second are commands followed by one
byte, either in or out. They operate on the Status
Register. The third group includes commands for
memory transactions followed by address and one or
more bytes of data.
WRDI – Write Disable
The WRDI command disables all write activity by
clearing the Write Enable Latch. The user can verify
that writes are disabled by reading the WEL bit in
the Status Register and verifying that WEL=0.
Figure 6 illustrates the WRDI command bus
configuration.
S
0
0
1
0
2
0
3
4
5
1
6
0
7
0
C
Table 1. Op-code Commands
Name
Description
Op-code
00000110b
00000100b
00000101b
00000001b
00000011b
00001011b
00000010b
10111001b
10011111b
Set Write Enable Latch
WREN
WRDI
RDSR
WRSR
READ
FSTRD
WRITE
SLEEP
RDID
0
0
D
Q
Write Disable
Read Status Register
Write Status Register
Read Memory Data
Fast Read Memory Data
Write Memory Data
Enter Sleep Mode
Read Device ID
Hi-Z
Figure 6. WRDI Bus Configuration
RDSR – Read Status Register
The RDSR command allows the bus master to
verify the contents of the Status Register. Reading
Status provides information about the current state
of the write protection features. Following the
RDSR op-code, the FM25V20 will return one byte
with the contents of the Status Register. The Status
Register is described in detail in the section below.
WREN – Set Write Enable Latch
The FM25V20 will power up with writes disabled.
The WREN command must be issued prior to any
write operation. Sending the WREN op-code will
allow the user to issue subsequent op-codes for write
operations. These include writing the Status Register
(WRSR) and writing the memory (WRITE).
Sending the WREN op-code causes the internal Write
Enable Latch to be set. A flag bit in the Status
Register, called WEL, indicates the state of the latch.
WEL=1 indicates that writes are permitted.
Attempting to write the WEL bit in the Status
Register has no effect on the state of this bit.
Completing any write operation will automatically
clear the write-enable latch and prevent further writes
Rev. 3.0
August 2012
Page 5 of 17
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STM32H743技术深度剖析与应用案例探索
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