SPI Modes 0 and 3 (00 and 11)
Functional Description (Continued)
FM25C160U supports both Mode 0 and Mode 3 of operations.
The difference between Mode 0 and Mode 3 is determined by the
state of the SCK clock signal when a SPI cycle starts (when /CS
is driven low) as well as when the SPI cycle ends (when /CS is
drivenhigh).UnderMode0ofoperation,theSCKsignalisheldlow
both at the start and at the end of a SPI cycle. Under Mode 1 of
operation, the SCK signal is held high both at the start and at the
end of a SPI cycle. However in both of these two modes, the input
data(SI)issampled(latchedin)attherisingedgeoftheSCKclock
signal and the output data (SO) is driven after the falling edge of
the SCK clock signal. See Figure 1 and Figure 2.
SPI communication
As mentioned before, serial communication with the EEPROM is
enabled when the /CS pin is held low and the /HOLD pin is held
high. Input data (Instruction Opcodes, Array addresses and Data)
on the SI pin is latched in on the rising edges of SCK clock signal,
startingfromthefirstrisingedgeafterthe/CSpingoeslow. During
the time the SI data is input into the EEPROM, the SO pin remains
in high impedance state. If the intended instruction is of read
nature (Array read and Status register read), then data from the
EEPROM is driven out actively on the SO pin from every falling
edge of the SCK after the last input data (SI) is latched in. During
the time the SO data is output from the EEPROM, the data on the
SI pin is ignored. Figure 2 illustrates the above. Refer Figure 1 for
timing information.
READ SEQUENCE (READ)
Reading the memory via the serial SPI link requires the following
sequence. The/CS pin is pulled low to select the EEPROM. The
READ opcode is transmitted on the SI pin followed by two bytes of
address, "High byte addr" (A15-A8) and "Low byte addr" (A7-A0).
After this is done, data on the SI pin becomes don't care. The data
(D7-D0)formtheaddressspecifiedisthenshiftedoutontheSOpin.
If only one byte is to be read, the /CS pin can be pulled back to the
high level. It is possible to continue the READ sequence as the byte
address is automatically incremented and data will continue to be
shifted out as clock pulses are continuously applied. When the end
ofmemoryarrayisreached(lastbytelocation),theaddresscounter
rolls over to the start of memory array (first byte location) allowing
the entire memory to be read in one continuous READ cycle. See
Figure 5. Note that only A10-A0 address bits are treated valid by
FM25C160U while bits A15-A11 are ignored.
HOLD function
An active communication with the EEPROM can be temporarily
suspended by bringing the /HOLD pin low when a EEPROM is
selected (/CS pin should be low) and a serial sequence with the
EEPROM is currently underway. To suspend the communication,
/HOLDpinmustbedrivenlowwhileSCKislow,otherwisetheHold
function will not be invoked until the next SCK high to low
transition. The EEPROM must remain selected during this se-
quence. TransitionsontheSCKandSIpinsareignoredduringthe
time the part is suspended and the SO pin will be in high
impedance state. Releasing the /HOLD pin back to high state will
allow the operation to resume from the point it was suspended.
/HOLD pin must be driven high while the SCK pin is low, otherwise
serial communication will not resume until the next SCK high to
low transition. Asserting a low on the /HOLD pin at any time will tri-
state the SO pin. Figure 3 illustrates Hold timing.
FIGURE 5. Read Sequence
CS
Read
Opcode
High Byte
Addr
Low Byte
Addr
SI
System Configuration
WhenmultipleSPIperipherals(fore.g.EEPROMs)arepresenton
the bus, the SI, SO and the SCK signals can be tied together.
Figure 4 illustrates a typical system configuration with respect to
/CS, SCK, SI and SO pins.
Data
(1)
Data
(2)
Data
(n)
SO
READ STATUS REGISTER (RDSR):
The Read Status Register (RDSR) instruction provides read
access to the status register. As mentioned before, of the 8bits of
data, only the LSB 4bits are valid and they indicate Block Protec-
tion information (BP1 and BP0), Write Enable status (WEN) and
Busy/Ready status (/RDY) of the EEPROM. MSB 4bits of are
invalid (Don’t cares) Following is the format of RDSR data:
FIGURE 4. System Configuration
MASTER MCU
FM25Cxxx
DATA OUT (MOSI)
SI
DATA IN (MISO)
SO
SERIAL CLOCK (SPICK)
SCK
TABLE 3. Status Register Format
SS0
SS1
SS2
SS3
/CS
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
Bit
0
SPI
SI
SO
SCK
/CS
CHIP
SELECTION
X
X
X
X
BP1
BP0 WEN RDY
Bit3 (BP1) and Bit2 (BP0) together indicate Block write protection
previously set on the EEPROM. Refer Table 2.
SI
Bit1 (WEN) indicates the Write enable status of the EEPROM.
This bit is a read-only bit and is read by executing RDSR
instruction. If this bit is “1” then the EEPROM is write enabled. If
this bit is “0” then the EEPROM is write disabled.
SO
SCK
/CS
SI
SO
SCK
/CS
7
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