Chip Select (CS)
This is an active high input pin to FM93C66 EEPROM (the device)
and is generated by a master that is controlling the device. A high
level on this pin selects the device and a low level deselects the
device. All serial communications with the device is enabled only
when this pin is held high. However this pin cannot be permanently
tied high, as a rising edge on this signal is required to reset the
internal state-machine to accept a new cycle and a falling edge to
initiate an internal programming after a write cycle. All activity on the
SK, DI and DO pins are ignored while CS is held low.
Serial Clock (SK)
This is an input pin to the device and is generated by the master that
is controlling the device. This is a clock signal that synchronizes the
communication between a master and the device. All input informa-
tion (DI) to the device is latched on the rising edge of this clock input,
while output data (DO) from the device is driven from the rising edge
of this clock input. This pin is gated by CS signal.
Serial Input (DI)
This is an input pin to the device and is generated by the master
that is controlling the device. The master transfers Input informa-
tion (Start bit, Opcode bits, Array addresses and Data) serially via
this pin into the device. This Input information is latched on the
rising edge of the SCK. This pin is gated by CS signal.
Serial Output (DO)
This is an output pin from the device and is used to transfer Output
data via this pin to the controlling master. Output data is serially
shifted out on this pin from the rising edge of the SCK. This pin is
active only when the device is selected.
A typical communication on the Microwire bus is made through the
CS, SK, DI and DO signals. To facilitate various operations on the
Memory array, a set of 7 instructions are implemented on FM93C66.
The format of each instruction is listed under Table 1.
Each of the 7 instructions is explained under individual instruction
This is a 1-bit field and is the first bit that is clocked into the device
when a Microwire cycle starts. This bit has to be “1” for a valid cycle
to begin. Any number of preceding “0” can be clocked into the
device before clocking a “1”.
This is a 2-bit field and should immediately follow the start bit.
These two bits (along with 2 MSB of address field) select a
particular instruction to be executed.
This is an 8-bit field and should immediately follow the Opcode bits.
In FM93C66, all 8 bits are used for address decoding during READ,
WRITE and ERASE instructions. During all other instructions, the
MSB 2 bits are used to decode instruction (along with Opcode bits).
This is a 16-bit field and should immediately follow the Address
bits. Only the WRITE and WRALL instructions require this field.
D15 (MSB) is clocked first and D0 (LSB) is clocked last (both
during writes as well as reads).
Table 1. Instruction set
Start Bit Opcode Field
1 10 A7 A6 A5 A4 A3 A2 A1 A0
1 00 1 1 X X X X X X
1 01 A7 A6 A5 A4 A3 A2 A1 A0 D15-D0
1 00 0 1 X X X X X X D15-D0
1 00 0 0 X X X X X X
1 11 A7 A6 A5 A4 A3 A2 A1 A0
1 00 1 0 X X X X X X
FM93C66 Rev. C.1