Synchronous Serial Interface (SSI)
225
March 22, 2006
Preliminary
In this configuration, during idle periods:
SSIClk
is forced High
SSIFss
is forced High
The transmit data line
SSITx
is arbitrarily forced Low
When the SSI is configured as a master, it enables the
SSIClk
pad
When the SSI is configured as a slave, it disables the
SSIClk
pad
If the SSI is enabled and there is valid data within the transmit FIFO, the start of transmission is
signified by the
SSIFss
master signal being driven Low. The master
SSITx
output pad is enabled.
After a further one-half
SSIClk
period, both master and slave data are enabled onto their
respective transmission lines. At the same time,
SSIClk
is enabled with a falling edge transition.
Data is then captured on the rising edges and propagated on the falling edges of the
SSIClk
signal.
After all bits have been transferred, in the case of a single word transmission, the
SSIFss
line is
returned to its idle high state one
SSIClk
period after the last bit has been captured.
For continuous back-to-back transmissions, the
SSIFss
pin remains in its active Low state, until
the final bit of the last word has been captured, and then returns to its idle state as described
above.
For continuous back-to-back transfers, the
SSIFss
pin is held Low between successive data
words and termination is the same as that of the single word transfer.
12.2.4.7
National Semiconductor MICROWIRE Frame Format
Figure 12-10 shows the National Semiconductor MICROWIRE frame format, again for a single
frame. Figure 12-11 shows the same format when back-to-back frames are transmitted.
Figure 12-10. National Semiconductor MICROWIRE Frame Format (Single Frame)
MICROWIRE format is very similar to SPI format, except that transmission is half-duplex instead
of full-duplex, using a master-slave message passing technique. Each serial transmission begins
with an 8-bit control word that is transmitted from the SSI to the off-chip slave device. During this
transmission, no incoming data is received by the SSI. After the message has been sent, the off-
chip slave decodes it and, after waiting one serial clock after the last bit of the 8-bit control
message has been sent, responds with the required data. The returned data is 4 to 16 bits in
length, making the total frame length anywhere from 13 to 25 bits.
In this configuration, during idle periods:
SSIClk
is forced Low
SSIFss
is forced High
The transmit data line
SSITx
is arbitrarily forced Low
SSIClk
SSIFss
LSB
MSB
SSIRx
4 to 16 bits
outputdata
0
SSITx
MSB
LSB
8-bit control