UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
1015 of 1269
NXP Semiconductors
UM10503
Chapter 39: LPC43xx SSP0/1
After all bits have been transferred, in the case of a single word transmission, the SSEL
line is returned to its idle HIGH state one SCK period after the last bit has been captured.
For continuous back-to-back transmissions, the SSEL pins 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. In general, for continuous back-to-back transfers the SSEL pin is
held LOW between successive data words and termination is the same as that of the
single word transfer.
39.7.3 National Semiconductor Microwire frame format
shows the Microwire frame format for a single frame.
shows the
same format when back-to-back frames are transmitted.
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 SSP to the
off-chip slave device. During this transmission, no incoming data is received by the SSP.
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:
•
The SK signal is forced LOW.
•
CS is forced HIGH.
•
The transmit data line SO is arbitrarily forced LOW.
A transmission is triggered by writing a control byte to the transmit FIFO.The falling edge
of CS causes the value contained in the bottom entry of the transmit FIFO to be
transferred to the serial shift register of the transmit logic, and the MSB of the 8-bit control
frame to be shifted out onto the SO pin. CS remains LOW for the duration of the frame
transmission. The SI pin remains tristated during this transmission.
The off-chip serial slave device latches each control bit into its serial shifter on the rising
edge of each SK. After the last bit is latched by the slave device, the control byte is
decoded during a one clock wait-state, and the slave responds by transmitting data back
to the SSP. Each bit is driven onto SI line on the falling edge of SK. The SSP in turn
Fig 127. Microwire frame format (single transfer)
SK
CS
SO
4 to 16 bits
output data
SI
8-bit control
MSB
LSB
0
MSB
LSB