UM10413
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© NXP B.V. 2011. All rights reserved.
User manual
Rev. 1 — 16 December 2011
166 of 268
NXP Semiconductors
UM10413
MPT612 User manual
Microwire format is 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 is 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:
•
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 pin SO. CS remains LOW for the duration of the frame
transmission. Pin SI remains tri-stated 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 line SI on the falling edge of SK. The SSP in turn
latches each bit on the rising edge of SK. At the end of the frame, for single transfers, the
CS signal is pulled HIGH one clock period after the last bit is latched into the receive serial
shifter, which causes the data to be transferred to the receive FIFO.
Remark:
The off-chip slave device can tri-state the receive line either on the falling edge
of SK after the LSB is latched by the receive shifter, or when pin CS goes HIGH.
For continuous transfers, data transmission begins and ends in the same manner as a
single transfer. However, the CS line is continuously asserted (held LOW) and
transmission of data occurs back to back. The control byte of the next frame follows
directly after the LSB of the received data from the current frame. Each of the received
values is transferred from the receive shifter on the falling edge of SK after the LSB of the
frame has been latched into the SSP.
Fig 49. Microwire frame format (single transfer)
SK
CS
SO
MSB
LSB
SI
LSB
0
MSB
4 to 16 bits
output data
8 bit control
aaa-000612