UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 415 of 841
NXP Semiconductors
UM10360
Chapter 18: LPC176x/5x SSP0/1
For device configured as a master in this mode, CLK and FS are forced LOW, and the
transmit data line DX is tri-stated whenever the SSP is idle. Once the bottom entry of the
transmit FIFO contains data, FS is pulsed HIGH for one CLK period. The value to be
transmitted is also transferred from the transmit FIFO to the serial shift register of the
transmit logic. On the next rising edge of CLK, the MSB of the 4-bit to 16-bit data frame is
shifted out on the DX pin. Likewise, the MSB of the received data is shifted onto the DR
pin by the off-chip serial slave device.
Both the SSP and the off-chip serial slave device then clock each data bit into their serial
shifter on the falling edge of each CLK. The received data is transferred from the serial
shifter to the receive FIFO on the first rising edge of CLK after the LSB has been latched.
18.5.2 SPI frame format
The SPI interface is a four-wire interface where the SSEL signal behaves as a slave
select. The main feature of the SPI format is that the inactive state and phase of the SCK
signal are programmable through the CPOL and CPHA bits within the SSPCR0 control
register.
18.5.2.1 Clock Polarity (CPOL) and Phase (CPHA) control
When the CPOL clock polarity control bit is 0, it produces a steady state low value on the
SCK pin. If the CPOL clock polarity control bit is 1, a steady state high value is placed on
the CLK pin when data is not being transferred.
a. Single frame transfer
b. Continuous/back-to-back frames transfer
Fig 76. Texas Instruments Synchronous Serial Frame Format: a) Single and b) Continuous/back-to-back Two
Frames Transfer
CLK
FS
DX/DR
4 to 16 bits
MSB LSB
CLK
FS
DX/DR
LSBMSB LSBMSB
4 to 16 bits 4 to 16 bits
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