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366 SA-1110 Developer’s Manual
Peripheral Control Module
11.12.7.1 Frame Format
Each data frame is between 4 and 16 bits long depending on the size of data programmed, and is
transmitted starting with the MSB. There are three basic frame types that can be selected: Motorola
SPI, Texas Instruments synchronous serial, and National Microwire. For all three formats, the serial
clock (SCLK) is held low or inactive, while the SSP is idle and transitions at the programmed
frequency only during active transmission of data. For Motorola SPI and National Microwire frame
formats, the serial frame (SFRM) pin is active low, and is asserted (pulled down) during the entire
frame’s transmission. In these modes, the SFRM pin is used to select the off-chip slave serial device,
enabling it for transmission. For Texas Instruments format, the SFRM pin is pulsed for one serial
clock period starting at its rising edge, prior to each frame’s transmission. The type of serial clock
edges used to drive and sample data are different for all three modes. For National Microwire format,
both the SSP and the off-chip slave device drive their output data on the falling edge of SCLK, and
latch data from the other device on the rising edge. For Texas Instruments format, both the SSP and
the off-chip slave device drive their output data on the rising edge of SCLK, and latch data from the
other device on the falling edge. For Motorola SPI format, the user has the option of which edge of
SCLK to drive and sample data, as well as the phase of the SCLK signal (whether it is shifted
one-half period to the left or right during the frame transmission).
Unlike the full-duplex transmission of the other two frame formats, the National Microwire format
uses a special master-slave messaging technique that operates at half-duplex. In this mode, when a
frame begins, an 8-bit control message is transmitted to the off-chip slave. During this transmit, no
incoming data is received by the SSP. After the message has been sent, the off-chip slave decodes it
and responds with the requested data after waiting one serial clock after the last bit of the 8-bit
control message has been sent. The returned data can be 4 to 16 bits in length, making the total
frame length anywhere from 13 to 25 bits.