MC68HC08AZ0 Datasheet, PDF (230/444 Pages) Motorola, Inc – Advance Information

Freescale Semiconductor, Inc.
Serial Peripheral Interface Module (SPI)
Transmission
initiation latency
When the SPI is configured as a master (SPMSTR = ‘1’), transmissions
are started by a software write to the SPDR. CPHA has no effect on the
delay to the start of the transmission, but it does affect the initial state of
the SCK signal. When CPHA = ‘0’, the SCK signal remains inactive for
the first half of the first SCK cycle. When CPHA = ‘1’, the first SCK cycle
begins with an edge on the SCK line from its inactive to its active level.
The SPI clock rate (selected by SPR1:SPR0) affects the delay from the
write to SPDR and the start of the SPI transmission. See Figure 7. The
internal SPI clock in the master is a free-running derivative of the internal
MCU clock. It is only enabled when both the SPE and SPMSTR bits are
set to conserve power. SCK edges occur halfway through the low time
of the internal MCU clock. Since the SPI clock is free-running, it is
uncertain where the write to the SPDR will occur relative to the slower
SCK. This uncertainty causes the variation in the initiation delay shown
in Figure 7. This delay will be no longer than a single SPI bit time. That
is, the maximum delay is two MCU bus cycles for DIV2, eight MCU bus
cycles for DIV8, 32 MCU bus cycles for DIV32, and 128 MCU bus cycles
for DIV128.
MC68HC08AZ0
228
Serial Peripheral Interface Module (SPI)
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