mc68hc08qa24 Freescale Semiconductor, Inc, mc68hc08qa24 Datasheet - Page 254

• Manufacturer Part Number: mc68hc08qa24
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Serial Peripheral Interface (SPI)
18.6.4 Transmission Initiation Latency
18.7 Error Conditions
Technical Data
252
When the SPI is configured as a master (SPMSTR = 1), transmissions
are started by a software write to the SPDR ($0012). 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
derivative of the internal MCU clock. It is only enabled when both the
SPE and SPMSTR bits (SPCR) 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
than a single SPI bit time. That is, the maximum delay between the write
to SPDR and the start of the SPI transmission 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.
Two flags signal SPI error conditions:
1. Overflow (OVRFin SPSCR) — Failing to read the SPI data register
2. Mode fault error (MODF in SPSCR) — The MODF bit indicates
before the next byte enters the shift register sets the OVRF bit.
The new byte does not transfer to the receive data register, and
the unread byte still can be read by accessing the SPI data
register. OVRF is in the SPI status and control register.
that the voltage on the slave select pin (SS) is inconsistent with the
mode of the SPI. MODF is in the SPI status and control register.
18-7.) The internal SPI clock in the master is a free-running
Serial Peripheral Interface (SPI)
Figure
18-7. This delay will be no longer
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MC68HC08QA24