Motorola ColdFire MCF5281 - Manual (Page 416)

Queued Serial Peripheral Interface (QSPI)
22-12 Freescale Semiconductor
the least significant bits of the RAM. Unused bits in a receive queue entry are set to zero upon completion
of the individual queue entry. Receive RAM is not writeable.
QWR[CPTQP] shows which queue entries have been executed. The user can query this field to determine
which locations in receive RAM contain valid data.
22.4.1.2 Transmit RAM
Data to be transmitted by the QSPI is stored in the transmit RAM segment located at addresses 0x0 to 0xF.
The user normally writes 1 word into this segment for each queue command to be executed. The user
cannot read data in the transmit RAM.
Outbound data must be written to transmit RAM in a right-justified format. The unused bits are ignored.
The QSPI copies the data to its data serializer (shift register) for transmission. The data is transmitted most
significant bit first and remains in transmit RAM until overwritten by the user.
22.4.1.3 Command RAM
The CPU writes one byte of control information to this segment for each QSPI command to be executed.
Command RAM, referred to as QCR0–15, is write-only memory from a user’s perspective.
Command RAM consists of 16 bytes, each divided into two fields. The peripheral chip select field controls
the QSPI_CS signal levels for the transfer. The command control field provides transfer options.
A maximum of 16 commands can be in the queue. Queue execution proceeds from the address in
QWR[NEWQP] through the address in QWR[ENDQP].
The QSPI executes a queue of commands defined by the control bits in each command RAM entry that
sequence the following actions:
• Chip-select pins are activated.
• Data is transmitted from the transmit RAM and received into the receive RAM.
• The synchronous transfer clock QSPI_CLK is generated.
Before any data transfers begin, control data must be written to the command RAM, and any out-bound
data must be written to the transmit RAM. Also, the queue pointers must be initialized to the first and last
entries in the command queue.
Data transfer is synchronized with the internally generated QSPI_CLK, whose phase and polarity are
controlled by QMR[CPHA] and QMR[CPOL]. These control bits determine which QSPI_CLK edge is
used to drive outgoing data and to latch incoming data.
22.4.2 Baud Rate Selection
The maximum QSPI clock frequency is one-fourth the clock frequency of the internal bus clock (f
sys
).
Baud rate is selected by writing a value from 2–255 into QMR[BAUD]. The QSPI uses a prescaler to
derive the QSPI_CLK rate from the internal bus clock divided by two.
Table 22-10 shows the QSPI_CLK
frequency as a function of internal bus clock and baud rate.
A baud rate value of zero turns off the QSPI_CLK.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3