Motorola MCF5282, MCF5281 user manual Baud Rate Selection, Transmit RAM, Command RAM

Operation

22.4.1.2Transmit 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.

Out-bound 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.

22.4.1.3Command 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 with each byte 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 which sequence the following actions:

•Chip-select pins are activated

•Data is transmitted from 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 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 system clock. 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 system clock divided by two.

A baud rate value of zero turns off the QSPI_CLK.

22-6 MCF5282 User’s Manual MOTOROLA