EP7212 Datasheet, PDF (35/136 Pages) Cirrus Logic – HIGH-PERFORMANCE, LOW-POWER SYSTEM-ON-CHIP WITH LCD CONTROLLER AND DIGITAL AUDIO INTERFACE(DAI)

EP7212
PRDY signals. When the PRDY signal is de-assert-
ed (i.e., low), it indicates that the CL-PS6700 is
busy accessing its card. If a PC CARD access is at-
tempted while the device is busy, the PRDY signal
will cause the EP7212’s CPU to be stalled. The
EP7212’s CPU will have to wait for the card to be-
come available. DMA transfers to the LCD can still
continue in the background during this period of
time (as described below). The EP7212 can access
the registers in the CL-PS6700, regardless of the
state of the PRDY signal. If the EP7212 needs to
access the PC CARD via the CL-PS6700, it waits
until the PRDY signal is high before initiating a
transfer request. Once a request is sent, the PRDY
signal indicates if data is available.
In the case of a PC Card write, writes can be posted
to the CL-PS6700 device, with the same timing as
CL-PS6700 internal register writes. Writes will
normally be completed by the CL-PS6700 device
independent of the EP7212 processor activity. If a
posted write times out, or fails to complete for any
other reason, then the CL-PS6700 will issue an in-
terrupt (i.e., a WR_FAIL interrupt). In the case
where the CL-PS6700 write buffer is already full,
the PRDY signal will be de-asserted (i.e., driven
low) and the transaction will be stalled pending an
available slot in the buffer. In this case, the
EP7212’s CPU will be stalled until the write can be
posted successfully. While the PRDY signal is de-
asserted, the chip select to the CL-PS6700 will be
de-asserted and the main bus will be released so
that DMA transfers to the LCD controller can con-
tinue in the background.
In the case of a PC Card read, the PRDY signal
from the CL-PS6700 will be de-asserted until the
read data is ready. At this point, it will be reasserted
and the access will be completed in the same way
as for a register access. In the case of a byte access,
only one 16-bit data transfer will be required to
complete the access. While the PRDY signal is de-
asserted, the chip select to the CL-PS6700 will be
de-asserted, and the main bus will be released so
that DMA transfers to the LCD controller can con-
tinue in the background.
The EP7212 will re-arbitrate for control of the bus
when the PRDY signal is reasserted to indicate that
the read or write transaction can be completed. The
CPU will always be stalled until the PC Card ac-
cess is completed.
A card read operation may be split into a request
cycle and a data cycle, or it may be combined into
a single request/data transfer cycle. This depends
on whether the data requested from the card is
available in the prefetch buffer (internal to the CL-
PS6700).
The request portion of the cycle, for a card read, is
similar to the request phase for a card write (de-
scribed above). If the requested data is available in
the prefetch buffer, the CL-PS6700 asserts the
PRDY signal before the rising edge of the third
clock and the EP7212 continues the cycle to read
the data. Otherwise, the PRDY signal is de-assert-
ed, and the request cycle is stalled. The EP7212
may then allow the DMA address generator to gain
control of the bus, to allow LCD refreshes to con-
tinue. When the CL-PS6700 is ready with the data,
it asserts the PRDY signal. The EP7212 then arbi-
trates for the bus and, once the request is granted,
the suspended read cycle is resumed. The EP7212
resumes the cycle by asserting the appropriate chip
select, and data is transferred on the next two
clocks if a word read (one clock if a byte read).
There is no support within the EP7212 for detecting
time-outs. The CL-PS6700 device must be pro-
grammed to force the cycle to be completed (with
invalid data for a read) and then generate an inter-
rupt if a read or write access has timed out (i.e.,
RD_FAIL or WR_FAIL interrupt). The system
software can then determine which access was not
successfully completed by reading the status regis-
ters within the CL-PS6700.
The CL-PS6700 has support for DMA data trans-
fers. However, DMA is supported only by software
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