CS8420 datasheet(74/80 Pages) CIRRUS

74 / 80 page

CS8420

DS245PP2

16.1.2

Reserving the first 5 bytes in the E

buffer

D to E buffer transfers periodically overwrite the

data stored in the E buffer. This can be a problem

for users who want to transmit certain channel sta-

tus settings which are different from the incoming

settings. In this case, the user would have to super-

impose his settings on the E buffer after every D to

E overwrite.

To avoid this problem, the CS8420 has the capabil-

ity of reserving the first 5 bytes of the E buffer for

user writes only. When this capability is in use, in-

ternal D to E buffer transfers will NOT affect the

first 5 bytes of the E buffer. Therefore, the user can

set values in these first 5 E bytes once, and the set-

tings will persist until the next user change. This

mode is enabled via the Channel Status Data Buffer

Control register.

16.1.3

Serial Copy Management System

(SCMS)

In software mode, the CS8420 allows read/modi-

fy/write access to all the channel status bits. For

consumer mode SCMS compliance, the host mi-

crocontroller needs to read and manipulate the Cat-

egory Code, Copy bit and L bit appropriately.

In hardware mode, the SCMS protocol can be fol-

lowed by either using the COPY and ORIG input

pins, or by using the C bit serial input pin. These

options are documented in the hardware mode sec-

tion of this data sheet (starting on14 page 49)

16.1.4

Channel Status Data E Buffer

Access

The E buffer is organized as 24 x 16-bit words. For

each word the MS Byte is the A channel data, and

the LS Byte is the B channel data (see Figure 39).

There are two methods of accessing this memory,

known as one byte mode and two byte mode. The

desired mode is selected via a control register bit.

16.1.5

One Byte mode

In many applications, the channel status blocks for

the A and B channels will be identical. In this situ-

ation, if the user reads a byte from one of the chan-

nel’s blocks, the corresponding byte for the other

channel will be the same. Similarly, if the user

wrote a byte to one channel’s block, it would be

necessary to write the same byte to the other block.

One byte mode takes advantage of the often identi-

cal nature of A and B channel status data.

When reading data in one byte mode, a single byte

is returned, which can be from channel A or B data,

depending on a register control bit. If a write is be-

ing done, the CS8420 expects a single byte to be in-

put to its control port. This byte will be written to

both the A and B locations in the addressed word.

One byte mode saves the user substantial control

port access time, as it effectively accesses 2 bytes

worth of information in 1 byte’s worth of access

time. If the control port’s auto-increment address-

ing is used in combination with this mode, multi-

byte accesses such as full-block reads or writes can

be done especially efficiently.

16.1.6

Two Byte mode

There are those applications in which the A and B

channel status blocks will not be the same, and the

user is interested in accessing both blocks. In these

situations, two byte mode should be used to access

the E buffer.

In this mode, a read will cause the CS8420 to out-

put two bytes from its control port. The first byte

out will represent the A channel status data, and the

2nd byte will represent the B channel status data.

Writing is similar, in that two bytes must now be

input to the CS8420’s control port. The A channel

status data is first, B channel status data second.

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