Motorola MC68824 User Manual page 59

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3.5.2 READ VALUE Command
The read value command returns the current operational values of TBC parameters to the host.
Not all of the TBC parameters are meaningful at all times and the TBC does not indicate whether
the value it returns is meaningful. The opcode of the parameter to be read is loaded into the
command parameter area word 0 by the host processor. A list of these parameter opcodes is
given in Table 3-6. The TBC moves the current value of the parameter into the command return
field. In most cases, a two-byte value is returned in CPA RETO, and a four-byte value is returned
in CPA RETO and CPA RET1. The length of the returned value is not a parameter of the command
but is a function of the parameter requested. When finished, the TBC gives confirmation of
command execution in CPA RESULT.
The following exceptions exist:
1. If the opcode is '00' (next station address) or '04' (previous station address), and the
address length is 48 bits, the resulting address will be returned as the high word in CPA
RETO, the medium word in CPA RET1, and the low word in CPA RET2. If the address is
16 bits, the result will be returned in CPA RET2.
2. If the opcode is '80' (internal token rotation time), the value is always returned in CPA
RET2. Note that this parameter is prescaled by three or six bits according to the prescaler
specified via the SET MODE 3 command.
3. The boolean 'nexLstation_known' is returned in CPA RESULT bit 15 if the host has
requested the next station address (opcode 00). If next station known is true, this bit is
zero and the next station address that is returned is valid; if false, this bit is one and the
next station value that is read is not meaningful.
The code for the READ VALUE command is 90 and its format is shown below.
o
I
0
I
0
I
0
3.5.3 SET VALUE Command
An array of commands, referred to as SET VALUE commands, are available to the user to initialize
or modify the TBC's parameters. The parameters which may be modified include the function
codes for buffer descriptors, frame descriptors, and receive and transmit data buffers. Also in-
cluded is the capability, to set the pad timer preset register (PTP), and some of the parameters
which reside in the private area (see Table 3-7 for a complete list). The mechanism used to pass
parameters for the SET VALUE commands is described in each case in the following subsections.
The coding for the SET VALUE commands is 81 through 87 and the format for each command is
shown below.
Where:
S S S
o
0 1
0 1 0
1 1 1
o
1 1
1 1 0
1 0 1
NOTE:
1
I
0
I
0
o
I
0
s
Function
Set Function Code of Buffer Descriptor
1
Set Function Code of Frame Descriptor
1
Set Function Code of RX and TX Data Buffers
1
Set Pad Timer Preset (PTP) Register
Set One Word Value
Set Two Word Value
1. If the signals FCO-FC3 are not used, these commands are not necessary.
MOTOROLA
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MC68824 USER'S MANUAL