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GR716-DS-UM, May 2019, Version 1.29
448
www.cobham.com/gaisler
GR716
The message header is composed of a Command token from the master and a Response token from
the slave. The message also contains optional data words and CRC checksum appended at the end that
are calculated for the data words transferred. The CRC is mandatory, if the message contains payload
data then the message is always appended with one word of CRC. The received messages are pro-
cessed by the SPI slave device and response and data are transferred as per the received command.
Also note some of the status bits inplatform
the response token are status for the previously received command.
45.6
Message Header - Command Token
The master transmits a message header that specifies the action need to be performed in slave. The
command token sent by the master device consist of two 16 bit words. The message header content
details are explained below.
Prefix and spare
The prefix bits are transmitted initially. In the command word#1 and #2 the prefix and spare bits have
fixed value in the current implementation, these are reserved bits. The spislave receives them and use
it for validating the token. If an invalid prefix and spare bits are received then Status illegal command
(SIC) status bit is enabled and also transmitted to master as part of the next response token.
Message Length
The number of payload words that will be transmitted in the current message. The number should not
include the command token and the CRC checksum appended at the end of the message.
Sub-address
This field provide additional sub-address location for write and read commands.
CRC-4
Table 595.
Example message format (read data)
Signal
Message Header
Payload
Payload CRC
MOSI
Command #1
Command #2
0x0000
0x0000
MISO
Response #1
Response #2
Data
CRC-16
Table 596.
Command word 1
MSB
Command Token Word #1
LSB
Prefix
Command Code
Spare
Message Length
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
’0’
’1’
C5
C4
C3
C2
C1
C0
’1’
’1’
L5
L4
L3
L2
L1
L0
Table 597.
Command word 2
MSB
Command Token Word #1
LSB
Prefix
Sub-Address
Spare
CRC-4
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
’0’
’1’
SA7
SA6
SA5
SA4
SA3
SA2
SA1
SA0
’1’
’1’
C3
C2
C1
C0