GR740-UM-DS, Nov 2017, Version 1.7
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GR740
16.6
Bus Monitor Operation
16.6.1 Overview
The Bus Monitor (BM) can be enabled by itself, or in parallel to the BC or RT. The BM acts as a pas-
sive logging device, writing received data with time stamps to a ring buffer.
Transfers can be filtered per RT address and per subaddress or mode code, and the filter conditions are
logically AND:ed. If all bits of the three filter registers and bits 2-3 of the control register are set to
’1’, the BM core will log all words that are received on the bus.
In order to filter on subaddress/mode code, the BM has logic to track 1553 words belonging to the
same message. All 10 message types are supported. If an unexpected word appears, the filter logic
will restart. Data words not appearing to belong to any message can be logged by setting a bit in the
control register.
The filter logic can be manually restarted by setting the BM enable bit low and then back to high. This
feature is mainly to improve testability of the BM itself.
16.6.2 No-response handling
Due to the nature of the MIL-STD-1553B protocol, ambiguity can arise when the subaddress or mode
code filters are used, an RT is not responding on a subaddress, and the BC then commands the same
RT again on subaddress 8 or mode code indicator 0 on the same bus. This can lead to the second com-
mand word being interpreted as a status word and filtered out.
The BM can use the instrumentation bit and reserved bits to disambiguate, which means that this case
will never occur when subaddresses 1-7, 16-30 and mode code indicator 31 are used. Also, this case
does not occur if only the RT address filter is used.
16.6.3 Log entry format
Each log entry is two 32-bit words.
16.7
Clocking and reset
The core operates in two clock domains, the AMBA clock domain and the 1553 codec clock domain,
with synchronization and handshaking between the domains. For the codec clock domain, a 20 MHz
clock must be supplied. The AMBA clock can be at any frequency but must be at a minimum of 10
Table 301.
GR1553B BM Log entry word 0 (offset 0x00)
31
30
24
23
0
1
Reserved
TIME
31
Always written as 1
30 : 24
Reserved - Mask out on read for forward compatibility
23 : 0
Time tag (TIME)
Table 302.
GR1553B BM Log entry word 1 (offset 0x04)
31
30
20
19
18
17
16
15
0
0
Reserved
BUS
WST
WTP
WD
31
Always written as 0
30 : 20
Reserved - Mask out on read for forward compatibility
19
Receive data bus (BUS) - 0:A, 1:B
18 : 17
Word status (WST) - 00=word OK, 01=Manchester error, 10=Parity error
16
Word type (WTP) - 0:Data, 1:Command/status
15 : 0
Word data (WD)