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Two SMCS332SpW can work on a common memory without any external arbitration logic. However, if more than two
SMCS332SpWs share a common communication memory an external arbiter is required.
The following describes the arbitration between two SMCS332SpWs. The rising edge of the RESET* signal loads the level
of the CAM-signal into the SMCS332SpW. The level of the CAM-signal determines which SMCS332SpW owns the COMI
BUS (is master and the other is slave).
The arbitration between the two SMCS332SpW follows a fair toggle and parking scheme. This means if SMCS332SpW -A
got the communication memory bus after arbitration (or after reset) it "parks" on it until SMCS332SpW -B sends a request
over his COCO output (active high) to the COCI input of SMCS332SpW -A. SMCS332SpW -B however sends only a
request signal to SMCS332SpW -A if it needs the COMI bus for memory transfers.
After completion of the current memory transfer from SMCS332SpW -A, the SMCS332SpW -A COM interface becomes
tristate and the SMCS332SpW -A COCO signal goes inactive (low). After SMCS332SpW -B receives the low level on its
COCI, it occupies the COMI bus and enables its COM interface (drives the signals).
Now, SMCS332SpW -B "parks" on the bus until there is a request from SMCS332SpW -A.
If a bus owner releases the bus after a request from the other SMCS332SpW, it has to wait for N-1 Cycles (N = value from
Register COMI_ACR), before it can request the bus again.
With the according values in the COMI_ACR register the bandwidth of the communication memory data bus can be split
between the two SMCS332SpWs.
Example:
COMI_ACR SMCS332SpW -A = 4
COMI_ACR SMCS332SpW -B = 8
both SMCS332SpW read data from the communication memory
datawidth = 32 bit
CLK = 25 MHz => cycletime = 40ns
overall bandwidth:
(4-1) + 1 arbitrationcycle + (8-1) + 1 arbitrationcycle = 12 cycles
readcycles: (4-1) + (8-1) = 10 cycles
bandwidth 25 MHz * 10 / 12 * 4 Byte/cycle => 83.3 Mbyte/s
bandwidth SMCS332SpW -A:
(8-1) = 7 readcycles
25 MHz * 7/12 * 4 Bytes/cycle = 58.33 Mbyte/s
bandwidth SMCS332SpW -B:
SMCS332SpW
User Manual
– All Rights Reserved – Copyright per DIN 34 –
EADS Astrium GmbH, ASE2
Doc No: SMCS_ASTD_UM_100
Issue:
1.4
Updated: 9-Sep-2006
Page:
46 of 131
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