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29.4.12.2 Transfer size and port size translation
With bursting disabled, any transfer larger than the port size breaks into multiple
individual transfers (e.g. <Addr><Data><Addr+1><Data><Addr+2><Data>). With
bursting enabled, any transfer larger than the port size results in a burst cycle of multiple
beats (e.g. <Addr><Data><Data><Data>). The following table shows the result of such
transfer translations.
Port size PS[1:0] Transfer size FB_TSIZ[1:0]
Burst-inhibited: Number of transfers
Burst enabled: Number of beats
01b (8 bit) 10b (16 bits) 2
00b (32 bits) 4
11b (16 bytes) 16
1Xb (16 bit) 00b (32 bits) 2
11b (16 bytes) 8
00b (32 bit) 11b (line) 4
The FlexBus can support X-1-1-1 burst cycles to maximize system performance, where X
is the primary number of wait states (max 63). Delaying termination of the cycle can add
wait states. If internal termination is used, different wait state counters can be used for the
first access and the following beats.
29.4.12.3 32-bit-Read burst from 8-Bit port 2-1-1-1 (no wait states)
The following figure shows a 32-bit read to an 8-bit external chip programmed for burst
enable. The transfer results in a 4-beat burst and the data is driven on FB_AD[31:24].
The transfer size is driven at 32-bit (00b) throughout the bus cycle.
Note
In non-multiplexed address/data mode, the address on FB_A
increments only during internally-terminated burst cycles. The
first address is driven throughout the entire burst for externally-
terminated cycles.
In multiplexed address/data mode, the address is driven on
FB_AD only during the first cycle for all terminated cycles.
Chapter 29 External Bus Interface (FlexBus)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
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