
Feature Property
Channel Not supported.
Error Not used.
Packet Not supported.
For more information about Avalon-ST interfaces, refer to the Avalon Interface Specifications.
Operation
The Avalon-ST SPI core waits for the nSS signal to be asserted low, signifying that the SPI master is
initiating a transaction. The core then starts shifting in bits from the input signal mosi. The core packs the
bits received on the SPI to bytes and checks for the following special characters:
• 0x4a—Idle character. The core drops the idle character.
• 0x4d—Escape character. The core drops the escape character, and XORs the following byte with 0x20.
For each valid byte of data received, the core asserts the valid signal on its Avalon-ST source interface
and presents the byte on the interface for a clock cycle.
At the same time, the core shifts data out from the Avalon-ST sink to the output signal miso beginning
with from the most significant bit. If there is no data to shift out, the core shifts out idle characters
(0x4a). If the data is a special character, the core inserts an escape character (0x4d) and XORs the data
with 0x20.
The data shifts into and out of the core in the direction of MSB first.
Figure 13-2: SPI Transfer Protocol
sclk
(CPOL = 0)
Sample I
MOSI/MISO
Change O
MISO pin
Change O
MOSI pin
nSS
TL
TT
TI
TL
SPI Transfer Protocol Notes:
• TL = The worst recovery time of sclk with respect with nSS.
• TT = The worst hold time for MOSI and MISO data.
• TI = The minimum width of a reset pulse required by Altera FPGA families.
Timing
The core requires a lead time (TL) between asserting the nSS signal and the SPI clock, and a lag time (TT)
between the last edge of the SPI clock and deasserting the nSS signal. The nSS signal must be deasserted
for a minimum idling time (TI) of one SPI clock between byte transfers. A TimeQuest SDC file (.sdc) is
13-2
Operation
UG-01085
2014.24.07
Altera Corporation
Avalon-ST Serial Peripheral Interface Core
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