Related Information
•
on page 19-12
•
Texas Instruments Synchronous Serial Protocol (SSP)
on page 19-13
•
National Semiconductor Microwire Protocol
on page 19-14
Serial Bit-Rate Clocks
SPI Master Bit-Rate Clock
The maximum frequency of the SPI master bit-rate clock (
sclk_out
) is one-half the frequency of SPI
master clock (
spi_m_clk
). This allows the shift control logic to capture data on one clock edge of
sclk_out
and propagate data on the opposite edge. The
sclk_out
line toggles only when an active
transfer is in progress. At all other times it is held in an inactive state, as defined by the serial protocol under
which it operates. †
Figure 19-3: Maximum sclk_out/spi_m_clk Ratio
MSB
Capture
Drive 1
Capture 1
Drive 2
Capture 2
Drive 3
Capture 3
spi_m_clk
sclk_out
txd/rxd
The frequency of
sclk_out
can be derived from the equation below , where <SPI clock> is
spi_m_clk
for the master SPI modules and
l4_main_clk
for the slave SPI modules. †
Fsclk_out = F
<SPI clock>
/ SCKDV
SCKDV is a bit field in the register
BAUDR
, holding any even value in the range 2 to 65,534. If SCKDV is 0,
then
sclk_out
is disabled. †
The following equation describes the frequency ratio restrictions between the bit-rate clock
sclk_out
and
the SPI master peripheral clock. The SPI master peripheral clock must be at least double the offchip master
clock. †
Table 19-1: SPI Master Peripheral Clock
•
SPI Master Peripheral Clock
Fspi_m_clk >= 2 x (maximum Fsclk_out) †
SPI Slave Bit-Rate Clock
The minimum frequency of
l4_main_clk
depends on the operation of the slave peripheral. If the slave
device is
receive only
, the minimum frequency of
l4_main_clk
is six times the maximum expected
frequency of the bit-rate clock from the master device (
sclk_in
). The
sclk_in
signal is double
SPI Controller
Altera Corporation
cv_54019
Serial Bit-Rate Clocks
19-4
2013.12.30