Functional Description
24.3.2.9 Clock Ratio Granularity
By default the clock division ratio is defined by the register MCSPI_CH(I)CONF[CLKD] with power of two
granularity leading to a clock division in range 1 to 32768, in this case the duty cycle is always 50%. With
bit MCSPI_CH(I)CONF[CLKG] the clock division granularity can be changed to one clock cycle, in that
case the register MCSPI_CH(I)CTRL[EXTCLK] is concatenated with MCSPI_CH(I)CONF[CLKD] to give a
12-bit width division ratio in range 1 to 4096.
When granularity is one clock cycle (MCSPI_CH(I)CONF[CLKG] set to 1), for odd value of clock ratio the
clock duty cycle is kept to 50-50 using falling edge of clock reference CLKSPIREF.
Table 24-7. CLKSPIO High/Low Time Computation
Clock Ratio F
ratio
CLKSPIO High Time
CLKSPIO Low Time
1
T
high
_ref
T
low
_ref
Even
≥
2
t_ref × (F
ratio
/2)
t_ref × (F
ratio
/2)
Odd
≥
3
t_ref × (F
ratio
/2)
t_ref × (F
ratio
/2)
T_ref = CLKSPIREF period in ns. T
high
_ref = CLKSPIREF high Time period in ns. T
low
_ref = CLKSPIREF
low Time period in ns. F
ratio
= SPI clock division ratio
F
ratio
= MCSPI_CH(I)CTRL[EXTCLK].MCSPI_CH(I)CONF[CLKD] + 1
For odd ratio value the duty cycle is calculated as below:
Duty_cycle = ½
Granularity examples: With a clock source frequency of 48 MHz:
Table 24-8. Clock Granularity Examples
MCSPI_CH
MCSPI_CH
MCSPI_CH
MCSPI_CH
MCSPI_CH
(I)CTRL
(I)CONF
(I)CONF
(I)CONF
(I)CONF
Thigh
Tlow
Tperiod
Duty
Fout
EXTCLK
CLKD
CLKG
F
ratio
PHA
POL
(ns)
(ns)
(ns)
Cycle
(MHz)
X
0
0
1
X
X
10.4
10.4
20.8
50-50
48
X
1
0
2
X
X
20.8
20.8
41.6
50-50
24
X
2
0
4
X
X
41.6
41.6
83.2
50-50
12
X
3
0
8
X
X
83.2
83.2
166.4
50-50
6
0
0
1
1
X
X
10.4
10.4
20.8
50-50
48
0
1
1
2
X
X
20.8
20.8
41.6
50-50
24
0
2
1
3
1
0
31,2
31,2
62.4
50-50
16
0
2
1
3
1
1
31,2
31,2
62.4
50-50
16
0
3
1
4
X
X
41.6
41.6
83.2
50-50
12
5
0
1
81
1
0
842,4
842,4
1684.8
50-50
0.592
5
7
1
88
X
X
915.2
915.2
1830.4
50-50
0.545
4013
SPRUH73H – October 2011 – Revised April 2013
Multichannel Serial Port Interface (McSPI)
Copyright © 2011–2013, Texas Instruments Incorporated