Rainbow Electronics ATmega8515L - Manual (Page 128)

128
ATmega8515(L)
2512A–AVR–04/02
• Bit 4 – MSTR: Master/Slave Select
This bit selects Master SPI mode when written to one, and Slave SPI mode when written
logic zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will
be cleared, and SPIF in SPSR will become set. The user will then have to set MSTR to
re-enable SPI Master mode.
• Bit 3 – CPOL: Clock Polarity
When this bit is written to one, SCK is high when idle. When CPOL is written to zero,
SCK is low when idle. Refer to Figure 61 and Figure 62 for an example. The CPOL func-
tionality is summarized below:
• Bit 2 – CPHA: Clock Phase
The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading
(first) or trailing (last) edge of SCK. Refer to Figure 61 and Figure 62 for an example.
The CPOL functionality is summarized below:
• Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0
These two bits control the SCK rate of the device configured as a master. SPR1 and
SPR0 have no effect on the Slave. The relationship between SCK and the Oscillator
Clock frequency f
osc
is shown in the following table:
Table 56. CPOL Functionality
CPOL Leading Edge Trailing Edge
0 Rising Falling
1 Falling Rising
Table 57. CPHA Functionality
CPHA Leading Edge Trailing Edge
0 Sample Setup
1 Setup Sample
Table 58. Relationship Between SCK and the Oscillator Frequency
SPI2X SPR1 SPR0 SCK Frequency
0 0 0 f
osc
/ 4
0 0 1 f
osc
/ 16
0 1 0 f
osc
/ 64
0 1 1 f
osc
/ 128
1 0 0 f
osc
/ 2
1 0 1 f
osc
/ 8
1 1 0 f
osc
/ 32
1 1 1 f
osc
/ 64