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2014-07-02 - Tiny Gecko Family - d0034_Rev1.20
199
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whether the transmitted word is for the left or right audio channel; A word transmitted while the word
clock is low is for the left channel, and a word transmitted while the word clock is high is for the right.
When operating in I2S mode, the CS pin is used as a the word clock. In master mode, this is automatically
driven by the USART, and in slave mode, the word clock is expected from an external master.
15.3.3.6.1 Word Format
The general I2S word format is 32 bits wide, but the USART also supports 16-bit and 8-bit words. In
addition to this, it can be specified how many bits of the word should actually be used by the USART.
These parameters are given by FORMAT in USARTn_I2SCTRL.
As an example, configuring FORMAT to using a 32-bit word with 16-bit data will make each word on the
I2S bus 32-bits wide, but when receiving data through the USART, only the 16 most significant bits of
each word can be read out of the USART. Similarly, only the 16 most significant bits have to be written
to the USART when transmitting. The rest of the bits will be transmitted as zeroes.
15.3.3.6.2 Major Modes
The USART supports a set of different I2S formats as shown in Table 15.9 (p. 199) , but it is not limited
to these modes. MONO, JUSTIFY and DELAY in USARTn_I2SCTRL can be mixed and matched to
create an appropriate format. MONO enables mono mode, i.e. one data stream instead of two which is
the default. JUSTIFY aligns data within a word on the I2S bus, either left or right which can bee seen in
figures Figure 15.18 (p. 200) and Figure 15.19 (p. 200) . Finally, DELAY specifies whether a new I2S
word should be started directly on the edge of the word-select signal, or one bit-period after the edge.
Table 15.9. USART I2S Modes
Mode
MONO
JUSTIFY
DELAY
CLKPOL
Regular I2S
0
0
1
0
Left-Justified
0
0
0
1
Right-Justified
0
1
0
1
Mono
1
0
0
0
The regular I2S waveform is shown in Figure 15.16 (p. 199) and Figure 15.17 (p. 200) . The first
figure shows a waveform transmitted with full accuracy. The wordlength can be configured to 32-bit,
16-bit or 8-bit using FORMAT in USARTn_I2SCTRL. In the second figure, I2S data is transmitted with
reduced accuracy, i.e. the data transmitted has less bits than what is possible in the bus format.
Note that the msb of a word transmitted in regular I2S mode is delayed by one cycle with respect to
word select
Figure 15.16. USART Standard I2S waveform
USn_CLK
USn_CS
(word select )
USn_TX/
USn_RX
MSB
Left channel
Right channel
Right channel
LSB
MSB
LSB
Summary of Contents for EFM32TG
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