EM6621
6.6.5 Detailed Functional Description
Master or Slave mode is selected in the control register
RegSCntl1.
In Slave mode, the serial clock comes from an external device and is input via the PSP[3] terminal as a
synchronous clock (SCLKIn) to the serial interface. The serial clock is ignored as long as the
Start
bit is not set.
After setting
Start,
only the eight following active edges of the serial clock input PSP[3] are used to shift the
serial data in and out. After eight serial clock edges the
Start
bit is reset. The PSP[1] terminal is a copy of the
(Start
OR Status)
bit values, it can be used to indicate to the external master, that the interface is ready to
operate or it can be used as a chip select signal in case of an external slave.
In Master mode, the synchronous serial clock is generated internally from the system clock. The frequency is
selected from one out of three sources (
MS0
and
MS1
bits in
RegSCntl1) .
The serial shifting clock is only
generated during
Start
= high and is output to the SCLK terminal as the Master Clock (SCLKOut). When
Start
is low, the serial clock output on PSP[3] is 0.
An interrupt request
IRQSerial
is generated after the eight shift operations are done. This signal is set by the
last negative edge of the serial interface clock on PSP[3] (master or slave mode) and is reset to 0 by the next
write of
Start
or by any reset. This interrupt can be masked with register
RegIRQMask3.
For more details
about the interrupt handling see chapter 10.
Serial data input on PSP[0] is sampled by the positive or negative serial shifting clock edge, as selected by the
Control Register
POSnNeg
bit. Serial data input is shifted in LSB first or MSB first, as selected by the Control
Register
MSBnLSB
bit.
6.6.6 Output Modes
Serial data output is given out in two different ways (Refer also to
-
OM[1]
= 1,
OM[0]
= 0 :
The serial output data is generated with the
selected shift register clock (POSnNeg). The
first data bit is available directly after the
Start
bit is set.
Figure 16 and Figure 17).
Figure 16. Direct or Re-Synchronized Output
SIN bit0 bit1 bit2 bit3 bit4 bit5 bit6 bit7
M
U
bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
X
+ve/-ve Edge
MSBnLSB
SOUT
Direct
Shift Out
-OM[1] = 0,
OM[0]
= 1 :
The serial output data is re-synchronized by
the positive serial interface clock edge,
independent of the selected clock shifting
edge. The first data bit is available on the first
positive serial interface clock edge after
Start=‘1’.
SIN bit0 bit1 bit2 bit3 bit4 bit5 bit6 bit7
M
bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0
U
X
+ve/-ve Edge
+ve Edge clock
SOUT
bit[n
Re-synchronised
shift out
Table 6.6.6 Output Mode Selection in RegSCntl2
OM[1]
0
0
1
1
OM[0]
0
1
0
1
Output mode
Tristate
Serial-
Synchronized
Serial-Direct
Parallel
Description
Output disable (tristate on PSP[3:0])
Re-synchronized positive edge data shift out
Direct shift pos. or neg. edge data out
Parallel port SP output
Tristate output is selected by default.
03/02 REV. D/444
Copyright
2002, EM Microelectronic-Marin SA
23
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EMMICRO [ EM MICROELECTRONIC - MARIN SA ]
