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PEB 20321
PEF 20321
Synchronous Serial Control (SSC) Interface
Data Sheet
182
2001-02-14
transferred to the shift register and the next transmission will start without any additional
delay. On the data line there is no gap between the two successive frames. For example,
two byte transfers would look the same as one 16-bit word transfer. This feature can be
used to interface with devices which can operate with or require more than 16 data bits
per transfer. The length of a total data frame is up to the software. This option can also
be used to interface to byte-wide and word-wide devices on the same serial bus.
Note: This feature only applies to multiples of the selected basic data width, since it
would require disabling/enabling of the SSC to re-program the basic data width
on-the-fly.
Port Control
The SSC uses three pins of the General Purpose Bus pins to communicate with the
external world. Pin GP15/MCLK serves as the clock line, while pins GP13/MRST (Master
Receive/Slave Transmit) and GP14/MTSR (Master Transmit/Slave Receive) serve as
the serial data I/O lines. The operation of these pins depends on the selected operating
mode (master or slave).
In order to enable the alternate output functions (in this case SSC functions) of these pins
instead of the general purpose I/O operation, the respective port latches of the General
Purpose Bus registers (refer to
Chapter 11.2.1
) have to be set to
‘
1
’
, since the port latch
outputs and the alternate output lines are ANDed. When an alternate data output line is
not used (function disabled), it is held at a high level, allowing I/O operations via the port
latch.
The direction of the port lines depends on the operating mode. The SSC will
automatically use the correct alternate input or output line of the ports when switching
the modes. The direction of the pins, however, must be programmed by the user, as
shown in
Table 17
. Using the open drain output feature helps to avoid bus contention
problems and reduces the need for hardwired hand-shaking or slave select lines. In this
case it is not always necessary to switch the direction of a port pin. The table below
summarizes the required values for the different modes and pins.
Table 17
Pin
Port Control of the SSC Interface
Master Mode
Function
SSC Clock Output
SSC Data Output
SSC Data Input
Port Latch
GPDATA.15 =
‘
1
’
GPDIR.15 =
‘
1
’
GPDATA.14 =
‘
1
’
GPDIR.14 =
‘
1
’
GPDATA.13 =
‘
x
’
GPDIR.13 =
‘
0
’
Slave Mode
GPDATA.15 =
‘
x
’
GPDIR.15 =
‘
0
’
Direction
GP15/MCLK
GP14/MTSR
GP13/MRST
GP15/MCLK
SSC Clock Input