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T48C862-R8 Datasheet

  • T48C862-R8

  • MCU

  • Atmel   Atmel

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General SSI Operation
The SSI is comprised essentially of an 8-bit shift register with two associated 8-bit buff-
ers 鈥?the receive buffer (SRB) for capturing the incoming serial data and a transmit
buffer (STB) for intermediate storage of data to be serially output. Both buffers are
directly accessable by software. Transferring the parallel buffer data into and out of the
shift register is controlled automatically by the SSI control, so that both single byte trans-
fers or continuous bit streams can be supported.
The SSI can generate the shift clock (SC) either from one of several on-chip clock
sources or accept an external clock. The external shift clock is output on, or applied to
the Port BP40. Selection of an external clock source is performed by the Serial Clock
Direction control bit (SCD). In the combinational modes, the required clock is selected
by the corresponding timer mode.
The SSI can operate in three data transfer modes 鈥?synchronous 8-bit shift mode, MCL
compatible 9-bit shift modes or 8-bit pseudo MCL protocol (without acknowledge-bit).
External SSI clocking is not supported in these modes. The SSI should thus generate
and has full control over the shift clock so that it can always be regarded as an MCL Bus
Master device.
All directional control of the external data port used by the SSI is handled automatically
and is dependent on the transmission direction set by the Serial Data Direction (SDD)
control bit. This control bit defines whether the SSI is currently operating in Transmit
(TX) mode or Receive (RX) mode.
Serial data is organized in 8-bit telegrams which are shifted with the most significant bit
first. In the 9-bit MCL mode, an additional acknowledge bit is appended to the end of the
telegram for handshaking purposes (see MCL protocol).
At the beginning of every telegram, the SSI control loads the transmit buffer into the shift
register and proceeds immediately to shift data serially out. At the same time, incoming
data is shifted into the shift register input. This incoming data is automatically loaded
into the receive buffer when the complete telegram has been received. Thus, data can
be simultaneously received and transmitted if required.
Before data can be transferred, the SSI must first be activated. This is performed by
means of the SSI reset control (SIR) bit. All further operation then depends on the data
directional mode (TX/RX) and the present status of the SSI buffer registers shown by
the Serial Interface Ready Status Flag (SRDY). This SRDY flag indicates the
(empty/full) status of either the transmit buffer (in TX mode), or the receive buffer (in RX
mode). The control logic ensures that data shifting is temporarily halted at any time, if
the appropriate receive/transmit buffer is not ready (SRDY = 0). The SRDY status will
then automatically be set back to 鈥?鈥?and data shifting resumed as soon as the applica-
tion software loads the new data into the transmit register (in TX mode) or frees the shift
register by reading it into the receive buffer (in RX mode).
A further activity status (ACT) bit indicates the present status of the serial communica-
tion. The ACT bit remains high for the duration of the serial telegram or if MCL stop or
start conditions are currently being generated. Both the current SRDY and ACT status
can be read in the SSI status register. To deactivate the SSI, the SIR bit must be set
high.
68
T48C862-R8
4590B鈥?BMCU鈥?2/03

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