T48C862x-Rf-TNQ datasheet(68/107 Pages) ATMEL

68 / 107 page

T48C862-R4 [Preliminary]

4551C–4BMCU–01/04

Figure 64.

Block Diagram of the Synchronous Serial Interface

General SSI Operation

The SSI is comprised essentially of an 8-bit shift register with two associated 8-bit buff-

ers – 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 – synchronous 8-bit shift mode, a

9-bit Multi-Chip Link Mode (MCL) ,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 Bus Protocol” on page 72).

At the beginning of every telegram, the SSI control loads the transmit buffer into the shift

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.

8-bit Shift Register

MSB

LSB

Shift_CL

SIC1

SIC2

SISC

Control

STB

SRB

Timer 2 / Timer 3

Output

INT3

I/O-bus

SSI-Control

TOG2

POUT

T1OUT

SYSCL

MCL_SC

MCL_SD

Transmit

Buffer

Receive

Buffer

SCI

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