SYNCHRONOUS SERIAL I/O (SSIO) PORT

8.5PROGRAMMING THE SSIO PORT

To use the SSIO port, you must configure the port pins to serve as special-function signals, then set up the SSIO channels.

8.5.1Configuring the SSIO Port Pins

Before you can use the SSIO port, you must configure the necessary port 6 pins to serve as their special-function signals. Handshaking mode requires that both the master and slave SCx pins be configured as open-drain outputs. (This configuration requires external pull-up resistors.) Table 8-1 on page 8-2 lists the pins associated with the SSIO port, and Table 8-2 lists the port configuration registers. See Chapter 6 for configuration details.

8.5.2Programming the Baud Rate and Enabling the Baud-rate Generator

The SSIO_BAUD register (Figure 8-5 on page 8-10) defines the baud rate and enables the baudrate generator. This register acts as a control register during write operations and as a downcounter monitor during read operations. The baud-rate generator provides an internal clock to the transceiver channels. The frequency ranges from FOSC/8 to FOSC/1024. With a 16-MHz oscillator frequency, this corresponds to a range from a maximum of 2.0 MHz to a minimum of 15.625 kHz. Table 8-3 lists SSIO_BAUD values for common baud rates.

Table 8-3. Common SSIO_BAUD Values at 16 MHz

Baud Rate	SSIO_BAUD Value†
(Maximum) 2.0 MHz	80H
100.0 kHz	93H
64.52 kHz	9DH
50.0 kHz	A7H
25.0 kHz	CFH
(Minimum) 15.625 kHz	FFH

† Bit 7 must be set to enable the baud-rate generator.

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8XC196Kx, Jx, CA USER’S MANUAL

SSIO_BAUD	Address:	1FB4H
	Reset State:	XXH

The synchronous serial port baud (SSIO_BAUD) register enables and disables the baud-rate generator and selects the SSIO baud rate. During read operations, SSIO_BAUD serves as the downcounter monitor. The down-counter is decremented once every four state times when the baud-rate generator is enabled.

7

0

BE

BV6

BV5

BV4

BV3

BV2

BV1

BV0

Bit

Bit

Function

Number

Mnemonic

7

BE

Baud-rate Generator Enable

This bit enables and disables the baud-rate generator.

For write operations:

0 = disable the baud-rate generator and clear BV6:0

1 = enable the baud-rate generator and start the down-counter

For read operations:

0 = baud-rate generator is disabled

1 = baud-rate generator is enabled and down-counter is running

6:0

BV6:0

Baud Value

For write operations:

These bits represent BAUD_VALUE, an unsigned integer that

determines the baud rate. The maximum value of BAUD_VALUE is 7FH;

the minimum value is 0. Use the following equation to determine

BAUD_VALUE for a given baud rate.

FOSC

BAUD_VALUE = ------------------------------------- – 1

Baud Rate × 8

For read operations:

These bits contain the current value of the down-counter.

Figure 8-5. Synchronous Serial Port Baud (SSIO_BAUD) Register

8.5.3Controlling the Communications Mode and Handshaking

The SSIOx_CON register (Figure 8-6) controls the communications mode and handshaking. The two least-significant bits indicate whether an underflow or overflow has occurred and whether the channel is ready to transmit or receive.

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SYNCHRONOUS SERIAL I/O (SSIO) PORT

SSIOx_CON	Address:	1FB1H, 1FB3H
x = 0–1	Reset State:	00H

The synchronous serial control x (SSIOx_CON) registers control the communications mode and handshaking. The two least-significant bits indicate whether an overflow or underflow has occurred and whether the channel is ready to transmit or receive.

7

0

M/S#

T/R#

TRT

THS

STE

ATR

OUF

TBS

Bit

Bit

Function

Number

Mnemonic

7†

M/S#

Master/Slave Select

Configures the channel as either master or slave.

0

= slave; SCx is an external clock input to SSIOx_BUF

1

= master; SCx is an output driven by the SSIO baud-rate generator

6†

T/R#

Transmit/Receive Select

Configures the channel as either transmitter or receiver.

0

= receiver; SDx is an input to SSIOx_BUF

1

= transmitter; SDx is an output driven by the output of SSIOx_BUF

5

TRT

Transmitter/Receiver Toggle

Controls whether receiver and transmitter switch roles at the end of each

transfer.

0

= do not switch

1

= switch; toggle T/R# and clear TRT at the end of the current transfer

Setting TRT allows the channel configuration to change immediately on

transfer completions, thus avoiding possible contention on the data line.

4

THS

Transceiver Handshake Select

Enables and disables handshaking. The THS, STE, and ATR bits must

be set for handshaking modes.

0

= disables handshaking

1

= enables handshaking

3

STE

Single Transfer Enable

Enables and disables transfer of a single byte. Unless ATR is set, STE is

automatically cleared at the end of a transfer. The THS, STE, and ATR

bits must be set for handshaking modes.

0

= disable transfers

1

= allow transmission or reception of a single byte

† The M/S# and T/R# bits specify four possible configurations: master transmitter, master receiver, slave transmitter, or slave receiver.

Figure 8-6. Synchronous Serial Control x (SSIOx_CON) Registers

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8XC196Kx, Jx, CA USER’S MANUAL

SSIOx_CON (Continued)	Address:	1FB1H, 1FB3H
x = 0–1	Reset State:	00H

The synchronous serial control x (SSIOx_CON) registers control the communications mode and handshaking. The two least-significant bits indicate whether an overflow or underflow has occurred and whether the channel is ready to transmit or receive.

7

0

M/S#

T/R#

TRT

THS

STE

ATR

OUF

TBS

Bit

Bit

Function

Number

Mnemonic

2

ATR

Automatic Transfer Re-enable

Enables and disables subsequent transfers. The THS, STE, and ATR bits

must be set for handshaking modes.

0

= allow automatic clearing of STE; disable subsequent transfers

1

= prevent automatic clearing of STE; allow transfer of next byte

1

OUF

Overflow/Underflow Flag

Indicates whether an overflow or underflow has occurred. An attempt to

access SSIOx_BUF during a byte transfer sets this bit.

For the master (M/S# = 1)

0

=

no overflow or underflow has occurred

1

= the core attempted to access SSIOx_BUF during the current transfer

For the slave (M/S# = 0)

0

= no overflow or underflow has occurred

1

=

the core attempted to access SSIOx_BUF during the current transfer

or the master attempted to clock data into or out of the slave’s

SSIOx_BUF before the buffer was available

0

TBS

Transceiver Buffer Status

Indicates the status of the channel’s SSIOx_BUF.

For the transmitter (T/R# =1)

0

=

SSIOx_BUF is full; waiting to transmit

1

= SSIOx_BUF is empty; buffer available

For the receiver (T/R# = 0)

0

= SSIOx_BUF is empty; waiting to receive

1

=

SSIOx_BUF is full; data available

† The M/S# and T/R# bits specify four possible configurations: master transmitter, master receiver, slave transmitter, or slave receiver.

Figure 8-6. Synchronous Serial Control x (SSIOx_CON) Registers (Continued)

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