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8XC196NT USER'S MANUAL
SC x
STE
MSB
SD x (out)
SD x (in)
SC x
(Handshake Mode)
8.4
SSIO HANDSHAKING
Handshaking (Figure 8-4) prevents a data underflow or overflow from occurring at the slave,
which enables a master device to perform SSIO data transfers using the PTS. Without handshak-
ing, data overflows and underflows would make it nearly impossible to use the PTS for transfer-
ring blocks of data. Handshaking takes place in hardware, using the clock pins, with no CPU
overhead. When the master is the transmitter and the slave is the receiver, the slave pulls the clock
line low until it is ready to receive a byte. This prevents a data overflow at the slave. In the oppo-
site configuration, the slave pulls the clock line low until its buffer is loaded with data. This pre-
vents a data underflow at the slave.
8.4.1
SSIO Handshaking Configuration
To use the PTS with the SSIO in handshaking mode, the SSIO channels must be configured as
follows:
•
Channels must be auto-enabled (both the ATR and STE bits in SSIOx_CON must be set).
•
Handshaking mode must be selected (the THS bit in SSIOx_CON must be set).
•
The clock pin, SCx, must be configured as a special-function, open-drain output in both
master and slave. (This requires an external pull-up resistor.)
8-6
1
2
3
D6
D5
valid
valid
valid
1
2
3
Figure 8-3. SSIO Transmit/Receive Timings
4
5
6
D4
D3
D2
valid
valid
valid
4
5
6
Slave Receiver Pulls SC x low
7
8
D1
D0
valid
valid
7
8
A0266-01
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