Intel 8XC196MD, 8XC196MH, 8XC196MC manual Synchronous Modes Modes 0

SERIAL I/O (SIO) PORT

7.3.1Synchronous Modes (Modes 0 and 4)

The 8XC196MH serial port has two synchronous modes, mode 0 and mode 4. Mode 0 is the syn- chronous mode available on all the 8XC196 devices that have serial ports. Mode 4 is an enhanced, full-duplex synchronous mode.

7.3.1.1Mode 0

The most common use of mode 0 is to expand the I/O capability of the device with shift registers (see Figure 7-2). In this mode, the TXDx pin outputs a set of eight clock pulses, while the RXDx pin either transmits or receives data. Data is transferred eight bits at a time with the least-signif- icant bit first. Figure 7-3 shows a diagram of the relative timing of these signals. Note that only mode 0 uses RXDx as an open-drain output.

Clock Inhibit			Shift / LOAD#
			Px.x
Serial In			VCC
			74HC05
			15kΩ
			Data
	Shift Register	Q#	RXD
			Clock
	74HC165
			TXD
	Inputs		Microcontroller
VCC	Outputs
Serial
In B			Serial In A
	Shift Register		Clock
Clear	74HC164
			Enable#
			Px.x
			A0264-03

Figure 7-2. Typical Shift Register Circuit for Mode 0

In mode 0, RXDx must be enabled for receptions and disabled for transmissions. (See “Program- ming the Control Register” on page 7-10.) When RXD x is enabled, either a rising edge on the RXDx input or clearing the receive interrupt (RI) flag in SPx_STATUS starts a reception. When RXDx is disabled, writing to SBUFx_TX starts a transmission.

Disabling RXDx stops a reception in progress and inhibits further receptions. To avoid a partial or undesired complete reception, disable RXDx before clearing the RI flag in SPx_STATUS. This can be handled in an interrupt environment by using software flags or in straight-line code by us- ing the interrupt pending register to signal the completion of a reception.

7-5