Registers

• Buffer registers
• Shift registers
• Controlled shift registers
• Three-state registers
• Buses

Buffer Register

• buffer -- can drive more loads than a regular TTL circuit can (already seen in project 2)
• register -- stores data
• a series (row) of D flip-flops, all clocked together
• on the rising edge of the clock the outputs, Q, store the inputs, X
• how can we control the loading? In other words, we wish to only load when an external signal tells us to load, and holding the value otherwise.

Controlled Register

• have a multiplexer (two ANDs and an OR) on each input
• if LOAD is high, load the D flip-flop from the outside input
• if LOAD is low, load the D flip-flop from its own output
• Simple-As-Possible computer (SAP-1): output register built from to 74LS173 circuits, drives LEDs (figure 8-3 p. 107)

Shift Registers

• feed the output of each D flip-flop into the input of the next flip flop to the left
• on each clock cycle, the word moves left
• for example if the rightmost input is 1 and the word in the register is 1110 0011, after one clock cycle the word in the register is 1100 0111
• right shift is similar
• arithmetic: multiplication, division by two
• serial adder: add one bit at a time from two shift registers, place result back into one of the shift registers (or another shift register)

Controlled shift registers

• one control line, SHL (or SHR)
• multiplexer on input to D flip-flop
• when SHL is high, load from the flip-flop to the right
• when SHL is low, load from our own output
• LSB (least significant bit) loads from the one-bit input
• figure 8-7 p. 109

In-Class Exercise

• work alone or with 1 other person
• 10 minutes
• design a shift register that will do all of the following:
  • if LOAD is high, loads from the inputs
  • if LOAD is low, and SHIFT is high, then:
    • if LEFT is low, shift right
    • if LEFT is high, shift left
  • if both LOAD and SHIFT are low, hold the data
• draw a circuit diagram of one bit slice of this shift register.

Three-state outputs

• The only way we've seen to combine outputs is through gates (e.g. multiplexers)
• it would be nice to be able to connect circuits (e.g., registers) together on a bus, and only one circuit would be enabled at a time: that circuit would write onto the bus, all others can read it
• a three-state output has three possible states: 0, 1, and "off" (high impedance, equivalent to high resistance)
• when a three-state output has high impedance, it neither sinks nor sources current.

Bus organization

• a single bus can connect all the inputs and all the outputs of all the registers
• when we want to read from a register, we raise the ENABLE line for that register to tell its three-state outputs to leave the high-impedance state
• we only ENABLE one register at a time, or we'd have multiple outputs driving the same line, leading to unpredictable (and possibly damaging) results
• when we want to write to a register, we raise its LOAD control line