TTL voltage profile; TTL-to-CMOS interface

Voltage profile of TTL

The voltage profile of TTL ($V_{CC} = 5$ V) shows the voltage ranges that represent logic 0 and logic 1 at the output and at the input of a gate:

  Output (driving)       Input (receiving)
 5.0 +--------+         5.0 +--------+
     | HIGH   |             | HIGH   |
 2.4 +--------+ VOH(min) 2.0 +--------+ VIH(min)
     |invalid |  NM_H=0.4   | indet. |
 0.4 +--------+ VOL(max) 0.8 +--------+ VIL(max)
     | LOW    |  NM_L=0.4   | LOW    |
 0.0 +--------+         0.0 +--------+

• Output HIGH ≥ $V_{OH(min)} = 2.4$ V; output LOW ≤ $V_{OL(max)} = 0.4$ V.
• Input read as HIGH if ≥ $V_{IH(min)} = 2.0$ V; as LOW if ≤ $V_{IL(max)} = 0.8$ V; 0.8–2.0 V is indeterminate.
• Noise margins: $NM_H = 2.4 - 2.0 = 0.4$ V and $NM_L = 0.8 - 0.4 = 0.4$ V.

TTL to CMOS interface

When a TTL output drives a CMOS input, the LOW levels match but the HIGH levels do not:

A TTL HIGH of 2.4–3.4 V may fall in CMOS's indeterminate region. Current is not a problem, since a CMOS input draws almost no current. Methods to interface:

1. Pull-up resistor (same 5 V supply): a resistor $R_P$ (about 1–10 kΩ) is connected from the TTL output to +5 V. When the TTL output goes HIGH, Q3 of the totem pole turns off as the voltage rises and $R_P$ pulls the line up to nearly 5 V, which is a valid CMOS HIGH. When the TTL output is LOW, Q4 sinks the current $5/R_P$ (e.g. 1 mA for 4.7 kΩ), well within its 16 mA rating.

            +5 V
             |
             RP (≈ 4.7k)
             |
 TTL gate ---+----> CMOS input
   (74xx)            (74HC / 4000)

2. CMOS supply higher than 5 V (e.g. $V_{DD} = 10$–15 V): use an open-collector TTL gate (e.g. 7406/7407, rated for high voltage) with the pull-up resistor connected to $V_{DD}$, or a level-shifter IC (e.g. 4504). The output transistor switches between about 0 V and $V_{DD}$.

3. TTL-compatible CMOS: use the 74HCT/74ACT series, whose input thresholds are TTL-like ($V_{IH} = 2.0$ V, $V_{IL} = 0.8$ V), so the TTL output can be connected directly.