M74HC4851YTTR channel selection starts with INH, followed by the address inputs in C, B, A order. With INH low, the address selects one of eight analog channels to connect to the common terminal. With INH high, none is selected. This explains the static channel choice; it does not establish signal accuracy, switching performance or suitability for a particular circuit.
Start with the exact ordering code
ST's M74HC4851 datasheet, Doc ID 8640 Rev 7, lists M74HC4851YTTR as the TSSOP16 automotive-grade, tape-and-reel option. Its operating temperature range is −40 to +125 °C. Use that complete code when connecting a BOM entry, quotation and technical review. A shared family name does not identify every ordering option.
The package-body marking is a separate identification field. If a shortened marking is all that is available, the guide to body markings and full part numbers explains why the missing ordering characters should remain unconfirmed. The channel-selection table helps interpret an identified device; it cannot authenticate a physical lot.
Separate channel numbers from physical pins
The common terminal is named COM OUT/IN, reflecting the multiplexer/demultiplexer function. Do not read the numbered channels as sequential package pins. ST's pin-description table gives this mapping:
Function | Physical pin or pins |
|---|---|
Common terminal, COM OUT/IN | 3 |
Inhibit, INH | 6 |
Address A, B, C | 11, 10, 9 respectively |
Channels 0, 1, 2, 3 | 13, 14, 15, 12 respectively |
Channels 4, 5, 6, 7 | 1, 5, 2, 4 respectively |
Ground, GND | 8 |
Positive supply, VCC | 16 |
Not connected, NC | 7 |
For a drawing review, keep three labels distinct: the board's signal name, the device's channel number and the physical pin. An address can identify the intended channel correctly while a drawing still points to the wrong package pin. Pin 7 is identified as NC here; do not interpret it as an additional supply terminal from familiarity with another multiplexer.
Read INH before decoding C, B and A
The truth table uses L for a low logic state, H for a high state and X for a state that does not affect that row's selection result.
INH | C | B | A | Selected channel |
|---|---|---|---|---|
L | L | L | L | 0 |
L | L | L | H | 1 |
L | L | H | L | 2 |
L | L | H | H | 3 |
L | H | L | L | 4 |
L | H | L | H | 5 |
L | H | H | L | 6 |
L | H | H | H | 7 |
H | X | X | X | None |
A is the least significant address bit in this table. As a datasheet-reading example, INH=L and CBA=001 selects channel 1, whose physical pin is 14, connecting it through the selected switch to common pin 3. Reversing the address-bit order changes the interpretation: CBA=100 selects channel 4, on pin 1.
The inhibited row overrides the address selection. It does not say that COM becomes a driven logic-low output. Likewise, X does not authorize leaving control inputs electrically undefined: it describes the selection result while INH is high.
Check what L and H mean at the supply voltage
A truth table assumes valid logic levels. It does not establish whether a particular controller's output meets them. Before treating two circuits as compatible, compare their specified electrical limits under the relevant supply, temperature and loading conditions.
For this device, Table 5 lists a recommended VCC range of 2 to 6 V, with control-input and analog I/O voltages from 0 to VCC. Table 5 also specifies 0 to 1.2 V for the static or dynamic voltage across the switch; its footnote explains that a larger drop across an on switch may draw excessive VCC current. Table 6 supplies the control-input thresholds; use those limits rather than interpreting L and H as arbitrary voltages.
Keep recommended operation separate from absolute maximum ratings. The latter identify stress limits, not a promise of normal operation at those limits. The injection-current-protection feature also needs its stated conditions; it is not blanket permission to apply any voltage to a channel.
Keep selection separate from analog performance
Selecting the right channel answers which switch is on. It does not answer how closely a signal will match the source after passing through the complete circuit.
The datasheet treats on-resistance, leakage and switching behavior separately from the truth table. Those specifications have test conditions. A reviewer must consider the signal range, source and load, control timing and required settling behavior before deciding whether the device meets an application. Do not turn a static selection example into a verified circuit or a replacement recommendation.
For procurement handover, retain the complete MPN, datasheet revision, intended channel, corresponding pin and any unresolved electrical questions. Keep a seller's identification records separate from the buyer's engineering approval. Neither the automotive-grade designation nor a correct truth-table reading constitutes approval of a finished vehicle system.
Frequently asked questions
Does INH high select channel 0?
No. INH high gives the “none” selection result regardless of C, B and A. Channel 0 requires INH low and all three address inputs low. A disabled switch path should not be described as a driven-zero output.
Which address input changes the channel number by one?
A is the lowest-order bit in ST's CBA table. Compare 000 with 001: only A changes, selecting channel 0 or 1. Preserve the manufacturer's column order when transcribing a table into a review note.
Is channel 7 connected to pin 7?
No. Channel 7 is on physical pin 4; pin 7 is NC. Use the pin-description table alongside the truth table. Channel numbering describes the switch choice, whereas pin numbering describes the package connection.
Can the truth table confirm GPIO compatibility?
No. It describes logical selection after valid input states are established. GPIO compatibility requires comparing the controller's output specifications with the multiplexer input requirements at the actual supply and operating conditions. A nominal logic-voltage label is insufficient.
Does a matching function make another 4051-family part interchangeable?
No. Preserve the proposed complete MPN and review its own pinout, supply requirements, electrical limits, package and applicable qualification information. Similar descriptions or selection tables do not replace the required engineering review and approval of a substitution.