I2C is a well-liked bidirectional serial communications bus having a clock and a knowledge line. Each line’s drivers encompass an open drain ground-referenced N-channel MOSFET with a pullup resistor linked to a provide starting from 1.8 V to five V. The pullup resistor should be sufficiently small to satisfy sure timing necessities within the presence of serious bus capacitance, however giant sufficient that the surprisingly weak lively driver (specified to drop lower than 0.4 V at 3 mA for traditional mode and fewer than 0.6 V at 6 mA for quick mode speeds) present shouldn’t be exceeded and that the logic low ranges are met. Assembly each wants could be a problem.
Determine 44 in part 7.24 of the UM10204 I2C-bus specification and person handbook presents a way of amelioration (Determine 1).
Determine 1: Switched-pullup circuit the place the analog change is activated at excessive bus voltages solely, paralleling a further resistor with the usual pullup. Supply: NXP
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An analog change is activated on the greater bus voltages solely, paralleling a further resistor with the usual pullup. This reduces rise time with out elevating the motive force’s achievable logic low stage. However when the motive force is activated, the quantity of enchancment is restricted by the presence of the extra resistor on the greater voltages—too small a further pullup, and the allowed driver present might be exceeded, and the required logic low stage is not going to be met. A greater method can be to attach the extra resistor solely when the sign is rising, that’s, when the motive force is off. The driving force would then not be preventing the extra pullup, which accordingly may very well be made extraordinarily small. That is the method taken with the next circuit.
In Determine 2, comparators U1 and U2 are set to modify on the logic high and low thresholds of a typical 1.8V I2C bus.
Determine 2 A schematic of simulated I2C drivers, pullup resistors and bus capacitances, with out (outdated) and with (new) connection to the autonomous non-linear pullup circuit.
When the motive force turns off and releases the sign “new” from a logic low, that sign rises by the low threshold. There’s a suitable propagation-delayed constructive output transition of U1 which clocks the 1Q output of D flipflop U3 to a logic excessive. This prompts U4, switching R5 in parallel with the usual pullup R6 and tremendously lowering rise time. Because the sign rises by the logic excessive stage, the output of U2 transitions to a logic low, clearing the 1Q output of U3, deactivating U4 and disconnecting R5. (On this occasion, the propagation delay is welcome. U2’s delay permits the sign time to achieve 1.8 V, courtesy of the extra pullup.) The circuit is now prepared for the motive force’s subsequent activation, which can occur with out it having to combat R5. Till activation, the circuit attracts negligible present. Determine 3 reveals the lowered rise time of the “new” circuit compared to that of the “outdated”, each having the identical bus capacitance and similar customary pullup. 100 pF is simply 25% of the utmost specified worth for I2C operation.
Determine 3 A comparability of the performances of normal (outdated) and an enhanced (new) I2C bus indicators. The indicators CLR, CLK, and Q swing between floor and +3.3 V are proven scaled for readability functions.
Though 1.8 V is a well-liked bus voltage (particularly for good battery IC’s), I used to be unable to search out suitably quick, adequately low provide present comparators which will be powered from this voltage. Happily, 3.3 V is usually obtainable in merchandise with 1.8 V buses, and an analog change serves admirably to bridge the hole between the 2 provides. If the bus runs at 3.3 V, the analog change will be changed with a PNP transistor whose emitter is linked to the bus’s provide, and its base pushed by a 3.3k resistor. Within the unlikely occasion of a 5 V bus, 5V will be linked to the PNP’s emitter, however a 5 V-supply-capable D flip-flop will have to be discovered to exchange U3.
Christopher Paul has labored in varied engineering positions within the communications trade for over 40 years.
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