TCA9509 I2C Repeater: 7 Checks for A-Side Current Sources and Topology
TCA9509 I2C Repeater: 7 Checks for A-Side Current Sources and Topology
TCA9509 checks for asymmetric bus sides, no A-side pull-ups, low-level contention, compatible supplies and repeater topology. This is a documentation-based planning guide, not a report of independent hands-on testing.
1. Treat the two bus sides as electrically different
The TCA9509 buffers SDA and SCL bidirectionally but uses different circuitry on its A and B sides. VCCA supports 0.9 V to 5.5 V and VCCB supports 2.7 V to 5.5 V within the documented operating and application constraints. Do not swap sides solely because the schematic symbol appears symmetric. It performs voltage translation and capacitance separation, not galvanic isolation or protocol conversion. Map each supply, bus segment, target and common ground before choosing the board topology. Manufacturer documentation.
2. Do not add conventional pull-ups to the A side
The A-side ports have integrated current sources, typically 1 mA, and the application guidance explicitly says not to place external pull-up resistors there. This is a major difference from repeaters that require conventional pull-ups on both sides. Audit pull-ups already fitted on sensor modules or the host board, not just newly drawn components. The B side still requires an appropriate pull-up network. A copied two-resistor-per-side recipe can alter the current-sensing behavior and prevent reliable low-level recognition. Manufacturer documentation.
3. Verify the A-side contention-low requirement
The A-side low output is offset from ground, approximately 0.2 V under the described conditions, with an input recognition level below the internally generated low. Every external A-side driver must pull below the specified contention-low threshold. Check the minimum threshold and the target’s guaranteed low level at the actual sink current. A typical low-voltage waveform is not sufficient evidence of worst-case compatibility. Hidden module pull-ups and weak open-drain drivers are especially important to review in this current-sensing arrangement. Manufacturer documentation.
4. Follow supported series and star connections
The documentation shows supported connections with B sides joined, and certain series connections with an A side facing a B side. It does not permit arbitrarily tying all A sides together. The stated series supply rule is VCCA no greater than VCCB minus 1 V; follow the complete application topology and voltage conditions. Repeater delay also accumulates with each stage. A topology that is electrically allowed at low speed can still violate timing when extended or operated faster. Manufacturer documentation.
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5. Change enable only when both segments are idle
EN is active high and must change state only when the global bus and repeater port are idle. Disabling is not a protocol-aware way to interrupt a transaction in progress. The internal drivers also depend on both supplies reaching their documented operating conditions. Powered-off high-impedance bus pins are useful, but do not establish isolation of every external board connection. Include supplies, pull-up rails and host GPIO states when reviewing startup, shutdown and a partially powered sensor assembly. Manufacturer documentation.
6. Check clock stretching at the hand-off
The TCA9509 supports arbitration and clock stretching, but the application guidance calls attention to overshoot during hand-off between controller and target. Appropriate B-side pull-up selection helps reduce that problem. Evaluate rise time and sink current together rather than weakening or strengthening resistors without a timing budget. Capture both segments during a real stretch-capable transaction. A successful address scan without stretching does not validate the condition most likely to expose an incompatible low-level or hand-off behavior. Manufacturer documentation.
7. Qualify the bus architecture at its intended rate
The part supports Standard-mode and Fast-mode applications, including the documented 400 kHz operation, but system rate remains limited by loading, accumulated delays and connected devices. Record supplies, topology, existing pull-ups, A-side low margin, B-side rise time and clock-stretch traces. Exercise idle enable, target reset and power sequencing with noncritical peripherals. The final acceptance should establish bidirectional traffic and recovery for that actual architecture, not imply that a repeater automatically makes long cables, arbitrary cascades or unsupported target speeds reliable. Manufacturer documentation.
Related reading: PCA9515A I2C Repeater: 7 Checks for Pull-Ups and Clock Stretching.
Catalog note: The exact-part search did not return a matching Epro listing during preparation. Check the current catalog; this guide makes no price, availability or included-accessory claim.