Electronic Design

Bypass Switch Speeds Up Data Through Daisy Or JTAG Chain

A daisy chain consisting of multiple devices can be quite cumbersome to control, especially when the only device that needs updating is the last one in the chain. Each device in the chain must be written to, resulting in a slow update rate. It's possible to speed up the chain by bypassing devices using buffers, FETs, and resistors. But this increases the component count, placement cost, and required board space.

A simpler and neater solution uses a single, strategically placed, SOT-23 packaged ADG3233 bypass switch. The switch can bypass one or more devices linked in a chain, thus reducing the number of devices through which the update must propagate. In the example shown in Figure 1, the ADG3233 is used to bypass Device 1 from a chain of three devices. The normal route of flow is as follows (Fig. 1a): Data flows from the data output of Device 0 into pin A1 of the bypass switch, through Device 1 (again through the bypass switch), and onward to Device 2. This action brings data to each device of the daisy chain in turn.

To switch out one device, or one block of devices, the enable line, —EN, is pulled low, enabling the bypass function. Figure 1b shows how Device 1 is eliminated from the data path.

Similarly, this type of device can be used to bypass devices in a JTAG chain (Fig. 2). In the event of a device's failure, the bypass switch can switch the faulty device out of the chain and let testing continue on the other devices.

In addition to its bypass function, the ADG3233 can provide digital level translation between devices operating from different supplies. Figure 1 shows the devices operating from supplies VCC0, VCC1, and VCC2. Because the bypass switch operates from two supply voltages, it allows level translation from low voltages to high voltages—and vice versa. The signal path is unidirectional, with data flowing from A to Y. Supply voltages can range from 1.65 to 3.6 V.

Note that the A1 and —EN inputs can accept inputs from any logic level between 1.65 and 3.6 V and can thus be driven from a device operating from a voltage anywhere in that range. The A2 logic input can accept logic levels that are dependent on the VCC1 voltage level. Output levels from the ADG3233 are between the supply rails VCC1 and VCC2, ensuring that both Device 1 and Device 2 can accept the data written to them.

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