4 Configuring the Level Shifter

Top

4.1 Selecting Shifting Level

There are two options for selecting the Target side logic level.

The first method is to use the power supplied to the Level Shifter Board to power the target device. In this method, the output voltage must be enabled by placing a jumper on the TPWR pins. Then a second jumper is used on one of the voltage selection pins illustrated in Figure 9. The pins are clearly labeled with the output voltage that they will set. If no jumper is set on these pins, then it will default to 1.2 V.

Figure 9: A jumper must be placed on the appropriately labeled header for the desired voltage level. If supplying power from the Level Shifter Board to the target, a jumper needs to be place on TPWR as well.

The second method is to match the power supplied to the Target side when using a self-powered device/board. When using this option, the logic levels will be set to match the voltage levels applied to TPWR as shown in Figure 7.

In this configuration, the TPWR jumper must be removed. Simply connect the power and ground of the target device to the power and ground of the Level Shifter Board Target side. In order to function properly the target’s voltage level should not exceed 3.3 V.

Top

4.2 I2C Level Shifting

The I2C level shifting circuit automatically handles the bi-directionality of I2C communication. Simply select the correct logic level (as described previously) and correctly connect the SCL and SDA lines of the master and slave.

Top

4.3 SPI Level Shifting

Unlike I2C, the SPI level shifting logic must be explicitly controlled for direction. The four headers on the right side of the board determine the direction of traffic. Selections are made by placing a jumper on one of the headers. Their uses are described below:

Figure 10: The Level Shifter Board must be configured from the correct mode of operation when using this board to interface with an SPI or an MDIO bus.

• MASTER: When selected, will configure the Adapter/Analyzer side of the board to behave as the SPI master. Thus, the SS#, SCK, and MOSI lines will be shifted down to the Target side logic level. The MISO line will be left in its default enabled state, and will be shifted up to the Adapter/Analyzer’s logic level.

• SLAVE: When selected, will configure the Adapter/Analyzer side of the board to behave as the SPI slave. Thus, the MISO line will be shifted down to the Target side logic level. The SS#, SCK, and MOSI line will be left in their default enabled state, and will be shifted up to the Adapter/Analyzer’s logic level.

• MONITOR: When selected, will leave all SPI lines in their default enabled state, and will therefore be shifted up to the Adapter/Analyzer side logic level. This option is used in situations where only a Beagle I2C/SPI/MDIO analyzer is plugged into the board, and no SPI adapter is being used.

• DISABLE: When selected, will disable all Target side SPI outputs, and set them to a high-impedance state. If no connection is set, the board will default to this state. However, if any of the previous configuration is set, it will take precedence on this one.

It is important to note that for most all cases only one of these connections should be set at a time. Using more than one may cause incorrect SPI operation. The only situation in which more than one jumper may be used is if the SPI lines are being used for GPIO.

Top

4.4 MDIO Level Shifting

The MDIO signal lines are shared with SPI. It is therefore not possible to bidirectionally shift MDIO signals, as the SPI level shifting does not have the infrastructure for this. However, MDIO monitoring of lower logic levels is still possible. Simply place the SPI control jumper on MONITOR, and set the Beagle I2C/SPI/MDIO analyzer to monitor MDIO.

Top

4.5 GPIO Level Shifting

GPIO signals may fall on either I2C lines or SPI lines, and each of these situations must be handled separately. When the GPIO lines coincide with the SPI lines, special care must be taken to ensure that the SPI level shifting circuit is configured to the correct direction. The SPI lines are logically broken up into two separate groups: the master-driven lines (SS#, SCK, MOSI) and the slave-driven line (MISO). The direction of each of these groups can be controlled independently, but all signals within a group will be in the same direction.

Placing a jumper on the MASTER header will enable the master-driven lines to down-shift, and the slave-driven line to up-shift. Placing a jumper on the SLAVE header will enable the slave-driven line to down-shift, and the master-driven lines to up-shift. Placing a jumper on the MONITOR header will enable all lines to up-shift. If there is ever a time when a line is configured to be both up-shifting and down-shifting, the down-shift will take precedence. Thus, if you want to use all the SPI lines as digital outputs from the host adapter, simply place jumpers on both the MASTER and SLAVE headers.

The I2C lines do not require any directional configuration as it is a bi-directional bus. Each I2C line may go in a different direction, and can also be swapped on the fly.