MNSC140CORE datasheet(83/88 Pages) FREESCALE

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MSC8122 Technical Data, Rev. 13

Freescale Semiconductor

4-1

Design Considerations

The following sections discuss areas to consider when the MSC8122 device is designed into a system.

4.1 Start-up Sequencing Recommendations

Use the following guidelines for start-up and power-down sequences:

•

Assert PORESET and TRST before applying power and keep the signals driven low until the power reaches the

required minimum power levels. This can be implemented via weak pull-down resistors.

•

CLKIN

can be held low or allowed to toggle during the beginning of the power-up sequence. However, CLKIN

must start toggling before the deassertion of PORESET and after both power supplies have reached nominal

voltage levels.

•

If possible, bring up VDD/VCCSYN and VDDH together. If it is not possible, raise VDD/VCCSYN first and then bring up

VDDH. VDDH should not exceed VDD/VCCSYN until VDD/VCCSYN reaches its nominal voltage level. Similarly, bring

both voltage levels down together. If that is not possible reverse the power-up sequence, with VDDH going down

first and then VDD/VCCSYN.

Note:

This recommended power sequencing for the MSC8122 is different from the MSC8102.

External voltage applied to any input line must not exceed the I/O supply VDDH by more than 0.8 V at any time,

including during power-up. Some designs require pull-up voltages applied to selected input lines during power-up

for configuration purposes. This is an acceptable exception to the rule. However, each such input can draw up to 80

mA per input pin per device in the system during start-up.

After power-up, VDDH must not exceed VDD/VCCSYN by more than 2.6 V.

4.2 Power Supply Design Considerations

When used as a drop-in replacement in MSC8102 applications or when implementing a new design, use the

guidelines described in Migrating Designs from the MSC8102 to the MSC8122 (AN2716) and the MSC8122

Design Checklist (AN2787) for optimal system performance. MSC8122 and MSC8126 Power Circuit Design

Recommendations and Examples (AN2937) provides detailed design information.

Figure 4-1 shows the recommended power decoupling circuit for the core power supply. The voltage regulator and

the decoupling capacitors should supply the required device current without any drop in voltage on the device pins.

The voltage on the package pins should not drop below the minimum specified voltage level even for a very short

spikes. This can be achieved by using the following guidelines:

•

For the core supply, use a voltage regulator rated at 1.2 V with nominal rating of at least 3 A. This rating does

not reflect actual average current draw, but is recommended because it resists changes imposed by transient

spikes and has better voltage recovery time than supplies with lower current ratings.

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