The high-speed microcontroller family supports in-circuit debugging of designs. The in-system disable
(ISD) feature allows the device to be three-stated for in-circuit emulation or board testing. During ISD
mode, the device pins will take on the following states:

DEVICE PIN

STATE DURING ISD

Port 0, 1, 2, 3 RST,

True three-state

ALE,

PSEN

Weak Pullup (~10kΩ)

XTAL1, XTAL2

Oscillator remains active

The following procedure is used to enter ISD mode:

1) Assert reset by pulling RST high.
2) Pull ALE low and pull PSEN high.
3) Verify that P2.7, P2.6, P2.5 are not being driven low.
4) Release RST.
5) Hold ALE low and PSEN high for at least two machine cycles.
6) Device is now in ISD mode. Release ALE and PSEN if desired.

Note that pins P2.7, P2.6, P2.5 should not be driven low when RST is released. This will place the device
into a reserved test mode. Because these pins have a weak pullup during reset, they can be left floating.
The test mode is only sampled on the falling edge of RST, and once RST is released their state will not
affect device operation. In a similar manner, the PSEN and RST pins can be released once ISD mode is
invoked, and their state will not affect device operation. The RST pin will also be in a three-state mode,
but asserting it in ISD mode will return the device to normal operation.