Advanced Parallel I/O - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 306

7.5.2 Port E
As shown in Figure 7.1, Port E pins are used for bus control and interrupt service request
signals. When a pin is not used for one of these specific functions, it can be used as a general-
purpose I/O. However, two of the Port E pins (PE[1:0]) can only be used for inp ut, and t he sta tes
of these pins can be read in the port data register even when they are used for IRQ and XIRQ.
PE0/XIRQ
PE1/IRQ
PE2/R/W
PE3/LSTRB/TAGLO
PE4/ECLK
PE5/MODA/IPIPE0
PE6/MODB/IPIPE1
PE7/NOACC/XCLKS
Figure 7.1 ■ Port E pins and their alternate functions
The PE7 pin serves three functions.
1. A general-purpose I/O pin
2. A signal that indicates that the MCU is not accessing external memory
3. A signal that selects between the external clock or crystal oscillator signal as the
clock input to the HCS12
The PE6 and PE5 pins serve three functions.
1. General-purpose I/O pins (in single-chip mode)
2. Signals that set the operation mode of the HCS12 after reset
3. Instruction queue tracking signals (in expanded mode)
The PE4 pin has two functions.
1. A general-purpose I/O pin (in single-chip mode)
2. The E-clock output (in expanded mode)
The PE3 pin has three functions.
1. A general-purpose I/O pin (in single-chip mode)
2. The LCD front-plane segment driver output pin (in H subfamily)
3. The low-byte strobe signal to indicate the type of access (in expanded mode)
The PE2 pin has two functions.
1. A general-purpose I/O pin (in single-chip mode)
2. A signal to indicate whether the current bus cycle is a read cycle or a write cycle (in
expanded mode)
The PE1 pin has two functions.
1. A ge nera l-purpose I/O pin
2. The IRQ input