MC9S12XD256MAL Datasheet, PDF (202/1348 Pages) Freescale Semiconductor, Inc – Freescale’s Scalable Controller Area Network

Chapter 6 XGATE (S12XGATEV2)
6.4 Functional Description
The core of the XGATE module is a RISC processor which is able to access the MCU’s internal memories
and peripherals (see Figure 6-1). The RISC processor always remains in an idle state until it is triggered
by an XGATE request. Then it executes a code sequence that is associated with the request and optionally
triggers an interrupt to the S12X_CPU upon completion. Code sequences are not interruptible. A new
XGATE request can only be serviced when the previous sequence is finished and the RISC core becomes
idle.
The XGATE module also provides a set of hardware semaphores which are necessary to ensure data
consistency whenever RAM locations or peripherals are shared with the S12X_CPU.
The following sections describe the components of the XGATE module in further detail.
6.4.1 XGATE RISC Core
The RISC core is a 16 bit processor with an instruction set that is well suited for data transfers, bit
manipulations, and simple arithmetic operations (see Section 6.8, “Instruction Set”).
It is able to access the MCU’s internal memories and peripherals without blocking these resources from
the S12X_CPU¹. Whenever the S12X_CPU and the RISC core access the same resource, the RISC core
will be stalled until the resource becomes available again¹.
The XGATE offers a high access rate to the MCU’s internal RAM. Depending on the bus load, the RISC
core can perform up to two RAM accesses per S12X_CPU bus cycle.
Bus accesses to peripheral registers or flash are slower. A transfer rate of one bus access per S12X_CPU
cycle can not be exceeded.
The XGATE module is intended to execute short interrupt service routines that are triggered by peripheral
modules or by software.
6.4.2 Programmer’s Model
Register Block
Program Counter
15
R7
0
15
PC
0
15
R6
0
15
R5
0
Condition
Code
Register
15
R4
0
NZVC
15
R3
0
3210
15
R2
0
15
R1_(Variable
0
Pointer)
15
R0 = 0
0
Figure 6-19. Programmer’s Model
1. With the exception of PRR registers (see Section “S12X_MMC”).
MC9S12XDP512 Data Sheet, Rev. 2.21
202
Freescale Semiconductor