mc68hc908qb4 Freescale Semiconductor, Inc, mc68hc908qb4 Datasheet - Page 30

• Manufacturer Part Number: mc68hc908qb4
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Memory
2.5 Random-Access Memory (RAM)
This MCU includes static RAM. The locations in RAM below $0100 can be accessed using the more
efficient direct addressing mode, and any single bit in this area can be accessed with the bit manipulation
instructions (BCLR, BSET, BRCLR, and BRSET). Locating the most frequently accessed program
variables in this area of RAM is preferred.
The RAM retains data when the MCU is in low-power wait or stop mode. At power-on, the contents of
RAM are uninitialized. RAM data is unaffected by any reset provided that the supply voltage does not drop
below the minimum value for RAM retention.
For compatibility with older M68HC05 MCUs, the HC08 resets the stack pointer to $00FF. In the devices
that have RAM above $00FF, it is usually best to reinitialize the stack pointer to the top of the RAM so the
direct page RAM can be used for frequently accessed RAM variables and bit-addressable program
variables. Include the following 2-instruction sequence in your reset initialization routine (where RamLast
is equated to the highest address of the RAM).
30
Vector Priority
LDHX
TXS
Highest
Lowest
#RamLast+1
Vector
IF22–
IF16
IF15
IF14
IF13
IF12
IF11
IF10
IF9
IF8
IF7
IF6
IF5
IF4
IF3
IF2
IF1
—
—
MC68HC908QB8 Data Sheet, Rev. 2
Table 2-1. Vector Addresses
Address
$FFDE,F
$FFEA,B
$FFEE,F
$FFFC,D
$FFE0,1
$FFE2,3
$FFE4,5
$FFE6,7
$FFE8,9
$FFFA,B
$FFFE,F
$FFF0,1
$FFF2,3
$FFF4,5
$FFF6,7
$FFD0–
$FFDC
—
—
;point one past RAM
;SP<-(H:X-1)
Not used
ADC conversion complete vector
Keyboard vector
SPI transmit vector
SPI receive vector
ESCI transmit vector
ESCI receive vector
ESCI error vector
Not used
TIM1 Channel 3 vector
TIM1 Channel 2 vector
TIM1 overflow vector
TIM1 Channel 1 vector
TIM1 Channel 0 vector
Not used
IRQ vector
SWI vector
Reset vector
Vector
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