ADE5166 datasheet(76/148 Pages) AD

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ADE5166/ADE5169/ADE5566/ADE5569

Preliminary Technical Data

Rev. PrB | Page 76 of 148

is that the use of the general-purpose RAM can be limited to

data storage. The use of the extended internal RAM can be

limited to the stack, or alternatively split between the stack and

data storage if more space is required. This separation limits the

chance of data corruption because the stack can be contained in

the upper section of the XRAM and does not overflow into the

lower section containing data. Data can still be stored in

extended RAM by using the MOVX command.

The default starting address for the stack is 0x100, electing the

upper 1792 bytes of XRAM for the stack operation. The starting

address can be reconfigured to reduce the stack by writing to

Bits SPH[5:3]. These three bits set the value of the three most

significant bits of the stack pointer. For example, setting the

SPH[5:3] to a value of 110b moves the default starting address

of the stack to 0x600, allowing the highest 512 bytes of the

XRAM to be used for stack operation. If the situation occurs

that the stack reaches the top of the XRAM and overflows, the

stack pointer rolls over to the default starting address that is

written in SPH[5:3]. Care should be taken if altering the default

starting address of the stack because unwanted overwrite

operations may occur should the stack overflow or underflow.

Stack Boundary Protection

As a warning signal that the stack pointer is extending outside

the specified range, a stack boundary protection feature is

included. This feature is controlled through the stack boundary

SFR (STCON, 0xBF) and is disabled by default. To enable this

feature, the boundary protection enable bit should be set in the

STCON SFR.

The stack boundary protection works in two ways to protect the

remainder of the XRAM from being corrupted. The waterline

detection feature monitors the top of the stack and warns the

user when the stack pointer is reaching the overflow point. By

setting STCON[7:3], the level of the water line below the top of

the XRAM can be set. For example, by setting STCON[7:3] to

the maximum value of 0x1F, the waterline is set to its minimum

value of 0x7FF − 0x1F = 0x7F0. Similarly, by setting STCON[7:3]

to 0x1, the waterline is set at the top of the RAM space, 0x7FE.

Note that if STCON[7:3] is set to 000b, then the feature is

effectively disabled and no interrupt or reset is generated.

The bottom of the stack is also preserved by the stack boundary

feature. Should the stack pointer be written to a value lower

than the default stack starting address defined in the SPH[5:3],

a warning is issued and the perpetrating command is ignored.

The protection for both the waterline and the stack starting

address are enabled simultaneously by setting Bit 1 in the

STCON SFR.

When enabled, the stack boundary protection can be configured

to either reset the part or trigger an interrupt when a stack viola-

tion occurs. The value of STCON[2] determines the response

of the part. When STCON[2] is set to 0x1 and the stack pointer

exceeds the waterline, the part resets immediately no matter

what other routines are in progress. If an attempt is made to

move the stack pointer below the default stack starting address

when STCON[2] is high, a reset also occurs. If an interrupt

response is selected, the watchdog interrupt service routine is

entered assuming that there is no higher level interrupt currently

being serviced. It should be noted that once STCON[1] is

enabled, an interrupt(or reset) triggers if the stack boundary is

violated, regardless of the status of the EA bit in the IE register.

This is because the watchdog interrupt is automatically configured

as a high priority interrupt and, therefore, is not disabled by

clearing EA. When STCON[1] is low, the feature is completely

disabled and no pending interrupts are generated.

There are two separate flags associated with the stack boundary

protection, allowing the cause of the violation to be determined.

When the waterline is exceeded, a flag is set in Bit 0 of the stack

boundary SFR (STCON, 0xBF) indicating that the reset/interrupt

was initiated by the stack waterline monitor. This flag remains

high until the stack pointer falls below the waterline and the

user clears the flag in software. A waterline or watchdog reset

alone does not clear the flag. To successfully clear the flag, the

software clear must occur while the stack pointer is below the

waterline.

Note that the stack pointer should never be altered while in the

interrupt service routine because this leads to the program

returning to a different section of the program and therefore

malfunctioning. An external reset also causes the waterline flag

to reset.

When an attempt has been made to move the stack pointer

below the stack starting address, a flag is set in the stack pointer

high SFR (SPH, 0xB7), indicating that the reset/interrupt was

initiated by the stack bottom monitor. Once again, a boundary

or watchdog reset alone does not clear this flag, and the user

must clear the flag in software to successfully acknowledge the

event. Note that if SPH[5:3] and SPH[2:0] are altered simulta-

neously to reduce the default stack starting address, when the

stack boundary condition is enabled, a stack violation condition

occurs and the stack bottom flag, SPH[6], is initiated. To avoid

this condition, it is recommended that the default stack starting

address remain at 0x100 or be increased further up the XRAM.

A useful implementation of the waterline feature is to determine

the amount of space required for the stack and allow a suitable

default starting address to be selected. This optimizes the use

of the additional XRAM space, allowing it to be used for data

storage. To obtain this information, the waterline should be set

to the estimated stack maximum and the interrupt enabled. If

the stack exceeds the estimated maximum, the interrupt triggers,

and the waterline level should be increased in the interrupt

service routine. Before returning to the main program, the

waterline interrupt status flag should be cleared. This program

continues to jump to the waterline service routine until the

stack no longer exceeds the waterline level and the maximum

stack level has been determined.

The stack boundary feature can be protected by clearing the

second most significant bit in Flash Location 0xF7FF (see the

Protecting the Flash section). If this protection bit is enabled,

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