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Рыбницкий Филиал Приднестровского Государственного Университета им.Т.Г.Шевченко

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AVR / datasheets / atmega_128.pdf

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ATmega128

Boundary-scan Chain

The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip connection.

Scanning the Digital Port Pins Figure 124 shows the Boundary-scan Cell for a bi-directional port pin with pull-up function. The cell consists of a standard Boundary-scan cell for the Pull-up Enable – PUExn

– function, and a bi-directional pin cell that combines the three signals Output Control – OCxn, Output Data – ODxn, and Input Data – IDxn, into only a two-stage Shift Register. The port and pin indexes are not used in the following description

The Boundary-scan logic is not included in the figures in the Data Sheet. Figure 125 shows a simple digital Port Pin as described in the section “I/O Ports” on page 63. The Boundary-scan details from Figure 124 replaces the dashed box in Figure 125.

When no alternate port function is present, the Input Data – ID corresponds to the PINxn Register value (but ID has no synchronizer), Output Data corresponds to the PORT Register, Output Control corresponds to the Data Direction – DD Register, and the Pullup Enable – PUExn – corresponds to logic expression PUD · DDxn · PORTxn.

Digital alternate port functions are connected outside the dotted box in Figure 125 to make the scan chain read the actual pin value. For Analog function, there is a direct connection from the external pin to the analog circuit, and a scan chain is inserted on the interface between the digital logic and the analog circuitry.

Figure 124. Boundary-scan Cell for Bi-directional Port Pin with Pull-Up Function.

Pullup Enable (PUE)

Output Control (OC)

ShiftDR	To Next Cell
FF2
0
D	Q
1
FF1
0
D	Q
1

EXTEST Vcc

0 LD2 1

D Q

LD1	0

D Q	1
D Q
G

Output Data (OD)

0	FF0	LD0	0
	0		Port Pin (PXn)
1	0		1
1	D Q	D Q	1
	D Q	D Q
	1	G
		G

Input Data (ID)

From Last Cell

ClockDR

UpdateDR

259

2467K–AVR–04/04

Figure 125. General Port Pin Schematic diagram

See Boundary-Scan description for details!

Boundary-scan and the Twowire Interface

PUExn					PUD
			Q	D
			DDxn
			Q CLR
			RESET		WDx
OCxn			RESET
OCxn
					RDx
Pxn			Q	D		BUS
Pxn			Q	D		DATA
ODxn			PORTxn			DATA
ODxn			PORTxn
			Q CLR
IDxn			RESET		WPx
			RESET
SLEEP					RRx
SYNCHRONIZER					RPx
					RPx
D	Q	D	Q
		PINxn
L	Q		Q
					CLK I/O

PUD:	PULLUP DISABLE	WDx:	WRITE DDRx
PUExn:	PULLUP ENABLE for pin Pxn	RDx:	READ DDRx
OCxn:	OUTPUT CONTROL for pin Pxn	WPx:	WRITE PORTx
ODxn:	OUTPUT DATA to pin Pxn	RRx:	READ PORTx REGISTER
IDxn:	INPUT DATA from pin Pxn	RPx:	READ PORTx PIN
SLEEP:	SLEEP CONTROL	CLK I/O :	I/O CLOCK

The two Two-wire Interface pins SCL and SDA have one additional control signal in the scan-chain; Two-wire Interface Enable – TWIEN. As shown in Figure 126, the TWIEN signal enables a tri-state buffer with slew-rate control in parallel with the ordinary digital port pins. A general scan cell as shown in Figure 130 is attached to the TWIEN signal.

Notes: 1. A separate scan chain for the 50 ns spike filter on the input is not provided. The ordinary scan support for digital port pins suffice for connectivity tests. The only reason for having TWIEN in the scan path, is to be able to disconnect the slew-rate control buffer when doing boundary-scan.

2.Make sure the OC and TWIEN signals are not asserted simultaneously, as this will lead to drive contention.

260 ATmega128

2467K–AVR–04/04

ATmega128

Figure 126. Additional Scan Signal for the Two-wire Interface

PUExn

Pxn

SRC

OCxn

ODxn

TWIEN

Slew-rate limited

IDxn

Scanning the RESET Pin The RESET pin accepts 5V active low logic for standard Reset operation, and 12V active high logic for High Voltage Parallel programming. An observe-only cell as shown in Figure 127 is inserted both for the 5V Reset signal; RSTT, and the 12V Reset signal; RSTHV.

Figure 127. Observe-only Cell

					To
				next
	ShiftDR				cell
From system pin						To system logic


			FF1
		0
		1	D	Q
		1

From ClockDR

cell

Scanning the Clock Pins The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscillator, External RC, External Clock, (High Frequency) Crystal Oscillator, Low-fre- quency Crystal Oscillator, and Ceramic Resonator.

Figure 128 shows how each Oscillator with external connection is supported in the scan chain. The Enable signal is supported with a general boundary-scan cell, while the Oscillator/Clock output is attached to an observe-only cell. In addition to the main clock, the Timer Oscillator is scanned in the same way. The output from the internal RC Oscillator is not scanned, as this Oscillator does not have external connections.

261

2467K–AVR–04/04

Figure 128. Boundary-scan Cells for Oscillators and Clock Options

XTAL1/TOSC1 XTAL2/TOSC2

Oscillator

ShiftDR

Cell

EXTEST

From Digital Logic

ShiftDR

cell

ENABLE

OUTPUT

To System Logic

FF1

From ClockDR

UpdateDR

From ClockDR

Cell

Table 103 summaries the scan registers for the external clock pin XTAL1, oscillators with XTAL1/XTAL2 connections as well as 32 kHz Timer Oscillator.

Table 103. Scan Signals for the Oscillators(1)(2)(3)

	Scanned Clock		Scanned Clock Line
Enable signal	Line	Clock Option	when not Used

EXTCLKEN	EXTCLK (XTAL1)	External Clock	0

OSCON	OSCCK	External Crystal	0
		External Ceramic Resonator

RCOSCEN	RCCK	External RC	1

OSC32EN	OSC32CK	Low Freq. External Crystal	0

TOSKON	TOSCK	32 kHz Timer Oscillator	0

Notes: 1. Do not enable more than one clock source as main clock at a time.

2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift between the Internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is preferred.

	3. The clock configuration is programmed by fuses. As a fuse is not changed run-time,
	the clock configuration is considered fixed for a given application. The user is advised
	to scan the same clock option as to be used in the final system. The enable signals
	are supported in the scan chain because the system logic can disable clock options
	in sleep modes, thereby disconnecting the Oscillator pins from the scan path if not
	provided. The INTCAP fuses are not supported in the scan-chain, so the boundary
	scan chain can not make a XTAL Oscillator requiring internal capacitors to run unless
	the fuse is correctly programmed.
Scanning the Analog	The relevant Comparator signals regarding Boundary-scan are shown in Figure 129.
Comparator	The Boundary-scan cell from Figure 130 is attached to each of these signals. The sig-
	nals are described in Table 104.
	The Comparator need not be used for pure connectivity testing, since all analog inputs
	are shared with a digital port pin as well.

262 ATmega128

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ATmega128

Figure 129. Analog comparator

BANDGAP

REFERENCE

ACBG

ACO

AC_IDLE

ACME

ADCEN

ADC MULTIPLEXER

OUTPUT

Figure 130. General Boundary-scan Cell used for Signals for Comparator and ADC

ShiftDR

Cell

EXTEST

From Digital Logic/

From Analog Ciruitry

To Analog Circuitry/

To Digital Logic

D Q

From ClockDR UpdateDR

Cell

263

2467K–AVR–04/04

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