11.11.2. Physical pin protocol

For a trace capture device to correctly interpret trace data from a SWO, it must know how to decode where data exists on the various pin protocols. This section describes how logic is interpreted and any wire protocols that must be decoded to establish the underlying transmit data.

Note

In Table 11.11 and Table 11.12, the terms HIGH and LOW are relative terms that describe how the pin can appear electrically:

  • LOW is typically 0V

  • HIGH is equivalent to the supply voltage.

Two types of encoding are used:

Manchester encoding

This protocol is supported if bit [10] of the Device ID Register, 0xFC8, is set. It is enabled when bits [1:0] of the Selected Pin Protocol Register, 0x00C, are set to 2'b01. When in Manchester encoding, the SWO outputs up to eight bytes of data between a start and a stop bit.

Figure 11.9 shows how a sequence of bytes is transmitted using the Manchester bit encoding.

Figure 11.9. SWO Manchester encoded data sequence


Table 11.11 shows the Manchester pin protocol encoding.

Table 11.11. Manchester pin protocol encoding

PinLogic '0'Logic '1'Idle State (no data)Valid Data
TRACESWO

LOW-HIGH

(01)

HIGH-LOW

(10)

LOW

(00)

  • start bit:

    HIGH-LOW transition/Logic'1'

  • between 1 and 8 bytes of data.

  • stop bit:

    output LOW, not a valid Manchester symbol


Figure 11.10 shows an example of Manchester encoding, transmitting a bit pattern of 01100101.

Figure 11.10. Manchester encoding example


UART encoding

This protocol is supported if bit [11] of the Device ID Register, 0xFC8, is set. It is enabled when bits [1:0] of the Selected Pin Protocol Register, 0x00C, are set to 2'b10. Data is sent out in packets of ten bits, with start and stop bits, as shown in Figure 11.11 and Table 11.12. Capture devices are expected to operate at the same clocking speed as the TRACESWO pin and synchronize by waiting for a start bit.

Figure 11.11 shows a UART encoded data sequence.

Figure 11.11. UART encoded data sequence


Table 11.12 shows the UART pin protocol encoding.

Table 11.12. UART pin protocol encoding

PinLogic '0'Logic '1'Idle State (no data)Valid Data
TRACESWOLOWHIGHHIGH

10 bit sequence:

  • Logic '0'

  • 8 data bits

  • Logic '1'

The structure is shown in Figure 11.11.


Copyright © 2004-2009 ARM. All rights reserved.ARM DDI 0314H
Non-Confidential