ArC TWO protocol
Introduction
ArC TWO reads instructions over USB port and collects them into an internal
command buffer. Instructions are always 9 WORDs long. A WORD in the context of
this document is an unsigned 32-bit value. The first WORD is the instruction
opcode, WORDs 2 to 8 are the arguments of the instruction and WORD #9 is the
end-of-instruction marker. This document illustrates the instructions ArC TWO
understands. Please note that these are provided for information and
documentation purposes only. If you want to interact with an ArC TWO board
please do so through our Python libraries or GUI Applications. Instructions
with arguments smaller than 8 words are padded with the Empty WORD which is
0x8000_8000. Please note that everything in this document is 0-indexed.
List of supported instructions
CLR: Clear ArC TWO State
Reset the hardware to default state. All switches open and DACs at 0.0 V
AMP PREP: Prepare Op-Amps for Transition
Position |
Name |
Length |
Description |
|---|---|---|---|
1 |
DAC Bitmask |
2 WORDs |
Selected channels to apply AMP PRP (LSB) |
Connect feedback resistors to the op-amp in a controlled manner to avoid transients on the selected channels (words 1 and 2). AMP PRP must be issued before transitions from and to High Speed pulsing configurations
UP CH: Update Channel Configuration
Position |
Name |
Length |
Description |
|---|---|---|---|
1 |
Digipot config (bits 0-9) |
10 bits |
Digipot resistor (1-Dv/1024)×20 kΩ |
Padding |
14 bits |
Fill space to 32 bits |
|
Current Source config (bits 24-32) |
8 bits |
Current Source State Code |
|
4 |
Channel state code |
4 WORDs |
Configuration for all 64 channels |
Set the state of the range and High Speed driver switches in the channels along with the state of the switches in the current source and the range of the digipot.
MOD CH: Modify Channel State
Position |
Name |
Length |
Description |
|---|---|---|---|
2 |
Channel Bitmask |
2 WORDs |
Selected channels to hard GND |
4 |
Channel Bitmask |
2 WORDs |
Selected channels to analogue GND |
6 |
Channel Bitmask |
2 WORDs |
Selected channels to current source |
The MODIFY CHANNEL instruction sets the state of GND, analogue GND and current source connect switches in the channels. It will not change the state of the switches previously set by the UPDATE CHANNEL instruction.
LD VOLT: Set DAC Voltages
Position |
Name |
Length |
Description |
|---|---|---|---|
1 |
Padding |
14 bits |
Pad argument to 1 WORD |
DAC Cluster Bitmask |
18 bits |
DAC Cluster selection (see below) |
|
2 |
Padding |
1 WORD |
|
3 |
Padding |
28 bits |
Pad argument to 1 WORD |
Channel bitmask |
4 bits |
Select channel in DAC cluster (see below) |
|
4 |
Voltage configuration |
4 WORDs |
Set voltage to cluster channels |
The LD VOLT instruction is the basic tool for setting channel voltages. In order for the instruction layout to make sense, some information is required on how the individual channels in ArC TWO are accessed. Each ArC TWO channel is connected to a DAC output. There are 8 channel DACs on board (so 8 channels per board) and 2 auxiliary DACs. We call each set of 8 channels connected to a DAC a channel cluster. Addressing of the channels is done on a half-cluster basis. Word #1 selects the half-cluster the instruction will operate. Similarly word #3 selects the channel within the cluster.
Danger
Beware that incorrect configuration of the DAC outputs might result in damage to the ArC TWO DACs. End users are heavily encouraged to use the user-facing libraries to interact with the hardware rather than issuing raw instructions.
The final 4 words set the voltage for each one of the channels selected in the channel bitmask (word #3). Bits not asserted in word #3 will leave corresponding channels unaffected. Each voltage is represented by a 32-bit value. Each channel can be differentially biased with the upper 16 bits of the voltage corresponding to the DAC+ output and the lower 16 bits corresponding to the DAC- output. For normal biasing situation (non high speed) both values are usually set to the desired voltage, whereas in high speed operations the values of the DAC+/DAC- parts depend on the type of biasing used. DAC+ should always be higher than DAC- to avoid potential damage to the instrument.
Assuming c is the index of the asserted bit in word #1 and i the index
of the asserted bit in word #3 the last four words will correspond to the
following channels. Bottom row indicates the bit in word #3 that needs to be
asserted in order for the voltage to be applied.
Word #4 |
Word #5 |
Word #6 |
Word #7 |
|
|---|---|---|---|---|
Corresponding channel |
4*c + 0 |
4*c + 1 |
4*c + 2 |
4*c + 3 |
Word #3 bit that needs asserting |
Bit 3 |
Bit 2 |
Bit 1 |
Bit 0 |
You can assert different half-cluster and channel bits to compile a suitable DAC configuration for a given biasing scheme. All biasing functions in pyarc2 will make a best effort to minimise the number of LD VOLT instructions required.
The DAC values are linearly distributed over 16 bits with 0x0000 being
-10 V on the standard range (-20 V on the extended range) and 0xFFFF being
+10 V on the standard range (+20 V on the extended range). For both ranges
value 0x8000 corresponds to 0 V. This translates to the following formula
where r is either 10 (for standard range) or 20 (for the extended
range) and V is the target voltage. Values should be rounded to the nearest
uint16. Since 16-bit precision is used for either case the effective resolution
for the standard range is 305 μV and for the extended range is 610 μV.
Note
Issuing a set of LD VOLT instructions is not sufficient to set the DAC voltages. In order to commit the changes to ArC TWO an UP DAC instruction must follow. You need only one UP DAC instruction per DAC configuration.
The upper 2 bits in the 18-bit bitmask in word #2 are reserved for setting up the auxiliary DACs on ArC TWO. These control the logic level, selector voltages and the four arbitrary analogue outputs. The layout for the auxiliary DACs is
Argument |
Bit 16 |
Description |
Bit 17 |
Description |
|---|---|---|---|---|
Word #4 upper |
SELL |
Selector low |
N/A |
|
Word #4 lower |
SELH |
Selector high |
N/A |
|
Word #5 upper |
ARB4 |
Arbitrary 4 |
LGC |
Logic level |
Word #5 lower |
ARB3 |
Arbitrary 3 |
N/A |
|
Word #6 upper |
ARB1 |
Arbitrary 1 |
N/A |
|
Word #6 lower |
ARB2 |
Arbitrary 2 |
N/A |
|
Word #7 upper |
CREF |
Current reference |
N/A |
|
Word #7 lower |
CSET |
Current value |
N/A |
Danger
To avoid damage to the ArC TWO current-related outputs (CSET and CREF) must be both set and within 1.0 V of each other. User-facing libraries include safeguards to ensure proper configuration of the auxiliary DACs.
Note
Logic level value must be set at ×2.62 the intended voltage. For 3.3 V this translates to 8.646 V (0xEEAB in the ±10 V range or 0xB755 in the ±20 V range).
Danger
To avoid unexpected behaviour logic voltage must ALWAYS be between 0 V and 13.5 V (equivalent logic voltage 0–5.15 V).
UP DAC: Update DAC configuration
Update DAC configuration with the previously loaded values. This typically follows any LD VOLT and/or LD OFF instructions. It is required for any DAC configuation to take effect. Please note that you only need on UP DAC per batch of LD VOLT or LD OFF instruction. You can queue multiple LD VOLT or LD OFF instructions and then update the instrument with the single UP DAC.
DELAY: High resolution delay
Position |
Name |
Length |
Description |
|---|---|---|---|
1 |
Delay counter |
1 WORDs |
Delay in 20 ns increments |
Add a delay in the execution chain of ArC TWO. The actual delay value is 320 ns plus the value specified in the instruction argument. Delay is calculated on a 50 MHz clock so the minimum interval is 20 ns. Maximum delay is (2³²−1) × 20 + 320 ns.